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Article

Phylotranscriptomics Shed Light on Intrageneric Relationships and Historical Biogeography of Ceratozamia (Cycadales)

by
Sadaf Habib
1,2,*,
Yiqing Gong
2,
Shanshan Dong
2,
Anders Lindstrom
3,
Dennis William Stevenson
4,
Hong Wu
1 and
Shouzhou Zhang
2,*
1
College of Life Sciences, South China Agricultural University, Guangzhou 510642, China
2
Key Laboratory of Southern Subtropical Plant Diversity, Fairy Lake Botanical Garden, Shenzhen and Chinese Academy of Sciences, Shenzhen 518004, China
3
Global Biodiversity Conservancy 144/124 Moo 3, Soi Bua Thong, Bangsalae, Sattahip, Chonburi 20250, Thailand
4
New York Botanical Garden, Bronx, NY 10458, USA
*
Authors to whom correspondence should be addressed.
Plants 2023, 12(3), 478; https://doi.org/10.3390/plants12030478
Submission received: 8 December 2022 / Revised: 10 January 2023 / Accepted: 16 January 2023 / Published: 19 January 2023
(This article belongs to the Special Issue Evolution of Land Plants)
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Abstract

:
Ceratozamia Brongn. is one of the species-rich genera of Cycadales comprising 38 species that are mainly distributed in Mexico, with a few species reported from neighboring regions. Phylogenetic relationships within the genus need detailed investigation based on extensive datasets and reliable systematic approaches. Therefore, we used 30 of the known 38 species to reconstruct the phylogeny based on transcriptome data of 3954 single-copy nuclear genes (SCGs) via coalescent and concatenated approaches and three comparative datasets (nt/nt12/aa). Based on all these methods, Ceratozamia is divided into six phylogenetic subclades within three major clades. There were a few discrepancies regarding phylogenetic position of some species within these subclades. Using these phylogenetic trees, biogeographic history and morphological diversity of the genus are explored. Ceratozamia originated from ancestors in southern Mexico since the mid-Miocene. There is a distinct distribution pattern of species through the Trans-Mexican Volcanic Belt (TMVB), that act as a barrier for the species dispersal at TMVB and its southern and northern part. Limited dispersal events occurred during the late Miocene, and maximum diversification happened during the Pliocene epoch. Our study provides a new insight into phylogenetic relationships, the origin and dispersal routes, and morphological diversity of the genus Ceratozamia. We also explain how past climatic changes affected the diversification of this Mesoamerica-native genus.

1. Introduction

Cycads, with 370 accepted species, are considered one of the oldest lineages of seed plants and are restricted to subtropical and tropical or warm temperate regions [1]. Cycads originated around 265 million years ago in the Permian period; however, extant cycads diversified as a result of rapid radiations in the last 11 to 20 Myr [2]. Four out of ten genera of cycads are distributed in the Neotropical region, i.e., Ceratozamia (38 species), Dioon (18 species), Microcycas (1 species) and Zamia (84 species) [1]. Among them, the former two genera have the greatest diversity in Mexico, with only few species described from neighboring regions of Guatemala and Belize and one outlier in each genus in Honduras [1,3,4].
Due to improved diagnostic methods and genetic data, knowledge about the systematics of Mexican cycads has been expanded greatly, with the number of representative species increasing in recent years. More than 20 cycad species have been described from Mexico in the past few years [3,5,6,7,8,9,10]. Increased understanding of species phylogenetic relationships and their biogeographic distribution in time and space is crucial for their conservation in the region because the majority of cycad species are considered threatened [11]. The primarily Mexican cycad genus, Ceratozamia Brongn. (Zamiaceae, Cycadales), contains ca. 38 species including some recently described species from Mexico [1,4,5,6]. Species of Ceratozamia have their greatest diversity in cloud forests, oak forests, and tropical rain forests on karstic rocks in Mexico and similarly with the three species that occur in Guatemala, Honduras, and Belize. The elevational range of the genus is from near sea level to ca. 2000 m [4].
Ceratozamia is monophyletic genus, sister to the genus Stangeria in a previous molecular data analysis [12]. In contrast, in other molecular analyses, Ceratozamia is resolved as the sister to a monophyletic clade composed of Stangeria and Microcycas + Zamia [2,13,14]. Morphological analyses have indicated that Ceratozamia is the sister to a clade comprising Microcycas + Zamia [15,16]. Several recent studies provide useful insights into the phylogenetic relationships within Ceratozamia using different molecular datasets [17,18]. Ceratozamia is either composed of two or three clades based on different datasets and comparative methods. Using three molecular markers (nrITS, trnL-F, D-SCAR), 16 species of Ceratozamia were divided into three main clades, with northern Mexico being resolved as an ancestral area for the genus [17]. A dataset of six molecular regions (nrITS, rpoC1, matK, rbcL, psbK/I, atpF/H) with an expanded coverage of 28 species revealed that Ceratozamia is composed of two major clades with low to moderately supported topologies [18]. Highly conserved sequences and low genetic diversity among Ceratozamia species has also been reported, using 31 species and MatK and ITS genetic regions [9]. This suggests that new molecular diagnostic characters such as single copy nuclear genes might be helpful to provide an improved understanding of infrageneric relationships as well as better resources for DNA barcoding for species identification in this genus and other cycad genera [13,19,20,21].
Protein-coding nuclear genes provide genetic information from both male and female inherence because they are biparentally inherited. Nuclear genes have been proven to be advantageous to resolve deep family/genus level relationships in all the major land plant groups [22,23,24,25,26,27]. These studies used either phylogenomics or phylotranscriptomics datasets with hundreds to thousands of genes, which demonstrate the great power of the nuclear genes toward addressing phylogenetic questions. A recent study demonstrated that extensive transcriptome data including thousands of single copy nuclear genes (SCGs) presented high resolution among closely related species in the cycad genus Macrozamia with relatively high evolutionary rates, while organellar protein-coding genes were less informative [19]. Therefore, we hypothesized that an extensive SCGs dataset would provide better resolved phylogenetic relationships within Ceratozamia, especially at the intrageneric level, and would further resolve the discrepancies found in earlier studies. Furthermore, a comparative dataset (nucleotide vs. amino acids) and analytical methods (concatenated vs. coalescent) would increase the confidence of the phylogenetic inferences among Ceratozamia species.
Within Ceratozamia, spatio-temporal diversification of the species has been studied recently [18]. According to this study, the cladogenesis events within the genus Ceratozamia started during the mid-Miocene (ca. 12 million years) and reached its maximum in the early Pleistocene (ca. 2 million years ago). Younger age estimates for the genus Ceratozamia have been provided indicating that the crown group was around 6 to 5.45 Ma, respectively, in those two previous reports [12,13]. Additionally, divergence times of Ceratozamia have been estimated in context of diversification ages of Cycadales using extensive phylogenomic analyses and expanded species coverage to 80% of the currently accepted Ceratozamia species, which had better node support at inter- or intra-generic levels [2,14]. Based on reliable sets of fossils records, the pattern of diversification in time and space among Macrozamia species was determined [19], which provided an explanation regarding species diversity in context of past climatic events. Thus, it is expected that these fossil calibrations and age estimations would also better explain the historical biogeography of the genus Ceratozamia and its current species distribution pattern. This in turn leads to a better understanding that spatial diversification has had an effect on environmental conditions and biogeographical history; thus, we explored the ancestral areas for the major clades and species within the genus Ceratozamia.
Ceratozamia are morphologically distinguishable in vegetative characters from closely related genera by the combination of two parallel adaxial canals along the rachis and entire leaflet margins. The presence of megasporophylls and microsporophylls with two distinct apical horns is an autapomorphy for the genus shown (Figure 1E,F) [4,28]. Species descriptions within the genus were traditionally based on morphological variations (Figure 1) such as leaf and leaflet morphology and the color and shape of strobilus indumentum [4,29,30]. In addition, some species diagnoses were based on geographical distribution and habitat differentiations, which is not a widely used approach and has limitations [4,31,32]. Several studies of the genus have evaluated the morphological variation among species populations with geographic and ecological affinities and, as a result, proposed the recognition of species complexes [6,7,18,33,34,35,36,37]. Additionally, morphological characters have been analyzed as quantitative and qualitative leaf morphological variations in the context of the species complexes [18].
Because previous efforts to establish species groups resulted in disparate results [9,17,18], an expanded sampling of species and markers is still needed to resolve infrageneric relationships within the species and species complexes. It has been proposed that in addition to traditionally used morphological characters of taxonomic utility from leaves, characters derived from reproductive structures should also be assessed to define relationships among species within the genus Ceratozamia [4]. Thus, an extensive exploration of morphological characters will provide further insight into their evolutionary history and ancestral states, especially the reproductive characters that have never been discussed in context of molecular phylogeny.
This study has four objectives: (1) Phylogenetic resolution of infrageneric relationships within the genus Ceratozamia using an extensive transcriptome dataset of SCGs, based on comparative dataset and analytical methods, (2) the historical biogeography and spatial distribution of this genus will be clarified based on the phylogenetic tree derived from the molecular evidence, (3) assessing the evolution of morphological characters in context of well-resolved phylogenetic tree, and (4) potential explanations regarding habitat variations impacting the current distribution pattern of Ceratozamia.

