Phylogeography of the Chydorus sphaericus Group (Cladocera: Chydoridae) in the Northern Palearctic

The biodiversity and the biogeography are still poorly understood for freshwater invertebrates. The crustacean Chydorus sphaericus-brevilabris complex (Cladocera: Chydoridae) is composed of species that are important components of Holarctic freshwater food webs. Recent morphological and genetic study of the complex has indicated a substantial species diversity in the northern hemisphere. However, we know little of the geographic boundaries of these novel lineages. Moreover, a large section of the Palearctic remains unexamined at the genetic level. Here we attempt to address the biodiversity knowledge gap for the Chydorus sphaericus group in the central Palearctic and assess its diversity and biogeographic boundaries. We sequenced nuclear (ITS-2) and mitochondrial (COI) gene regions of Chydorus specimens across the Palearctic and compared them with already available Holarctic sequences. We detected six main clades in the C. sphaericus group in the Palearctic, of which two of the groups are novel. Three of the more divergent clades are geographically widespread. The central portion of Eurasia (the Yenisey River basin) appears to be a narrow zone of secondary contact for phylogroups that expanded from European and Beringian refugia. As such, the previously unsampled central Palearctic represents an important region for understanding the evolutionary consequences of Pleistocene climatic oscillations on the Chydorus sphaericus group.

淡水无脊椎动物的生物多样性与生物地理学特征，目前仍存在大量未被阐明的认知空白。枝角亚目（Cladocera）圆囊溞科（Chydoridae）的球形短唇溞复合群（Chydorus sphaericus-brevilabris complex），其组成物种是全北界（Holarctic）淡水食物网的核心组成部分。针对该复合群的近期形态学与遗传学研究表明，北半球范围内该类群蕴藏着丰富的物种多样性。然而，学界对这些新演化支的地理分布边界仍知之甚少；此外，古北界（Palearctic）的大片区域尚未开展遗传学层面的研究。本研究旨在填补古北界中部区域球形短唇溞复合群（Chydorus sphaericus group）的生物多样性研究空白，并评估其物种多样性与生物地理分布边界。研究团队对分布于古北界各地的圆囊溞（Chydorus）标本的细胞核ITS-2与线粒体COI基因区域进行了测序，并与已公开的全北界序列开展比对分析。结果显示，古北界的球形短唇溞复合群中共检出6个主要演化支（clade），其中2个为首次发现的类群。3个分化程度较高的演化支具有广泛的地理分布范围。欧亚大陆中部区域（叶尼塞河流域）似乎是源自欧洲与白令陆桥避难所的系统发育类群的狭长次生接触带。综上，此前未被采样的古北界中部区域，是研究更新世（Pleistocene）气候波动对球形短唇溞复合群演化影响的关键区域。