1. Introduction
Cassava (
Manihot esculenta Crantz) is a globally significant commercial food crop that is widely cultivated in tropical and subtropical regions. It serves as an essential source of food, feed, starch and starch-related products worldwide [
1,
2]. In Southeast Asia, particularly in Thailand, Cambodia and Vietnam, cassava is primarily grown for industrial starch production. Over the past 40 years, efforts have been made to breed high-yield varieties that are well-adapted to local soil and climate conditions [
3,
4]. Despite the extensive crossing and breeding efforts, newly selected varieties face an increasing challenge to outperform released cultivars such as the highly successful KU50 [
3]. This situation suggests that alternative approaches may be necessary to enhance cassava breeding potential in Thailand.
Cassava breeding in Thailand has primarily involved inter-varietal crossing and selection at various trial stages [
3]. Traditionally, breeding decisions have been based on the individual performance of different crossing combinations, with little or no information regarding the segregating progenies at the family level. To date, quantitative genetic analysis, including diallel analysis and combining ability analysis, has not been performed in Thai cassava breeding populations. The lack of genetic information on parents and segregating progenies may be the underlying reason for the limitations in our breeding selections for superior progenies. With the increasing availability of outstanding breeding progenitors, it is crucial to obtain genetic information about the parents and the hybrids they produce. This will enable the effective allocation of resources toward specific crosses with the highest breeding potential for recurrent phenotypic selection.
The selection of suitable progenitors for hybridization is a critical aspect of cassava breeding programs, and a combining ability analysis is a proven method for selecting potential parents [
5]. Evaluating the performance of hybrid progenies commonly involves estimating the general (GCA) and specific (SCA) combining ability effects [
6]. While diallel crossing and combining ability analyses have been utilized in cassava breeding programs in South America and Africa, these methods have yet to be applied in Thai cassava breeding populations. Previous studies have shown that these genetic effects can significantly influence root yield and disease resistance, making this information valuable for guiding effective breeding selection [
7,
8,
9,
10,
11,
12,
13,
14]. Therefore, it is crucial to determine these genetic effects in the Thai cassava population to improve the breeding scheme.
Cassava breeding is a demanding process that requires significant investments in terms of time, labor and resources. However, insights into the genetic basis of breeding crosses can make the breeding process more efficient and streamline expected outcomes. In particular, the use of combining ability analysis can help identify key parents and mating pairs for targeted breeding populations. In this study, six Thai elite cassava varieties were selected for diallel crossing to estimate GCA and SCA effects for four productivity traits: fresh root yield (FRY), fresh shoot yield (FSY), harvesting index (HI) and starch content (SC). The 15 F1 progenies and parents were evaluated using an augmented RCBD design in Northeastern Thailand. Productivity trials were conducted over two consecutive years, with rainfall, temperature and soil conditions monitored. This study aims to provide guidelines for cassava breeding in Thailand by inferring genetic effects on the four traits. By leveraging the Thai elite varieties, the potential for improving breeding efficiency and streamlining the breeding process can be realized.
4. Discussion
In this study, we evaluated the performances and combining ability in a six-progenitor diallel analysis for four productivity traits over two consecutive years. The progenitors were elite Thai varieties. The experimental design employed an augmented randomized complete block design, and the storage roots were harvested at 12 MAP. The average values of F1 for FSY and HI were generally distributed between the respective parents, while the SC averages of F1 surpassed those of the progenitors in some cases. On the other hand, the average FRY values of F1 generally showed lower performance compared to their progenitors across all crosses. This outcome is not unexpected since the progenitors were selected for their exceptional performance over the years, while each F1 family represents a random sample of all potential genotypes that could result from a particular cross. Nevertheless, outstanding individual genotypes were observed for each of the four traits, providing opportunities to select desirable progenies.
The analysis of combining ability in our study confirmed that non-additive SCA effects were relatively more important for the FRY trait, while GCA (breeding value) effects were prevalent for FSY, HI and SC. The significance of non-additive genetic effects for FRY in cassava has been well documented in previous studies summarized by Ceballos et al. [
5]. Numerous diallel analysis studies [
7,
9,
12,
13,
27,
28] and combining ability analyses have consistently demonstrated higher ratios of SCA to GCA for FRY [
8,
10,
11,
14,
29,
30,
31,
32,
33,
34,
35,
36]. Epistasis effects, which involve the interaction of multiple genes, have also been shown to have a profound impact on FRY [
5]. Our study aligns with this consensus, as we observed a GCA:SCA ratio below 1.0 and a higher sum of squares of SCA compared to GCA. Our study, using locally developed germplasm grown under subtropical conditions in Southeast Asia, further supports this notion from studies conducted in South America or Africa. This highlights the importance of considering specific families or approved cross pairs for successful selection of high FRY. Furthermore, the relevance of non-additive genetic effects for FRY also emerged from ongoing efforts to implement genomic selection in cassava breeding [
37].
