Genomic insights into the evolution of the critically endangered soft-shelled turtle Rafetus swinhoei.

The 2019 death of the female Swinhoes soft-shelled turtle (Rafetus swinhoei) individual in China put this species in a dire conservation state. To avoid the loss of genetic information from the female individual and facilitate the protection and preservation of R. swinhoei, we sequenced and assembled its draft chromosome-level genome by combining BGISEQ short reads, Nanopore long reads, and Hi-C sequencing reads. We characterized its potential ZZ/ZW sex determination system. Phylogeny and divergence time analyses indicated that R. swinhoei has a close relationship with Pelodiscus sinensis, and they shared a common ancestor ~54.4 Ma. Population history analyses indicated that R. swinhoei has the lowest heterozygosity among all turtles examined, and its population size has declined dramatically over the past 10 million years. Many core genes involved in autophagy and DNA damage response, such as Rab5a and Parp1, were under positive selection and/or evolving rapidly, and these genes may make important contributions to the long lifespan and evolutionary adaptation of R. swinhoei. The loss of several tooth-related genes in turtle genomes explains the genetic basis of their toothless phenotype. This study not only provides a genomic resource for scientists to study the species using the strategy of comparative genomics, but also provides a data basis for us to find more potential R. swinhoei individuals in the wild.