2. Results

A list of investigated species of the genus Ceratozamia and outgroup taxa with data submission information is given in Table 1. A total of 3954 SCGs were identified for reconstructing the phylogeny of Ceratozamia. The average alignment length is ~5539 bp, and missing data are from 0 to 27.7%. The aligned length of the concatenated dataset of all nucleotides (nt) was 5,052,760 bp, with 312,760 (6.18%) variable sites and 186,324 (3.68%) parsimony-informative sites. The aligned nucleotide dataset with first and second codon positions (nt12) was 3,369,107 bp long with 200,812 (5.9%) being variable and only 80,283 (2.38%) as the most parsimonious sites. The percentage of variable sites in the amino acid (aa) dataset was 21% (364,453) of the total aligned length (1,684,249 bp), which was the highest among all three datasets. However, only 4% (73,867 bp) of the sites were parsimony-informative. For divergence time estimation, 50 SCGs were selected that had a higher species coverage (>50%) and nucleotide divergence. The final alignment of this 50-gene dataset was around 56 kb.

2.1. Phylogenetic Trees

The topologies produced from the ML tree of the concatenated dataset (nt, nt12, aa) were highly congruent, although with some conflicts of BS < 85 (Figure S1). When the topologies with BS < 85 were split into polytomies, we retrieved consistent results for the concatenated dataset (Figure 2). The phylogenetic inferences generated from the multi-species coalescent analyses in ASTRAL-III were mainly concordant among the three observed datasets (nt, nt12 and aa) and with the concatenated analyses (Figure S2). However, conflicts were retrieved for some internal nodes. The conflicting nodes with PP < 0.85 were split into polytomies (Figure 3), and only highly supported conflicts are discussed below.
Based on the concatenated ML tree with BS < 85 collapsed branches (ML-concat hereafter) and the multispecies MS tree with PP < 0.85 collapsed branches (MS-coal hereafter), Ceratozamia is divided into three major clades. The earliest diverging clade (clade I) consists of two species, i.e., Ceratozamia mixeorum and C. matudae (Figure 2 and Figure 3). The second clade (clade II) consists of 11 species and is further subdivided into two subclades. The interspecific relationships within these subclades were mostly consistent, but some discrepancies also occurred. The conflicting position of either C. whitelockiana or C. euryphyllidia as the earliest diverging species within a subclade of clade II is not resolved in the ML-concat tree (Figure 2). However, C. whitelockiana as the sister to all other species in this subclade is recovered in all three datasets (nt, nt12, aa) for MS-coal analyses (Figure 3 and Figure S2). For the second subclade within clade II, the position of C. alvarezii differed among the analyzed datasets. All ML-concat (BS = 100) and nt, aa datasets of the MS-coal tree (PP = 0.99) retrieved C. alvarezii as a sister to the other species within this subclade. However, in contrast, the nt12 dataset supported C. alvarezii as a sister to C. chimalapensis (PP = 0.99) at the inner most node within this subclade. The third major clade (clade III) consists of 17 species. Ceratozamia decumbens was a sister to all other species within this clade in all analyses. The other species within this major clade were divided into two subclades. Ceratozamia sabatoi, C. zaragozae and C. kuesteriana formed a weakly supported clade in the ML-concat datasets. Moreover, this subclade is maximally supported (PP = 1) in all the MS-coal datasets. The remaining 13 species were grouped into one subclade with several poor to highly supported relationships. Conflicting topologies were retrieved for the phylogenetic positions of C. hildae and C. huastecorum. The fairly supported alternative phylogenetic position of C. hildae was retrieved among the two different analytical methods (ML-concat and MS-coal) and in the different dataset of nt, nt12, aa within the MS-coal analyses (Figure 3 and Figure S2). In the case of C. huastecorum, the conflict occurred between the phylogenetic inferences from the ML-concat and MS-coal analyses. However, neither of these two positions retrieved well supported relationships with a BS > 75 and a PP > 0.85 with their immediate sister groups.
The quartet score calculated by ASTRAL (Figure S3) was higher in the main topology (q1) as compared to the alternative ones (q2, q3) for the three major clades (clade I: q1 = 0.98, q2 = 0.01, q3 = 0.01; clade II: q1 = 0.48, q2 = 0.3, q3 = 0.22; clade III: q1 = 0.4, q2 = 0.34, q3 = 0.25) and for the subclades within clade II (q1 = 0.64, q2 = 0.18, q3 = 0.19). In contrast, the phylogenetic inferences within clade III were low (>40%) at most of the internal nodes. All the species with conflicting positions among the ML-concat and the MS-coal approaches or among the nt, nt12, aa datasets also were revealed to have low quartet support for alternative topologies.

