Discovery of <scp>XY</scp> Sex Chromosomes in <i>Mauremys mutica</i> Provides Insights Into the Role of <i>KDM6B</i> Gene in Coexistence of Temperature‐Dependent and Genetic Sex Determination

The Asian yellow pond turtle (Mauremys mutica) has long been thought to lack dimorphic sex chromosomes, with prevailing theories suggesting a solely temperature-dependent sex determination (TSD) system. In this study, a male chromosome-level genomic sequence with a contig N50 of ~23.59 Mb was generated using a combination of both Nanopore and Hi-C sequencing technologies. We utilise a combination of bioinformatics and cytogenetic experimental validation to demonstrate that this species indeed possesses XY chromosomes, thereby correcting a longstanding misconception. The results suggest that the X chromosome of the Asian yellow pond turtle originated independently during later stages of evolution and underwent chromosomal rearrangements. Notably, it was observed that the sex chromosomes exhibited a significant repeat expansion, with 95.9% comprising repetitive sequences. This expansion is primarily driven by LINE/CR1 repeats, which account for 55.2% of the total length of the X chromosome. We found that the X chromosome underwent a lower rate of adaptive evolution, supporting the concept of the "slower-X" effect. We present a novel model concerning the KDM6B, which is located on both XY chromosomes, mediates a sex determination mechanism that coexists with TSD + XY in turtles. This study paves the way for further exploration into the complexities of sex determination and the evolutionary dynamics of sex chromosomes in turtles and potentially other reptiles.