Evolving in the darkness: Phylogenomics of Sinocyclocheilus cavefishes highlights recent diversification and cryptic diversity

Abstract

Troglomorphism-any morphological adaptation enabling life to the constant darkness of caves, such as loss of pigment, reduced eyesight or blindness, over-developed tactile and olfactory organs-has long intrigued biologists. However, inferring the proximate and ultimate mechanisms driving the evolution of troglomorphism (stygomorphism) in freshwater fish requires a sound understanding of the evolutionary relationships between surface and stygomorphic lineages. We use Restriction Site Associated DNA Sequencing (RADseq) to better understand the evolution of the Sinocyclocheilus fishes of China. With a remarkable array of derived stygomorphic traits, they comprise the largest cavefish diversification in the world, emerging as a multi-species model system to study evolutionary novelty. We sequenced a total of 120 individuals throughout the Sinocyclocheilus distribution. The data comprised a total of 646,497 bp per individual, including 4378 loci and 67,983 SNPs shared across a minimum of 114 individuals at a given locus. Phylogenetic analyses using either the concatenated RAD loci (RAxML) or the SNPs under a coalescent model (SVDquartets, SNAPP) showed a high degree of congruence with similar topologies and high node support (>95 for most nodes in the phylogeny). The major clades recovered conform to a pattern previously established using Sanger-based mt-DNA sequences, with a few notable exceptions. We now recognize six major clades in this group, elevating the blind cavefish S. tianlinensis and the micro-eyed S. microphthalmus as two new distinct clades due to their deep divergence from other clades. PCA plots of the SNP data also support the recognition of six major clusters of species congruent with the identified clades in ordination space. A Bayes factor delimitation (BFD) analysis showed support for 21 species, recognizing 19 previously described species and two putative new cryptic ones. Two species whose identities were previously disputed, S. furcodorsalis and S. tianeensis, are supported here as distinct species. In addition, our multi-species calibrated tree in SNAPP suggests that the genus Sinocyclocheilus originated around 10.16 Mya, with most speciation events occurring in the last 2 Mya, likely favored by the uplift of the Qinghai-Tibetan Plateau and cave occupation induced by climate-driven aridification during this period. These results provide a firm basis for future comparative studies on the evolution of Sinocyclocheilus and its adaptations to cave life.

Keywords: Cavefish; Diversification; Introgression; Phylogenomics; RADseq; Species delimitation.