It was thought that the few genes that fit the Y chromosome were used for little more than trigger the development of fetal testes to produce sperm. Two papers from the University of Lausanne and MIT (Massachusetts Institute of Technology, Boston) now show that the human Y chromosome originated at the time of separation between marsupials and mammals, hundreds of millions of years ago; and by then started losing genes at full speed until stabilized 25 million years ago, when we were monkeys. Aside from making testes and sperm, the Y chromosome is essential for the viability of the men, and their genes may explain much of the differences in disease susceptibility between men and women.
In mammals and many other animals, females have two X chromosomes (abbreviated XX), and males one X and one Y chromosome (XY abbreviated). The Y chromosome is much smaller, and evolved from an X through massive gene loss. This process is not only a theoretical deduction, but has been observed directly in model organisms like the fly ‘ Drosophila’, where it is possible to force the rapid development of a new chromosome Y.
The Y chromosome has been the most overlooked of the genome projects, for technical reasons: it is full of repetitive sequences and palindromic – that read the same forwards and backwards, which are the bane of any machine to read DNA. But researchers have drawn these technical pitfalls and have sequenced the Y chromosomes of the rat, mouse, monkey (marmoset or a small mono) and opossum (an Australian marsupial), and compared with each other and with three others were already sequenced (macaque, chimpanzee and human).
They deduced so that only 3% of the genes had the ancestral sex chromosome from which they come across them survive in at least one of today’s mammals. Most of this genetic slaughter occurred very early in the evolution of mammals, shortly after they split of marsupials, but once concluded that catastrophic archaic period, the remaining genetic content has remained fairly stable. This does not square with a random loss of material; rather aims to conventional selection processes, where what remains is so essential to the species. The results are presented in Nature.
There are 36 Y chromosome genes that are also present in the X, and are the same in all eight species analyzed. This ‘aristocracy ‘ genetics has been particularly stable over the last 25 million years of evolution (ie, having very few changes between humans and other primates). Many are “transcription factors” (genes that regulate other genes), and this is well-known for its “dose responsiveness” class unlike most genes, which works perfectly if inactivate one of the two chromosomes of a pair, dose responsive genes both copies need to do their job. This is probably the selective force that has kept them on the Y chromosome along the evoliución.
Perhaps the most unexpected of all data is that the Y chromosome is not merely a ‘switch’ that turns the male fetus – as – historically assumed, but affects the regulation of the entire genome for the rest of life. And the expected conclusion, however, is that we still do not fully understand the biological differences between men and women.