2.2. Biogeography and Habitat Description

As the ML tree topologies were mostly consistent among the three major clades and subclades included within them, we described the results from the constrained 50-gene ML tree to the topologies derived from only the ML-concat analyses. This 50-gene ML tree was then used for subsequent diversification analyses (Figure 4). The crown group of Ceratozamia was estimated to be 15.81 Ma (95% HPD: 10–15.92 Ma) during the mid-Miocene (Figure 4). The split between clade II and III occurred at 11.43 Ma (95% HPD: 6.95–12.93 Ma) during the late Miocene. Speciation within clade I, II and III started within the same sub-epoch of Miocene around 7.8 Ma (95% HPD: 4.62–10.68 Ma), 5.82 Ma (95% HPD: 3.22–7.44) and 7.98 Ma (95% HPD: 4.96–9.48 Ma), respectively (Figure 4). However, the major expansions of species occurred during the Pliocene (18 species) with fewer in the Pleistocene (6 species) (Figure 4).
The species within Ceratozamia are clearly divided into major clades based on their biogeographical distribution in the south and north at the Trans-Mexican Volcanic Belt (TMVB) in Mexico (Figure 5). The region at the south of the TMVB was found to be the ancestral area (Area C: 93% probability) for the entire genus (Table 2). The species of clades I and II are distributed in the south of the TMVB. The dispersal of species from southern to northern Mexico occurred though the TMVB, which is the ancestral area (Area B) of clade III with a higher probability (Area A: 65%) than the northern region (Area B: 33%) (Figure 5, Table 2). After reaching the north, the migration back to the TMVB occurred four times in the last 2–4 Ma and once toward the south through the TMVB.
The highest abundance of Ceratozamia species occurs in evergreen tropical forests, which is the most common habitat type in the south of the TMVB (Figure 5). Next in abundance is the “mountain cloud forest”, and the species were confined to the TMVB and its northern part in Mexico. Species present in coniferous forests have a wide range of distribution from the south to the north. Some species have a broader habitat range such as C. robusta, grown at all habitat ranges except for coniferous forests.

2.3. Character Evolution

Likelihood inferences of character states indicated that all 12 selected morphological characters (Table 3) are homoplastic in Ceratozamia. Among these, only a few such as “habit”, “leaflet shape”, “infertile portion shape” and “distal face of megasporophylls shape” provide distinctions to separate clades or subclades (Figure 6). Mapping of the alternating states for four characters, i.e., “leaflet direction”, “microsporophylls shape”, “angle between horns of megasporophylls”, and “sarcotesta color” appears to be extremely complex (Figure S4). Ancestral states for all of above-mentioned characters were uncertain for the whole genus, as none of the recorded states attain significant probabilities (>75%). On the other hand, “leaflet texture”, “formation of prickles”, “angle between horns of microsporophylls”, and “fertile portion shape” showed less variations (Figure S5).

3. Discussion

3.1. Phylogeny Based on Extensive Transcriptome Data

Increasing the number of genes provided high branch support for relationships within Ceratozamia, which were much lower, and not the optimal estimates, with fewer loci in previous studies [9,17,18,31]. Relationships among the three major clades were robust. There was a general agreement among trees generated from the comparative methods (coalescent vs. concatenated) and the datasets (nt, nt12, aa). The topological discordance between gene trees revealed a gene tree heterogeneity at shallow internal branches, especially for the species within clade III. The nearly identical quartet frequencies for alternative topologies in our datasets suggested that incomplete lineage sorting (ILS) is likely to impede phylogenetic resolution, which is evident from the very short branches for these topologies. Bootstrap support from the nucleotide dataset did not show an increase in branch support, compared to amino acid data, that is generally observed in cycads [19].
In a recent study, Ceratozamia was divided into two major clades: “Miqueliana” and “Mexicana”. C. matudae and C. mixeorum were distantly placed into these two clades, respectively [18]. In contrast, the current study placed these two taxa in clade I as a sister to all remaining taxa within the genus, i.e., Miqueliana clade (clade II) and Mexicana clade (clade III); thus, supporting a previous hypothesis based on a limited taxon sampling and dataset [17]. Our study provides reliable phylogenetic inferences from different comparative methods and datasets for the genus, based on 3954 SCGs including 30/38 species of Ceratozamia.

