Flatfish genes provide insight into sex chromosome evolution

The sex of the half-smooth tongue sole (Cynoglossus semilaevis), a marine flatfish cultured along the coast of China, is determined primarily by the inheritance of sex chromosomes, but can also be affected by environmental factors. At high temperatures, female tongue sole can develop into phenotypic males (pseudo-males).

To better understand the molecular mechanisms behind sex determination in these fish, researchers led by Songlin Chen from the Ministry of Agriculture in China, and including Jun Wang from BGI-Shenzhen and King Abdulaziz University in Saudi Arabia, sequenced the genome of both male (ZZ) and female (ZW) tongue sole, publishing their finding in Nature Genetics¹.

As they grow from the larval to the juvenile stage, tongue fish transition from an open sea habitat to become bottom dwellers. The team compared the full range of RNA molecules expressed by both stages to understand the adaptation of tongue sole to a benthic lifestyle, including the migration of eyes to the same side and increased light sensitivity.

The team then compared the relatively young sex chromosomes of tongue sole fish with the sex chromosomes of birds and mammals. "Sequencing the genome of the half-smooth tongue sole revealed insights into the evolution of the sex chromosomes," says Wang. "The gene, dmrt1, which in birds is the sex determining gene, was found to have a similar function in tongue sole."

This suggests that the sex chromosomes of tongue sole and birds are derived from the same ancestral protochromosome. "Future work needs to be done for the detailed functional study of this gene," adds Wang.

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