This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Why do humans rely on XY chromosomes to determine sex, while birds use ZW—and many reptiles use neither, letting temperature decide instead? Sex determination is remarkably diverse across the animal kingdom.
In genotypic sex determination (GSD), sex is encoded in the genome: XX individuals are female and XY males in humans and most mammals, while in the ZW system, found in birds and some reptiles, males are ZZ and females ZW. Many reptiles instead use environment-dependent sex determination (ESD), most commonly temperature-dependent sex determination (TSD). In leatherback turtles, higher incubation temperatures produce females, and in American alligators, males emerge only within intermediate temperatures.
Remarkably, XY and ZW systems have evolved independently many times from different ancestral chromosomes, raising the question of what drove evolution down one path rather than the other. The findings are published in the journal Science. Geckos are an ideal model for studying sex chromosome evolution because they exhibit TSD, XY-GSD and ZW-GSD, and their sex chromosomes span the full spectrum of differentiation, from young, nearly identical (homomorphic) pairs to ancient, highly distinct (heteromorphic) ones.
Geckos have also switched between ESD and GSD at least 23 times, making them a natural laboratory for tracking sex chromosome origins in real time. To capture this diversity, Professor Zhang Guojie's team at the Centre for Evolutionary & Organismal Biology, Zhejiang University School of Medicine, analyzed 22 gecko species—3 TSD, 6 XY-GSD and 11 ZW-GSD—together with two outgroups, collectively covering about 70% of known independent sex chromosome origins in geckos. Geckos diverged from other lizards and snakes about 191 million years ago, and their common ancestor carried 38 chromosomes (2n = 38), none of which were dedicated sex chromosomes.
By comparing genomes across species, the team traced the 22 distinct sex chromosome systems found in 20 gecko species with GSD back to just 17 ancestral chromosomes. Some ancestral chromosomes were recruited independently as sex chromosomes multiple times; ancestral chromosome 16, for example, evolved into a Z/W pair in four separate gecko lineages. Yet even sex chromosomes sharing the same ancestral origin often differ in exactly which regions became sex-differentiated (the sex-differentiated regions or SDRs), and the genes classically known to trigger sex determination in other vertebrates are usually absent from these SDRs, hinting that geckos may rely on entirely novel molecular switches to determine sex.
To find out when these sex chromosomes arose, the researchers estimated their ages from patterns of DNA sequence divergence. The results ranged widely, from under 6 million years in some species to more than 54 million years in others. Remarkably, five of 11 dated GSD species began differentiating their sex chromosomes within a narrow 7- to 12-million-year window, clustering around 9.6 million years ago.
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