3.2. Evolution of Species Complexes in Context of Phylogeny

Twenty-one species of Ceratozamia were divided into seven species complexes, based upon data from their geographic distribution, habitat preferences and gross morphology [30]. Later studies added several species to these species complexes [6,7,9,35,38]. Species of the Ceratozamia miqueliana complex and the Ceratozamia norstogii complex were grouped as two monophyletic subclades within the Miqueliana clade. The Ceratozamia miqueliana complex was described based on species distribution in the south and southeast of the TMVB, with wider leaves and leaflets [30,38,39]. These species are only found in evergreen tropical rain forest and are not generally found at elevations greater than 1000 m [40]. Previously, C. whitelockiana was suggested to be closely allied to C. miqueliana [38,41] based on shared cone and leaf characters. However, this species has also been considered to be part of the Ceratozamia robusta complex [30]. Our phylogenetic reconstruction affirmed the placement of C. whitelockiana within the Ceratozamia miqueliana complex in all the analyses and comparative methods.
The Ceratozamia norstogii complex sensu Vovides [29] includes C. norstogii, C. alvarezii and C. mirandae. Based on the current well-supported phylogeny, we recognize the inclusion of C. chimalapensis and C. vovidesii within this complex. This is further supported because these species share leaf and cone morphological features with the species of the Ceratozamia norstogii complex [42,43]. Cerastozamia chimalapensis and C. vovidesii occur in evergreen tropical forests and mountain cloud forests, respectively. The other three species, C. norstogii, C. alvarezii, and C. mirandae, are restricted to coniferous forests. Ceratozamia matudae also shares morphological characters with the species of the C. norstogii complex [42]. However, current distant phylogenetic placement (Figure 4) of the species in the matudae clade shows its distinction from this complex. Previous studies also considered C. matudae as a separate species complex from the extreme south of Chiapas into Guatemala [29].
Within the Mexicana clade, C. decumbens is a sister to two subclades (Figure 2). One of these is the kuesteriana subclade with the three species (C. kuesteriana sister to C. zaragozae + C. sabatoi) belonging to the Ceratozamia kuesteriana complex, which share narrow lanceolate to linear leaflets and close geological proximity in northeastern Mexico [29]. The monophyly of this complex was highly supported in the ML-coal analyses. The second subclade is the latifolia subclade, which is a sister to the kuesteriana subclade (Figure 2). This subclade also includes species of the Ceratozamia mexicana complex and C. robusta complex. None of these two species complexes formed the monophyletic group. The circumscription of the C. robusta complex has remained problematic historically and was partially disentangled in some taxonomic studies [7,33]. All other taxa, which never included any of the other species complexes within Ceratozamia, were also within this clade (Figure 2). Among the seven described species complexes within Ceratozamia [29,33,36,37,44], our study confirmed the monophylly of only three species complexes, i.e., C. norstogii complex, C. miqueliana complex and C. kuesteriana complex, whereas all others are either para- or polyphyletic. In light of the current evidence, we suggest the following six infrageneric groups within three major clades of the genus Ceratozamia: (1) Matudae clade (I), (2) norstogii subclade (Miqueliana clade, II), (3) miqueliana subclade (Miqueliana clade, II), (4) C. decumbens (Mexicana clade, III), (5) kuesteriana subclade (Mexicana clade, III), and (6) latifolia subclade (Mexicana clade, III). The broader circumscription of the latifolia subclade as a single group is suggested because the phylogenetic position of species within this subclade showed several media to poorly supported relationships (Figure 3). Furthermore, newly described species within this group are morphologically similar to the species from different species complexes placed within this monophyletic subgroup. For instance, C. totonacorum shares morphological affinities with C. mexicana, C. tenuis, C. delucana, C. fuscoviridis and C. morettii, all of which are placed within the latifolia subclade [35]. Similarly, Ceratozamia brevifrons is morphologically similar to C. robusta (of the C. robusta complex) and C. mexicana (of the C. mexicana complex) [45].

3.3. Age Estimations and Biogeographic Associations

The divergence between Ceratozamia and its immediate sister group (a clade consisting of Microcycas, Zamia and Stangeria) occurred during the early Cretaceous around 102 Ma [2]. Most of the diversity of the genus Ceratozamia is in Southern Mexico with 21 out of the 38 species occurring there. Previously, phylogeographic investigation of the genus reported that Ceratozamia diverged during the mid-Miocene (ca. 12 Ma), and major cladogenesis events started within the genus during the Pliocene (ca. 5 Ma), reaching the maximum rate during the Pleistocene (2.5 Ma). Furthermore, it was believed that diversification of the “Mexicana clade” started earlier than the “Miqueliana clade”, and there was a free exchange of species among the three biogeographical areas of Ceratozamia distribution [18,32]. In contrast to the above cited study, our study reported three major clades within the genus with slightly older age estimates of crown age dated to the mid-Miocene (ca. 16 Ma). The highest species divergence occurred during the Pliocene, and the speciation rates have been slowing down toward the present, as only six speciation events occurred during the Pleistocene. It is affirmed that the Ceratozamia species originated from an ancestor from the south of TMVB, with a secondary colonization at TMVB and its northern part. There were multiple migrations between the north and at the TMVB; however, species dispersal back to the south from the north or at the TMVB was limited. The earlier diverging C. decumbens inhabited the TMVB in mountain tropical forests during the Late Miocene, at ~8 Ma. Its presence as a sister to the taxa situated at and north of the TMVB appeared to be the product of a recent vicariance event [31]. Our divergence age estimates revealed the rapid diversification among Ceratozamia species in a short time frame (at 11.43 Ma) and fast overland dispersal between, at, or north of the TMVB, and ancestral southern Mexico taxa. The spatio-temporal diversification pattern of Ceratozamia is shaped by fluctuating climatic factors, which arose due to geological events. The vegetation history of southern Mexico to Belize indicated that these regions retained relict floral elements of great age [46,47,48]. Refuges of flora and fauna of ancient times (40,000 years) have been postulated in southern Mexico, which were apparently absent at the Neovolcanic Mountain Range and its northern region [49,50]. The last warm epoch was present in the late and mid-Miocene followed by consecutive cooling events [51]. Palynomorph studies in Mexico mentioned fluctuating climate that became increasingly temperate with semiarid regions intermixed with sub-humid regions, and increased climatic seasonality were responsible for the expansion and divergence of tropical deciduous forests [52,53,54]. Further evidence drawn from lithological studies indicated that the mountainous regions became more arid while the low-lying areas received maximum precipitation [55]. This evidence supports the dispersal time and genetic divergence of a closely related taxa of Ceratozamia into present-day Honduras and bordering eastern Guatemala and western Belize.

