Daily Weird News Report
Geckos appear to have evolved their remarkably varied sex-determination systems through a combination of environmental pressure and genomic predisposition, according to research reported by Phys.org.
The study, published in Science, examined 22 gecko species alongside two outgroup species. The sample included geckos whose sex is influenced by incubation temperature, as well as species with genetically determined XY or ZW systems. Geckos have shifted between environmental and genetic sex determination at least 23 times, making them a useful group for examining how sex chromosomes arise and change.
The researchers traced 22 sex-chromosome systems found in 20 gecko species with genetic sex determination back to 17 ancestral chromosomes. Some of those chromosomes were recruited repeatedly. One ancestral chromosome, numbered 16, independently became a Z/W pair in four gecko lineages. However, even systems with the same chromosomal origin did not always develop sex-differentiated regions in the same locations.
The study also found that the sex-determining regions usually lacked the genes known to initiate sex determination in other vertebrates. This suggests that geckos may use different molecular triggers.
Estimates of chromosome age ranged from less than 6 million years to more than 54 million years. Five of the 11 dated genetically sex-determined species showed chromosome differentiation within a narrower period, roughly 7 million to 12 million years ago. That cluster coincided with the Middle Miocene Climatic Transition, a period associated in the report with global cooling, increasing aridity and habitat fragmentation.
The researchers propose that such disruption may have repeatedly favored a move away from temperature-sensitive sex determination toward genetically fixed systems. They call this proposed concentration of sex-chromosome origins a “sex chromosome birth pulse.”
The direction of evolution may also depend on the chromosome’s original gene content. Chromosomes containing more genes preferentially active in testes were more likely to become Z chromosomes, while those with fewer such genes tended to become X chromosomes. The pattern was also observed across 39 vertebrate species, particularly among animals with internal fertilization.
As Y and W chromosomes lose genes after recombination stops, many geckos compensate by increasing expression from the remaining chromosome copy. The researchers’ model describes four stages: climate-linked transition to genetic sex determination, chromosomal bias toward X or Z, differentiation caused by suppressed recombination, and dosage compensation after gene loss.
