By Marisa Sanchez, Molecular and Cellular Biology ‘15
The genomes of male and female mammals differ by one chromosome. The Y chromosome is only present in males, and is responsible for initiating the physiological and morphological differences between the sexes. This has not always been the case though; at one point, the X and Y were identical, and over time the Y chromosome began to differentiate from the X chromosome and shrink in size. The Y chromosome today only has 20 genes, whereas the X chromosome has over 1,000 genes.
Until recently it was unknown when the Y chromosome originated. The team of Henrik Kaessmann, Associate Professor at the CIG and group leader at Swiss Institute of Bioinformatics, have found that the first “sex genes” originated 175 million years ago in mammals. These scientists have discovered this by studying samples of several male tissues, in particular male gonads, from different species in the three major mammalian lineages, placentals, marsupials, and monotremes (egg laying mammals).
The scientists compared genetic sequences from male and female tissues.To avoid analyzing the whole genome, they then eliminated all sequences that both sexes had in common, which allowed the scientists to focus on only male-specific genes. After almost 30,000 hours of computing, they found that the common ancestor of the placentals and marsupials had the same sex-determining gene, named SRY, which emerged 180 million years ago. In monotremes another gene, AMHY, is responsible for the development of Y chromosomes and this gene appeared 175 million years ago. The development of the Y chromosome was essential for mammalian evolution, which is why knowing these time scales is so important. Now the question is: what determined sex before the Y chromosome evolved? Some think that it may have been environmental factors that determined sex, but as of now no one truly knows.
Diego Cortez, Ray Marin, Deborah Toledo-Flores, Laure Froidevaux, Angélica Liechti, Paul D. Waters, Frank Grützner, Henrik Kaessmann. Origins and functional evolution of Y chromosomes across mammals. Nature, 2014; 508 (7497): 488 DOI: 10.1038/nature13151