Key Laboratory of National Forestry and Grassland Administration for Orchid Conservation and Utilization, College of Landscape Architecture, Fujian Agriculture and Forestry University, Fuzhou, 350002, China.
State Key Laboratory of Subtropical Silviculture, Bamboo Industry Institute, Zhejiang Agriculture and Forestry University, Hangzhou, 311300, China.
BMC Plant Biol. 2024 Nov 29;24(1):1146. doi: 10.1186/s12870-024-05827-6.
Myrmecophytes, characterized by specialized structures like hollow stems that facilitate mutualistic relationships with ants, serve as an important system for studying ant-plant interactions and the adaptation mechanisms. Caularthron and Myrmecophila are exemplary myrmecophytes within Orchidaceae. Previous studies suggested a genetic relationship between these two genera, placing them within Laeliinae (Epidendreae), yet the precise phylogenetic positioning remained uncertain. The absence of available plastome resources has hindered investigations into plastome evolution and phylogeny.
In this study, we sequenced and assembled the complete plastomes of Caularthron bicornutum and Myrmecophila thomsoniana to elucidate their plastome characteristics and phylogenetic relationships. The determined plastome sizes were 150,557 bp for C. bicornutum and 156,905 bp for M. thomsoniana, with GC contents of 37.3% and 37.1%, respectively. Notably, M. thomsoniana exhibited a distinctive IR expansion and SSC contraction, with the SSC region measuring only 4532 bp and containing five genes (ccsA, ndhD, rpl32, psaC, and trnL-UAG), a unique feature observed for the first time in Epidendreae. Comparative analyses with species from the related genus Epidendrum revealed that C. bicornutum plastome exhibited conserved genome size, GC content, gene content, and gene order. A total of 32 and 33 long sequence repeats, 50 and 40 tandem repeats, and 99 and 109 SSRs were identified in the plastomes of C. bicornutum and M. thomsoniana, respectively. The RSCU analysis demonstrated a consistent pattern in both plastomes, with 29 out of 30 codons with RSCU values greater than 1 featuring A/U at the third codon position. Leucine was the most prevalent amino acid, while Cysteine was the least common. Four potential DNA barcoding regions with Pi values exceeding 0.07, namely ycf1, ccsA-psaC, petN-psbM, and accD-psaI, were identified for subsequent phylogenetic reconstructions within Laeliinae. Phylogenetic analysis underscored the close relationships among Caularthron, Epidendrum, and Myrmecophila.
This study represents the first comprehensive analysis of the plastome characteristics of Caularthron bicornutum and Myrmecophila thomsoniana. Through our characterization and phylogenetic analyses, we unveiled the unique IR expansion/SSC contraction and further elucidated their phylogenetic positions. Our research contributes significant data and insights into the dynamic evolution of ant-orchid plastomes and the phylogeny of the Laeliinae.
蚁栖植物以具有空心茎等特殊结构为特征，这些结构有利于与蚂蚁建立互利关系，是研究蚂蚁-植物相互作用和适应机制的重要系统。卡特兰属和拟蚁兰属是兰科中典型的蚁栖植物。先前的研究表明，这两个属之间存在遗传关系，将它们置于拟兰族（萼兰亚族）中，但它们的系统发育位置仍不确定。缺乏可用的质体基因组资源阻碍了对质体进化和系统发育的研究。
本研究对卡特兰属的二角卡特兰和拟蚁兰属的汤姆逊拟蚁兰的完整质体基因组进行了测序和组装，以阐明它们的质体特征和系统发育关系。确定的质体大小分别为二角卡特兰的 150557bp 和汤姆逊拟蚁兰的 156905bp，GC 含量分别为 37.3%和 37.1%。值得注意的是，汤姆逊拟蚁兰表现出独特的 IR 扩张和 SSC 收缩，其 SSC 区域仅长 4532bp，包含五个基因（ccsA、ndhD、rpl32、psaC 和 trnL-UAG），这是在萼兰亚族中首次观察到的特征。与相关属的物种进行比较分析表明，二角卡特兰的质体基因组具有保守的基因组大小、GC 含量、基因含量和基因顺序。在二角卡特兰和汤姆逊拟蚁兰的质体中分别鉴定出 32 个和 33 个长序列重复、50 个和 40 个串联重复以及 99 个和 109 个 SSR。RSCU 分析表明两个质体都呈现出一致的模式，30 个密码子中有 29 个在第三位密码子上具有大于 1 的 RSCU 值，并且均为 A/U。亮氨酸是最常见的氨基酸，而半胱氨酸是最不常见的氨基酸。在拟兰族内进行系统发育重建时，鉴定出四个潜在的 DNA 条形码区域，其 Pi 值均超过 0.07，分别为 ycf1、ccsA-psaC、petN-psbM 和 accD-psaI。系统发育分析强调了卡特兰属、萼兰属和拟蚁兰属之间的密切关系。
本研究首次全面分析了二角卡特兰和汤姆逊拟蚁兰的质体特征。通过我们的特征描述和系统发育分析，揭示了它们独特的 IR 扩张/SSC 收缩，并进一步阐明了它们的系统发育位置。我们的研究为蚁兰质体的动态进化和拟兰族的系统发育提供了重要的数据和见解。
