**1. Introduction**

The traits of trophic specialization in all parasitic plants are described as "parasitic reduction syndrome". At the genetic level, parasitic reduction syndrome includes the functional and physical reduction of heterotrophs' plastid genomes, where rampant gene loss and an acceleration of molecular evolutionary rates occur [1,2]. Considering the partial or complete absence of their photosynthetic capacity, parasitic plants have to absorb organic nutrients, inorganic nutrients, and water from their hosts [3]. Most parasitic plants are included in the order Santalales and the families Orobanchaceae and Orchidaceae [2]. The first complete chloroplast genome of a parasitic plant was obtained from *Epifagus virginiana*, and all of its photosynthesis and energy producing genes have been lost [4]. Petersen et al. reported the complete plastome sequences of one species of *Osyris* and three species of *Viscum*. These researchers found that these four species have experienced rearrangements, and a

number of protein-coding genes and two tRNA genes have been pseudogenised or completely lost [5]. The complete chloroplast genome of *Schoepfia jasminodora* has been reported; *S. jasminodora* represents the early stages of chloroplast genome degradation along with its transition to heterotrophy in related taxa [6]. Li et al. determined the complete chloroplast genome sequences of *Taxillus chinensis* and *Taxillus sutchuenensis*. The results showed that all *ndh* genes, three ribosomal protein genes, seven tRNA genes, four *ycf* genes, and the *infA* gene of these two species have been lost [7]. Previous studies have reported that *Ra*ffl*esia lagascae* only contains small fragments of plastid sequences at low coverage depth, and they cannot recover any substantial portions of the chloroplast genome [8]. In the parasitic family Orobanchaceae, the complete chloroplast genomes of some species, including *Cistanche deserticola* [9], *Aureolaria virginica*, *Lindenbergia philippensis* [10], and *Lathraea squamaria* [11], have been reported. These chloroplast genomes have shown physical and functional gene loss or pseudogenization. The *Balanophora* plastid genomes of *Balanophora laxiflora* and *Balanophora reflexa* [12], at 15.5 kb in size with only 19 genes, are the most reduced plastomes reported thus far, except for the 11.3 and 15.2 kb genomes of two holoparasitic species of *Pilostyles* [13] and the 12.8 kb genome of the myco-heterotroph *Sciaphila thaidanica* [14]. *Rhopalocnemis phalloides* [15], which belongs to the family Balanophoraceae, has also shown highly plastid genome reduction with 18.6 kb in length. In addition, gene loss has also been found in myco-heterotrophs [16], where carbon is obtained from fungi, thus forming mycorrhizal symbiosis with their roots. Photosynthesis-related genes are lost first, followed by housekeeping genes, which eventually results in a highly reduced genome [17].

The chloroplast is an important organelle in plant cells, and it primarily carries out photosynthesis and carbon fixation. The chloroplast genome is independent of nuclear genes, and the chloroplast possesses its own independent transcription and transport system [18,19]. A typical chloroplast genome of most angiosperms consists of four parts, namely a pair of inverted repeats (IRa and IRb), a large single-copy (LSC) region and a small single-copy (SSC) region [20]. The chloroplast genome sequences are highly conserved in gene order and content [21], and they are thus ideal research models for the study of molecular markers [22,23], species identification [24–26], and species evolution [27].

*Macrosolen* plants are parasitic shrubs that belong to the family Loranthaceae. There are approximately 40 species of *Macrosolen,* and most of them are distributed in Southern and Southeastern Asia, whereas five species of *Macrosolen* are dispersed in China [28]. *Macrosolen cochinchinensis*, *Macrosolen tricolor,* and *Macrosolen bibracteolatus* have been used as folk medicines in China for a long time. *M. cochinchinensis* is used to clear heat and fire, remove blood stasis, and relieve pain. *M. tricolor* is used to dissipate heat and relieve coughing. *M. bibracteolatus* is used to invigorate the liver and kidney, expel wind, remove dampness, and strengthen tendons and bones [29–31]. These species exhibit different medicinal effects. However, they have similar morphologies when they are not in fluorescence (Figure 1), resulting in an extreme difficulty in their identification on the basis of morphological features. The limited reports on *Macrosolen* hinder the related research and development. In this study, we determined the complete chloroplast genome sequences of *M. cochinchinensis*, *M. tricolor* and *M. bibracteolatus*. To reveal the phylogenetic positions of the three species and the evolution of *Macrosolen* within Santalales, we conducted phylogenetic trees using the maximum parsimony (MP) and maximum likelihood (ML) methods on the basis of common protein-coding genes from 16 species. Our results can provide important genetic resources for the study of *Macrosolen*.