3.4. Evolution of Morphological Characters

The evolution of morphological characters is extremely complex in Ceratozamia. Some important diagnostic characters showed homoplasy in character states across the phylogenetic tree. Only four (characters # 1, 2, 8, 11) out of the 12 characters provide some evidence to define one or more major clade, whereas others exhibit considerable convergent evolution (Figure 6, Figures S4 and S5). These four characters are habit, leaflet shape, infertile portion shape and the shape of the distal face of megasporophylls. Most of the species within Ceratozamia have an “epigeous” habit; however, “semi-hypogeous” stems are dominant in the miqueliana and kuesteriana subclades. In addition, this character state has evolved once in the norstogii subclade and thrice in the latifolia subclade (Figure 6A). The ancestral state of leaflet shape among six groups within Ceratozamia is uncertain, and the probability of the two alternating states is low (elliptic–lanceolate = 55%, vs. oblong to oblanceolate = 45%). The leaves of the Matudae clade and the norstogii and kuesteriana subclades are elliptic–lanceolate, and this state also arose once in the miqueliana subclade and four times in the latifolia subclade (Figure 6B). Leaflet shape in Ceratozamia appears to be primarily related to their habitat types. With respect to reproductive characters, “linear” infertile portion shape is more common in the norstogii and miqueliana subclade, and it independently evolved once in the matudae and kuesteriana subclades and in C. decumbens. Most of the species in these subclades either have “orbicular” or “rounded” infertile portions (Figure 6C). The distal face of the megasporophyll is “prominent” in most of the species in the latifolia, norstogii and miqueliana subclades, whereas C. decumbens and the kuesteriana subclade have a “truncated” distal face of the megasporophylls (Figure 6D).
In other cycad genera, as in the genus Ceratozamia, both qualitative and quantitative variation of vegetative characters derived mainly from leaf morphology are widely used for species identification, e.g., [4,9,32,56,57,58,59,60]. This is the first study to evaluate reproductive morphological characters in addition to vegetative characters in broad phylogenetic context within Ceratozamia. We confirmed the extremely complex pattern of vegetative as well as reproductive characters in the genus because they are uninformative either because of little variation or because of high polymorphism within and between populations. Nevertheless, there are some morphological distinctions to represent the phylogenetic groups within the genus (Figure 4). In light of the above-mentioned issues, we affirmed the fact that the infrageneric phylogenetic groups within the genus do not have morphological distinction, but rather other factors such as biogeographical distinction, as in other cycads genera [3,19,61].

4. Materials and Methods

4.1. Sampling, Transcriptome Data Processing and Phylogenomic Analyses

A comprehensive transcriptomes analysis of data of 339 cycad species was conducted in our recent research to explore the early evolution of seed plants along with whole genome sequencing of Cycas panzhihuaensis [2]. We retrieved the transcriptome data of 30 Ceratozamia species from this study to investigate the phylogenetic relationships within Ceratozamia. Additionally, three species (Microcycas calocoma, Stangeria eriopus and Zamia furfuracea) were selected from closely related genera as an outgroup. RNA-seq data were submitted to the National Center for Biotechnology Information (NCBI) under the BioProject ID PRJNA922610. All the species included in our study were collected from the Nong Nooch Tropical Botanical Garden, Pattaya, Thailand (NNTBG), and voucher specimens were submitted at The Singapore Botanic Gardens Herbarium (SING), Singapore and Natural History Museum (W), Vienna Austria (Table 1).
Data processing mainly followed the recent phylotranscriptomic study of the cycad genus Macrozamia [19]. The clean reads were de novo assembled using the Trinity pipeline [62]. TransDecoder (https://github.com/TransDecoder, accessed on 12 March 2022) was used for predicting coding regions of each putative transcript and annotation for assembled transcriptomes. In the next step, orthologs were recognized from annotated transcripts using OrthoFinder v2.5.4 [63]. Then, single-copy genes were selected for phylogenetic reconstruction using Software KinFin v1.1 [64] with default parameters. Gene families containing at least 24 in-group species (>80%) were selected, which then obtained 3954 candidate homologous clusters. Gene dataset was filtered using a local version of program TranslatorX vLocal.pl [65] in the following four steps: (1) use the standard genetic code to translate the nucleotide sequences into amino acids sequences; (2) align these peptide sequences of each putative SCG with MAFFT v.7.505 [66]; (3) further trim the amino acid sequences that exclude ambiguous portions using Gblocks [67]; (4) convert the alignments into the corresponding nucleotide alignments.
For phylogenomic analysis, we used two comparative methods for the SCG dataset, i.e., coalescence- and concatenation-based methods. For the coalescent-based method, single-gene maximum likelihood (ML) trees were reconstructed in IQ-TREE 2 [68], with ultra-fast bootstrap analysis of 100 replicates. All those trees were used as input in ASTRAL-III [69] for species tree inference. To calculate quartet support (q) for each branch including the main topology and the first and second alternatives, option “-t 8” was used in ASTRAL-III. For the concatenation method, genes of all the datasets were concatenated using SeqKit [70], and then, we applied the ML method as implemented in IQ-TREE 2 [68]. The best fitting model is selected by ModelFinder [71], in IQ-TREE 2. Finally, we obtained phylogenetic trees for three datasets, i.e., nucleotide data including all codon positions (nt), nucleotide data from first and second codon position (nt 12) and amino acid alignments (aa) from each of the coalescent and concatenated analyses. Due to the possibility of incongruence among trees from comparative datasets and analyses, the poorly supported branches with <85% support were collapsed using TreeCollapserCL4 [72].

