Novel sex‐determining genes in fish and sex chromosome evolution

Abstract

Although the molecular mechanisms underlying many developmental events are conserved across vertebrate taxa, the lability at the top of the sex-determining (SD) cascade has been evident from the fact that four master SD genes have been identified: mammalian Sry; chicken DMRT1; medaka Dmy; and Xenopus laevis DM-W. This diversity is thought to be associated with the turnover of sex chromosomes, which is likely to be more frequent in fishes and other poikilotherms than in therian mammals and birds. Recently, four novel candidates for vertebrate SD genes were reported, all of them in fishes. These include amhy in the Patagonian pejerrey, Gsdf in Oryzias luzonensis, Amhr2 in fugu and sdY in rainbow trout. These studies provide a good opportunity to infer patterns from the seemingly chaotic picture of sex determination systems. Here, we review recent advances in our understanding of the master SD genes in fishes.

Did sex chromosome turnover promote divergence of the major mammal groups?

“…Again, for argument's sake, I will propose that monotremes originally had a bird‐like ZW system driven by dosage of DMRT1 . The first translocation to this ancestral Z (probably in a ZZ male) added a region that contained the AMH gene, an important player in the sex determining network in reptiles, and the sex determining gene in some fish 16, 71. I propose that AMH took over as a male‐dominant sex determining gene, defining a neo‐Y.…”

“…For instance, in medaka and its close relatives with similar karyotypes, different chromosomes show sex linkage, as a result of the rapid evolution of different sex determining genes 15, 16, 17. In sticklebacks, too, novel sex genes have rapidly arisen 18, 19, and in cichlid fish of Lake Victoria, rampant speciation is often accompanied by sex chromosome turnover 20.…”

“…However, this need not represent identity by descent, for a flatfish also has a ZW sex pair with homology to the bird ZW, and uses DMRT1 dosage to determine sex 32. DMRT1 has also been pressed into service independently by spawning copies that became a dominant male‐determiner in medaka 16, 65 or female determiner in Xenopus 66; clearly this gene is simply good at sex determination and has been independently recruited in several lineages 67.…”

Germline stem cells are critical for sexual fate decision of germ cells

“…Nonetheless, I postulate the conservation of foxl3 , because it is more conserved than the genes that are involved in the sexual fate decision of somatic cells (genes that are conventionally referred to as “sex determination genes”). The sex determination genes tend to change more rapidly, and are typically not conserved, even within a single class 55, 56. However, the foxl3 gene is found in the teleost, amphibian, reptile, bird, and marsupial genomes.…”

Expression characterization of testicular DMRT1 in both Sertoli cells and spermatogenic cells of polyploid gibel carp

“…Increasing evidences have indicated that most of the known sex determination genes discovered so far in vertebrates are derived from the duplicated homologs of Dmrt1 gene (Chong et al, 2013;Gamble and Zarkower, 2012;Gempe and Beye, 2011;Graves, 2013;Herpin and Schartl, 2011;Kikuchi and Hamaguchi, 2013;Matson and Zarkower, 2012;Siegal and Baker, 2005;Zhou and Gui, 2010). For gibel carp, as a unisexual lineage that is able to utilize gynogenesis as a unisexual reproduction mode, how male individuals arise and how male determination genes evolve will be the merit for further investigation.…”