Previous studies have consistently indicated that GCA effects play a more significant role than SCA effects for HI [
7,
10,
14,
28,
36] and FSY [
10]. Although our results align with this conclusion, the mode of inheritance for HI and FSY is rather inconsistent, as SCA effects were reported to be more important for these traits in some trial locations [
11,
33,
36]. In contrast, our two-year combining ability analysis strongly indicates that SC is influenced by additive effects. To select progenies with desirable FSY, HI and SC, one can test parents with desirable GCA effects for each trait. For example, HB80 and R5 can be utilized for high SC; R1 and HB80 for low FSY; and R1 for high HI. The substantial genetic gains achieved for SC over the years in Southeast Asia provide strong evidence for the influence of additive effects for this trait [
38,
39].
Cultivation conditions may affect the evaluation of genetic effects. Poor growing conditions or unsuitable environments may result in lower SCA effects and a closer resemblance to GCA effects [
10]. The study by Calle et al. [
28] also demonstrated a strong genotype–environment interaction for FRY in cassava. In our study, we observed notable differences between the two years of evaluation. Specifically, in the second year, there was a prolonged dry period and a decrease in FRY. These conditions resulted in no significance in SCA and an almost equal value of GCA. This disparity between the two years could be primarily attributed to differences in rainfall patterns during the critical period for starch deposition in the storage root. The poor performance in root yield under such conditions may impact the expression of the genotype.
Cassava breeding involves making direct crosses between desirable varieties, followed by clonal propagation of selected genotypes through stem cuttings. As a result, inter-varietal crossing results in a huge genetic segregation that requires large numbers of individuals to isolate exceptional genotypes. The presence of extensive within-family genetic variation further weakens the predictive value of GCA [
3]. In addition, the evaluation of traits such as FRY, SC, HI, dry matter content and those related to diseases and environmental changes requires long-term clonal evaluation, further prolonging the breeding and selection schemes. Some of the parental lines used in this study have been partly evaluated by [
40], which reported that more than 40% of selected hybrids are coming from crosses between R90, KU50, R60 and R5, demonstrating their potential as good breeding parents.
Determining the combining ability of parents through mating design analysis is a valuable approach that enhances the likelihood of systematically generating high-yielding breeding lines while minimizing the number of unsuccessful crosses. This study provides insights into the combining ability of six Thai cassava elite varieties for breeding purposes. The findings of this study emphasize the potential for improving cassava productivity through strategic parent selection based on desirable traits, focusing on traits such as FRY as well as FSY and HI for optimal root production and SC for the highest value of root products. However, obtaining an individual with multiple desired traits such as disease resistance and environmental adaptability remains challenging.
The difference in rainfall during the two-year trial allowed analyzing the impact of drought during the 9-11 MAP on the four traits. Crosses such as R1 × R5, R5 × HB80 and KU50 × HNT were identified as potential sources of drought tolerance based on the FRY result in the second year under drought conditions. R1 and HB80 have been identified as general combiners for FSY; R1 as a general combiner for HI and HB80; and R5 as a general combiner for SC.
The findings of this study have important implications for cassava breeding programs, providing valuable insights into the selection of parental lines. By understanding the genetic control of yield and related traits in Thai cassava germplasm, breeders can make informed decisions when designing breeding strategies and selecting suitable parents to improve cassava varieties. These insights contribute to the advancement of cassava breeding efforts, ultimately leading to the development of high-yielding and superior cassava cultivars.
In summary, this study assessed the combining ability of six Thai cassava varieties for FRY, FSY, HI and SC in a diallel-mating design over two years. The results demonstrate that non-additive SCA effects play a crucial role in determining FRY, while GCA effects are predominant for FSY, HI and SC. The study also highlights the influence of growing conditions on genetic effects and the need to consider specific crossing pairs for a successful selection of high FRY. The findings provide insights for selecting parental lines in cassava breeding programs and understanding the genetic control of yield and related traits in Thai cassava germplasm.