4.2. Molecular Dating, Ancestral Area Reconstruction, and Habitat Assessment

The dating analysis was performed in the following steps: (1) (1) 50 genes were manually selected to produce a 50-gene ML tree for dating analyses with the highest species coverage and sufficient variable sites. (2) Those 50-genes were aligned using MAFFT v.7.505 [66], and the ML trees were reconstructed by RAxML v.8.2.12 [73] under the GTR + G model. (3) Keeping in view the possible conflict among tree topologies from coalescent and concatenated datasets, the tree topologies were constrained initially to the nt tree that resulted from both analyses. (4) We used TreePL [74] to date the divergence times of species within the genus Ceratozamia with the 100 generated from the RAxML bootstrap replicates. For this analysis, we constrained both the root (min: 76.29, max: 110.96) and the crown ages of the genus (min: 10.00, max: 16.29) by using the time estimations found in MCMCTree dating analysis of our previous study of cycads diversification [2]. (5) The maximum clade credibility (MCC) trees were then constructed with median ages and 95% highest posterior density (HPD) intervals on nodes using TreeAnnotator v2.6.2 [75]. We used FigTree v.1.4.4 (https://tree.bio.ed.ac.uk/software/figtree; accessed on 2 May 2022) to visualize the final chronogram.
The final chronogram was also used as input to search and execute the best model in the R package “BioGeoBEARS” [76], which was DIVALIKE + J. Distribution data for Ceratozamia species were mainly derived from herbarium records given in [4] and online at the Global Biodiversity Information Facility (GBIF, http://www.gbif.org/; accessed on 8 May 2022). The Trans-Mexican Volcanic Belt (TMVB) in Mexico is considered an important barrier shaping the spatial patterns and geographic diversification of Ceratozamia [18,31]. Following the approach used in [18], three spatial regions were assigned to 30 Ceratozamia species based on their distribution patterns with respect to TMVB, i.e., Area A: at TMVB, Area B: north of TMVB, Area C: south of TMVB. We allowed the inferred ancestors to occupy a maximum of two areas (sampled tips occupy only one area).
We assessed the distribution pattern of vegetations types on the phylogenetic tree (Figure 5). There are five vegetation ranges where the Ceratozamia species are distributed: (1) evergreen tropical forest, (2) coniferous forest, (3) mountain cloud forest, (4) deciduous tropical forest, and (5) oak forests. The data of vegetation type were recorded from [4,16].

4.3. Morphological Characters State Mapping

Initially, we designed a character matrix including around 20 morphological characters from stems, leaves, pollen and ovulate strobili and seeds. We removed characters if there was little variation among species such as species autapomorphies. For example, all but one species had “recurved” distal face of microsporophylls in Ceratozamia; thus, this character was removed. The final data analyses included 12 morphological characters (Table 2). The character states and coding mainly followed the most recent monograph of Ceratozamia [4]. The ML tree from the concatenated dataset was used as input to infer the character states using the ML approach as implemented in Mesquite v. 3.70 [77]. The Markov k-state one-parameter model of evolution for discrete unordered characters was used [78]. Outgroup species were pruned from the tree because most morphological characters evaluated here were unknown/not present in those taxa.

5. Conclusions

Due to variable molecular markers and analytical methods investigated earlier, phylogenetic relationships within Ceratozamia have been a contentious issue. Based on the coalescent and concatenated approaches utilizing 3954 SCGs, our study yielded a reliable phylogenetic tree of Ceratozamia. It comprises six major branches/subclades with three major clades. The main cause of conflicts observed among the gene trees is suggested to be a rapid evolutionary divergence and ILS, which is evident from the low quartet support values among the gene trees. Molecular dating and diversification analysis based on the 50 concatenated SCGs suggests that Ceratozamia originated around 15.81 Mya, during the mid-Miocene in southern Mexico, which is considered to be a biodiversity hotspot of neotropical cycads. Our study showed that the major groups within the genus are consistent with their geographical distribution and sometimes with morphological variations. On the basis of combined molecular, biogeographical and morphological evidence, the six monophyletic subclades could be proposed as distinct groups in a formal infrageneric classification of the genus.
We concluded that global climate changes have been intensified now, which predict the extinction of the cycad tree of life, as a considerable decline in speciation occurred toward the present [19,79]. Therefore, extensive studies are required to understand the evolutionary fate of cycad species in their respective biodiversity hotspots, which would provide future guidelines for conservation and management practices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/plants12030478/s1. Figure S1:Phylogenetic relationships within Ceratozamia based on con-catenated dataset of (A) nt, (B) nt12, (C) aa, from 3954 Single Copy Nuclear genes (SCGs); Figure S2: Phylogenetic relationships within Ceratozamia based on coalescent dataset of (A) nt, (B) nt12, (C) aa, from 3954 Single Copy Nuclear genes (SCGs); Figure S3: Species trees of the genus Ceratozamia generated in ASTRAL-III for nucleotide dataset with 1st and 2nd codon position (nt12). Support values from quartet analysis are mentioned for each node. The low support values of the con-sensus highlight the amount of incongruence among the gene trees; Figure S4: Optimization of morphological characters with complex pattern, based on Mk1 model implemented in Mesquite. (A) Leaflet direction (character 3), (B) Microsporophyll shape (character 6), (C) Angle between horns of megasporophylls (character 10), and (D) Sarcotesta color (character 12) inferred to ML tree of concatenated dataset with all codon position included (nt dataset), with poorly supported branches (BS < 85) collapsed; Figure S5: Optimization of less variable morphological characters, based on Mk1 model implemented in Mesquite. (A) Leaflet texture (character 4), (B) Leaflet di-rection (character 5), (C) Angle between horns of microsporophyll (character 7), and (D) Fertile portion shape (character 9) inferred to ML tree of concatenated dataset with all codon position included (nt dataset), with poorly supported branches (BS < 85) collapsed.

Author Contributions

Conceptualization, S.H. and S.Z.; Data curation, Y.G.; Formal analysis, S.H., Y.G. and S.D.; Funding acquisition, S.Z.; Investigation, S.D. and A.L.; Methodology, S.H.; Project administration, H.W. and S.Z.; Resources, S.D. and A.L.; Software, S.H., Y.G. and S.D.; Supervision, H.W. and S.Z.; Validation, S.Z.; Writing—original draft, S.H.; Writing—review and editing, S.H., D.W.S. and A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This project was financially supported by the “Scientific Foundation of Urban Management Bureau of Shenzhen (no. 202105 to Y.Q., 202208 to S.Z., 202106 to S.D.), and the Biodiversity Survey and Assessment Project of the Ministry of Ecology and Environment, China (no. 2019HJ2096001006 to S.Z.).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The clean reads data are available at NCBI database under the Bioproject ID PRJNA922610, and SRA accession number of SRR23044168—SRR23044200.

