Differentiation of Dmrt1 Z and W Homologs Occurred Independently in Two Gekko hokouensis Populations

Gekko hokouensis is a gecko species widely distributed across East Asia. Although most of the Japanese populations possess ZW sex chromosomes (female heterogamety), the degree of sex chromosome differentiation varies among populations. The gene encoding for Dmrt1, a transcription factor involved in testis development in vertebrates, is located on the Z and W sex chromosomes of this species and is therefore a candidate of the sex-determining gene. In this study, we investigated the gene structure of the Z and W homologs of Dmrt1 in two populations of Gekko hokouensis from the Ishigaki Island and Okinawa Island. In the Ishigaki population, the ZW chromosome pair is morphologically undifferentiated, whereas in the Okinawa population the ZW pair is heteromorphic. In the Okinawa population, promoter and exon sequences were nearly identical between the Z and W homologs, and no non-synonymous substitution was detected. In contrast, the W homolog in the Ishigaki population exhibited 42 bp and 12 bp deletions in exon 2. The predicted three-dimensional protein structure revealed a rearrangement of the C-terminal region in the W homolog that may interfere with target site binding. These results indicate that differentiation between Z and W homologs of Dmrt1 has occurred independently in each population. Our findings highlight the diversity of sex chromosome evolution and sex-determining mechanisms even within a single species.