Birth of a Male Sex Chromosome in Atlantic Herring

The evolution of sex chromosomes is of crucial importance in biology as it stabilizes the mechanism underlying sex determination and usually results in an equal sex ratio. An international team of scientists, led by researchers from Uppsala University in Sweden, now reports that they have been able to reconstruct the birth of a male sex chromosome in the Atlantic herring. The male-specific region is tiny and contains only three genes: a sex-determining factor and two genes for sperm proteins. The study was publishd recently in PNAS. It is hard to study the early evolution of sex chromosomes because it usually happened a long time ago and the sex-determining chromosomes usually rapidly degenerate and accumulate repetitive sequences. For instance, humans have an X/Y system of sex determination and the presence of Y determines male sex. The human Y chromosome, which was established more than 100 million years ago, evolved from a chromosome identical to the X chromosome, but has since lost most of the genes present on X and is now only about a third the size of the X chromosome. The Atlantic herring also has an X/Y system but it is young and evolved much more recently. In the herring, X and Y are almost identical in gene content, the only difference being that the Y has three additional genes: a sex-determining factor (BMPR1BBY) and two sperm protein genes predicted to be essential for male fertility. “The unique feature of this study is that we have been able to reconstruct the birth of a sex chromosome. The evolution of the herring Y chromosome in fact resembles the process when my son makes a construction with pieces of Lego,” says Nima Rafati, PhD, a bioinformatician at Uppsala University and first author on the paper.