Acknowledgments

We acknowledge Nong Nooch Tropical Botanical Garden, Thailand, especially the President Kampon Tansacha for providing fully documented material for this study. The authors thank the two anonymous reviewers whose excellent comments and suggestions greatly improved the manuscript.

Conflicts of Interest

The authors declare that they have no competing interests.

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Figure 1. Species of Ceratozamia showing morphological variations. (A) C. norstogii, adapted for a shady, high-altitude, seasonal dry forest and displaying a twisted rachis with extremely narrow leaflets; (B) C. hondurensis adapted to a shady rainforest environment, and displaying broad leaflets; (C) C. robusta with distinct reddish-orange to dark maroon newly emerging leaflets; (D) C. mirandae from high-altitude open oak forest with narrow leaflets and a spreading leaf arrangement; (E,F) C. tenuis male vs. female cones displaying the distinct “horn-like” extensions that distinguish Ceratozamia from all other cycad genera. Photographs by Anders Lindstrom.
Figure 1. Species of Ceratozamia showing morphological variations. (A) C. norstogii, adapted for a shady, high-altitude, seasonal dry forest and displaying a twisted rachis with extremely narrow leaflets; (B) C. hondurensis adapted to a shady rainforest environment, and displaying broad leaflets; (C) C. robusta with distinct reddish-orange to dark maroon newly emerging leaflets; (D) C. mirandae from high-altitude open oak forest with narrow leaflets and a spreading leaf arrangement; (E,F) C. tenuis male vs. female cones displaying the distinct “horn-like” extensions that distinguish Ceratozamia from all other cycad genera. Photographs by Anders Lindstrom.
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Figure 2. Congruent tree topology of Ceratozamia found by the maximum likelihood (ML) approach from the concatenated dataset of 3954 SCGs for 30 species. Numbers above branches are ML bootstrap (BS) from the nt/nt12/aa dataset, respectively. Only support values < 100% are shown. Branches with BS < 85 are collapsed. Seven species complexes adapted from Vovides [25,37] are marked in context of phylogenetic tree. A phylogram overview is shown in the upper left-hand corner with the scale bar indicating the average number of nucleotide substitutions per site.
Figure 2. Congruent tree topology of Ceratozamia found by the maximum likelihood (ML) approach from the concatenated dataset of 3954 SCGs for 30 species. Numbers above branches are ML bootstrap (BS) from the nt/nt12/aa dataset, respectively. Only support values < 100% are shown. Branches with BS < 85 are collapsed. Seven species complexes adapted from Vovides [25,37] are marked in context of phylogenetic tree. A phylogram overview is shown in the upper left-hand corner with the scale bar indicating the average number of nucleotide substitutions per site.
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Figure 3. Maximum likelihood topology of the Ceratozamia species relationships inferred from the coalescence analyses of 3954 SCGs and the three comparative datasets (aa/nt12/nt). Posterior probability < 1.0 is given on the nodes. Branches with PP < 0.85 are collapsed. The six groups within the genus proposed in the current study are labeled.
Figure 3. Maximum likelihood topology of the Ceratozamia species relationships inferred from the coalescence analyses of 3954 SCGs and the three comparative datasets (aa/nt12/nt). Posterior probability < 1.0 is given on the nodes. Branches with PP < 0.85 are collapsed. The six groups within the genus proposed in the current study are labeled.
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Figure 4. Chronogram for Ceratozamia reconstructed based on the combined data matrix of 50 SCGs constrained to the ML tree of the concatenated dataset with all codon positions included (nt dataset). Node bar on each node represents the minimum and maximum ranges of dates from both low and high calibration. The map shows the geographical extents of the three major clades among the three distinct areas of its distribution in Mexico and its adjacent regions.
Figure 4. Chronogram for Ceratozamia reconstructed based on the combined data matrix of 50 SCGs constrained to the ML tree of the concatenated dataset with all codon positions included (nt dataset). Node bar on each node represents the minimum and maximum ranges of dates from both low and high calibration. The map shows the geographical extents of the three major clades among the three distinct areas of its distribution in Mexico and its adjacent regions.
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Figure 5. Ancestral area reconstruction for the genus Ceratozamia based on the combined data matrix of 50 single-copy nuclear genes constrained to the ML tree of the concatenated dataset with all codon positions included (nt dataset). The geographical extents of the three areas with reference to the Trans-Mexican Volcanic Belt (TMVB) as (A) at TMVB, (B) north of TMVB, and (C) south of TMVB considered in the analysis. The colored circles on the nodes show the most likely ancestral areas reconstructed by BioGeoBEARS. Current distributions are indicated at the branch tips. Habitat diversity for each Ceratozamia species is given to the right of the species name.
Figure 5. Ancestral area reconstruction for the genus Ceratozamia based on the combined data matrix of 50 single-copy nuclear genes constrained to the ML tree of the concatenated dataset with all codon positions included (nt dataset). The geographical extents of the three areas with reference to the Trans-Mexican Volcanic Belt (TMVB) as (A) at TMVB, (B) north of TMVB, and (C) south of TMVB considered in the analysis. The colored circles on the nodes show the most likely ancestral areas reconstructed by BioGeoBEARS. Current distributions are indicated at the branch tips. Habitat diversity for each Ceratozamia species is given to the right of the species name.
