The ZW sex chromosomes of Gekko hokouensis (Gekkonidae, Squamata) represent highly conserved homology with those of avian species.

Populations of the gecko lizard Gekko hokouensis (Gekkonidae, Squamata) on Okinawajima Island and a few other islands of the Ryukyu Archipelago, Japan, have the morphologically differentiated sex chromosomes, the acrocentric Z chromosome and the subtelocentric W chromosome, although the continental representative of this species reportedly shows no sex chromosome heteromorphism. To investigate the origin of sex chromosomes and the process of sex chromosomal differentiation in this species, we molecularly cloned the homologues of six chicken Z-linked genes and mapped them to the metaphase chromosomes of the Okinawajima sample. They were all localized to the Z and W chromosomes in the order ACO1/IREBP-RPS6-DMRT1-CHD1-GHR-ATP5A1, indicating that the origin of ZW chromosomes in G. hokouensis is the same as that in the class Aves, but is different from that in the suborder Ophidia. These results suggest that in reptiles the origin of sex chromosomes varies even within such a small clade as the order Squamata, employing a variety of genetic sex determination. ACO1/IREBP, RPS6, and DMRT1 were located on the Z long arm and the W short arm in the same order, suggesting that multiple rearrangements have occurred in this region of the W chromosome, where genetic differentiation between the Z and W chromosomes has been probably caused by the cessation of meiotic recombination.