Cryptic diversity has been underestimated, but many examples have been documented in the scientific literature in recent decades. Researchers often highlight the importance of establishing a stable classification and strong evolutionary relationships for preserving organisms, yet they rarely emphasize the significance of accurately defining taxonomy. The Rhabdophis nuchalis group, which includes species like R. nuchalis, R. chiwen, and R. pentasupralabialis, has attracted attention for its diversity and geographic distribution. However, unresolved issues persist regarding its systematic classification and taxonomy.
In a recent study by Qin et al. (2023), the phylogenetic connections and hidden diversity within this group were re-evaluated using comprehensive genetic data, encompassing single nucleotide polymorphisms (SNPs) from ddRAD-seq and mitochondrial DNA (mtDNA). Their findings generally align with previous research but offer new insights. The analysis of SNP and mtDNA data demonstrated that while the three species within the R. nuchalis group were not strictly monophyletic, each species was clearly supported as a distinct lineage in SNP-based assessments. The study also revealed instances of genetic mixing between certain pairs of species. Notably, a population in eastern Yunnan, China, was identified as potentially constituting a cryptic species, subsequently described based on its physical characteristics. In light of these findings and earlier investigations, the geographic distribution boundaries for each species within this group were redefined.
The history of Rhabdophis nuchalis taxonomy involves several taxonomic shifts. Described initially as Tropidonotus nuchalis by Boulenger in 1891, it was later moved to Natrix by Schmidt in 1925 and subsequently classified under the genus Rhabdophis by Malnate in 1960, containing various species including R. nuchalis. The recognition of R. n. pentasupralabialis as a subspecies by Jiang and Zhao in 1983 was eventually elevated to full species status due to significant and consistent differences. Zhao’s work 1998 limited the distribution of R. pentasupralabialis to Sichuan and Yunnan. Recent research by Piao et al. (2020) refined this taxonomy, designating a previously identified population as a distinct species, R. chiwen.
In a study by Zhu et al. (2022), a molecular analysis of the R. nuchalis group, which includes R. nuchalis, R. pentasupralabialis, R. chiwen, and R. leonardi, was conducted using genetic data from mitochondrial and nuclear genes. The results supported the distinct lineages of the four species, but the exact relationships between them remained uncertain. Notably, the three species within the R. nuchalis complex did not form a single coherent group.
The R. nuchalis complex poses challenges for morphological identification due to its similar appearances. Apart from R. nuchalis, which has a broad distribution, the other two species are limited to the Hengduan Mountains and adjacent areas. This study utilized an integrative approach, combining ddRAD-seq, mtDNA analysis, population genetics, and morphological comparisons to address several key questions: 1) the presence of hidden species within the complex, 2) the geographic range of each species, and 3) the potential occurrence of hybridization between species.
Citations
Citation
Qin L, Bin L, Xinhong X, Yangmei Z, Peng G. 2023. Genomic evidence sheds new light on phylogeny of Rhabdophis nuchalis (sensu lato) complex (Serpentes: Natricidae). Molecular Phylogenetics and Evolution. 2023 Aug 2:107893.
Schmidt KP. 1925. New reptiles and a new salamander from China. The Trustees of American Museum of Natural History; 1925.
Zhu, G et al. 2022. Cryptic diversity and phylogeography of the Rhabdophis nuchalis group (Squamata: Colubridae), Molecular Phylogenetics and Evolution, 166, https://doi.org/10.1016/j.ympev.2021.107325
