Chrysosplenium L. (Saxifragaceae) is a genus of plants widely distributed in Northern Hemisphere and usually found in moist, shaded valleys and mountain slopes. This genus is ideal for studying plant adaptation to low light conditions. Although some progress has been made in the systematics and biogeography of Chrysosplenium, its chloroplast genome evolution remains to be investigated. To fill this gap, we sequenced the chloroplast genomes of six Chrysosplenium species and analyzed their genome structure, GC content, and nucleotide diversity. Moreover, we performed a phylogenetic analysis and calculated non-synonymous (Ka) /synonymous (Ks) substitution ratios using the combined protein-coding genes of 29 species within Saxifragales and two additional species as outgroups, as well as a pair-wise estimation for each gene within Chrysosplenium. Compared with the outgroups in Saxifragaceae, the six Chrysosplenium chloroplast genomes had lower GC contents; they also had conserved boundary regions and gene contents, as only the rpl32 gene was lost in four of the Chrysosplenium chloroplast genomes. Phylogenetic analyses suggested that the Chrysosplenium separated to two major clades (the opposite group and the alternate group). The selection pressure estimation (Ka/Ks ratios) of genes in the Chrysosplenium species showed that matK and ycf2 were subjected to positive selection. This study provides genetic resources for exploring the phylogeny of Chrysosplenium and sheds light on plant adaptation to low light conditions. The lower average GC content and the lacking gene of rpl32 indicated selective pressure in their unique habitats. Different from results previously reported, our selective pressure estimation suggested that the genes related to photosynthesis (such as ycf2) were under positive selection at sites in the coding region.

中文翻译：

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对六个叶绿体基因组的分析提供了对 Chrysosplenium（虎耳草科）进化的深入了解。


Chrysosplenium L.（虎耳草科）是广泛分布于北半球的植物属，通常发现于潮湿、阴凉的山谷和山坡。该属非常适合研究植物对弱光条件的适应。尽管金脾属的系统学和生物地理学取得了一些进展，但其叶绿体基因组进化仍有待研究。为了填补这一空白，我们对六种金脾属物种的叶绿体基因组进行了测序，并分析了它们的基因组结构、GC 含量和核苷酸多样性。此外，我们使用虎耳草目内 29 个物种和另外两个物种作为外类群的组合蛋白质编码基因进行了系统发育分析并计算了非同义 (Ka)/同义 (Ks) 替换率，以及对金脾内的每个基因。与虎耳草科的外类群相比，金脾属6个叶绿体基因组的GC含量较低；它们还具有保守的边界区域和基因内容，因为四个金脾叶绿体基因组中仅丢失了 rpl32 基因。系统发育分析表明，金脾属分为两个主要分支（相对组和交替组）。 Chrysosplenium 物种基因的选择压力估计（Ka/Ks 比）表明 matK 和 ycf2 受到正选择。这项研究为探索金脾属的系统发育提供了遗传资源，并为植物适应弱光条件提供了线索。较低的平均GC含量和rpl32基因的缺失表明其独特栖息地的选择压力。 与之前报道的结果不同，我们的选择压力估计表明与光合作用相关的基因（例如 ycf2）在编码区位点受到正选择。