Sex Chromosome Evolution in Turtles

Reptiles display a wide diversity of sex determining mechanisms that encompass temperature-dependent sex determination (TSD) and genotypic sex determination (GSD) with either male (XY) or female (ZW) heterogamety.  Despite this astounding variability, the origin, structure and evolution of sex chromosomes remains poorly understood.  TSD is purportedtly ancestral in turtles while GSD has evolved repeatedly, once in a single family of side-neck turtles (Suborder Pleurodira), and multiple times independently within and across several families of hide-neck turtles (Suborder Cryptodira). We tested whether independent (XY or ZW) or morphologically divergent heterogametic sex chromosome systems evolved in tryonichid turtles (Cryptodira) using the GSD spiny softshell turtle, Apalone spinifera, a species with previously unidentified sex chromosomes. Comparative genomic hybridization (CGH), Giemsa/DAPI staining and 18S rRNA FISH permitted the detection of a ZZ/ZW system in A. spinifera that involves a pair of micro-chromosomes. This heteromorphic ZZ/ZW micro-sex chromosome system is identical to that found in another tryonichid, the Chinese softshell turtle Pelodiscus sinensis. Using a phylogenetic context to compare these findings to previous studies from our lab and others revealed a striking sex chromosome conservation over ~95 million years of tryonichid evolution, in contrast to the morphological divergence observed among younger XX/XY systems in side-neck turtles (Pleurodira). We discuss the need to understand the drivers behind the distinct lability and conservation that characterizes sex chromosomes in different lineages if we are to decipher sex chromosomes evolution in this and other groups of organisms.