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Figure 6. Optimization of key morphological characters based on the Mk1 model implemented in Mesquite. (A) Habit (character 1), (B) leaflet shape (character 2), (C) infertile portion shape (character 8), and (D) distal face of megasporophyll (character 11) inferred from the ML tree of the concatenated dataset with all codon positions included (nt dataset) and with poorly supported branches (BS < 85) collapsed.
Figure 6. Optimization of key morphological characters based on the Mk1 model implemented in Mesquite. (A) Habit (character 1), (B) leaflet shape (character 2), (C) infertile portion shape (character 8), and (D) distal face of megasporophyll (character 11) inferred from the ML tree of the concatenated dataset with all codon positions included (nt dataset) and with poorly supported branches (BS < 85) collapsed.
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Table
Table 1. List of Ceratozamia species and the three outgroup taxa included in this study for phylogenetic analyses.
Table 1. List of Ceratozamia species and the three outgroup taxa included in this study for phylogenetic analyses.
No.Sample ID *Taxon NamesRaw Data (Gb)Clean Reads (Gb) *Accession No. *Herbarium Code
1SAMN32670532Ceratozamia alvarezii63.32SRR23044200SING
2SAMN32670533Ceratozamia becerrae63.32SRR23044199SING
3SAMN32670534Ceratozamia brevifrons63.35SRR23044188W
4SAMN32670535Ceratozamia chimalapensis63.26SRR23044177SING
5SAMN32670536Ceratozamia decumbens63.29SRR23044173W
6SAMN32670537Ceratozamia delucana63.47SRR23044172SING
7SAMN32670538Ceratozamia euryphyllidia63.21SRR23044171SING
8SAMN32670539Ceratozamia fuscoviridis62.94SRR23044170W
9SAMN32670540Ceratozamia hildae126.49SRR23044169W
10SAMN32670541Ceratozamia hondurensis63.40SRR23044168SING
11SAMN32670542Ceratozamia huastecorum63.38SRR23044198SING
12SAMN32670543Ceratozamia kuesteriana63.09SRR23044197W
13SAMN32670544Ceratozamia latifolia63.36SRR23044196W
14SAMN32670545Ceratozamia matudae63.31SRR23044195SING
15SAMN32670546Ceratozamia mexicana63.35SRR23044194SING
16SAMN32670547Ceratozamia microstrobila63.38SRR23044193W
17SAMN32670548Ceratozamia miqueliana63.27SRR23044192W
18SAMN32670549Ceratozamia mirandae63.35SRR23044191W
19SAMN32670550Ceratozamia mixeorum63.46SRR23044190W
20SAMN32670551Ceratozamia morettii63.44SRR23044189SING
21SAMN32670552Ceratozamia norstogii63.41SRR23044187W
22SAMN32670553Ceratozamia robusta63.31SRR23044186SING
23SAMN32670554Ceratozamia sabatoi63.33SRR23044185W
24SAMN32670555Ceratozamia subroseophylla63.38SRR23044184W
25SAMN32670556Ceratozamia tenuis63.91SRR23044183SING
26SAMN32670557Ceratozamia totonacorum63.34SRR23044182W
27SAMN32670558Ceratozamia vovidesii63.23SRR23044181W
28SAMN32670559Ceratozamia whitelockiana63.4SRR23044180SING
29SAMN32670560Ceratozamia zaragozae63.35SRR23044179W
30SAMN32670561Ceratozamia zoquorum63.43SRR23044178W
31SAMN32670562Microcycas calocoma126.6SRR23044176W
32SAMN32670563Stangeria eriopus126.85SRR23044175W
33SAMN32670564Zamia furfuracea127.79SRR23044174W
* NCBI database.
Table
Table 2. Ancestral area probability (% age) based on Figure 4 for each of the key nodes of the genus Ceratozamia.
Table 2. Ancestral area probability (% age) based on Figure 4 for each of the key nodes of the genus Ceratozamia.
Area A: At TMVBArea B: North of TMVBArea C: South of TMVB
Whole genus2.671.6194.22
Matudae clade00100
Miqueliana + Mexicana5.943.590.23
Miqueliana clade00100
norstogii subclade00100
miqueliana subclade00100
Mexicana clade61.5737.24>1
kuesteriana subclade10000
latifolia subclade72.4426.451.09
Table
Table 3. List of morphological characters traced on the phylogenetic tree of the genus Ceratozamia.
Table 3. List of morphological characters traced on the phylogenetic tree of the genus Ceratozamia.
No.CharactersStates
012
Stem1HabitEpigeoussemi-hypogeous
Leaves2Leaflet shapeelliptic-lanceolateoblong to oblanceolateobovate
3Leaflet directionDescendingascending
4Leaflet textureCoriaceouspapyraceousmembranaceous
5Formation of prickles (Petiole)Robustthin
Microsporangiate strobili6Microsporophyll shapeDiscoidobconicelliptic
7Angle between horns of microsporophyllObtuseacuteright
8Infertile portion shapeOrbicularroundedlinear
9Fertile portion shapeLobatedeeply lobate
Megasporangiate strobili10Angle between horns of megasporophyllsObtuseacuteright
11Distal face of megasporophylls shapeTruncateprominent
Seed12Sarcotesta colorlight-brownwhitish-yellowwhitish-red
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Habib, S.; Gong, Y.; Dong, S.; Lindstrom, A.; Stevenson, D.W.; Wu, H.; Zhang, S. Phylotranscriptomics Shed Light on Intrageneric Relationships and Historical Biogeography of Ceratozamia (Cycadales). Plants 2023, 12, 478. https://doi.org/10.3390/plants12030478

AMA Style

Habib S, Gong Y, Dong S, Lindstrom A, Stevenson DW, Wu H, Zhang S. Phylotranscriptomics Shed Light on Intrageneric Relationships and Historical Biogeography of Ceratozamia (Cycadales). Plants. 2023; 12(3):478. https://doi.org/10.3390/plants12030478

Chicago/Turabian Style

Habib, Sadaf, Yiqing Gong, Shanshan Dong, Anders Lindstrom, Dennis William Stevenson, Hong Wu, and Shouzhou Zhang. 2023. "Phylotranscriptomics Shed Light on Intrageneric Relationships and Historical Biogeography of Ceratozamia (Cycadales)" Plants 12, no. 3: 478. https://doi.org/10.3390/plants12030478

APA Style

Habib, S., Gong, Y., Dong, S., Lindstrom, A., Stevenson, D. W., Wu, H., & Zhang, S. (2023). Phylotranscriptomics Shed Light on Intrageneric Relationships and Historical Biogeography of Ceratozamia (Cycadales). Plants, 12(3), 478. https://doi.org/10.3390/plants12030478

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