Female male genes: About mice, humans, and the sex chromosome
Of the 23 pairs of human chromosomes, one is special: in women they are the same, X and X, and in men there is their own small Y-chromosome. It is she who carries the genes of traits that make a man a man, determine sex and sexual development - and this is not only the case with us, but also with mammals in general. However, a new study by American geneticists has shown that a “female” X chromosome is necessary for true masculinity.
In fact, for a normal sexual development, even a woman has only one X chromosome, so her second copy is inactive. Such a “disconnection” of one copy of the chromosome was supposed to drastically reduce the variability of its genes. Therefore, even half a century ago, it was suggested that these genes are highly conserved, so if we compare them in different mammals, we will find that they evolved slower than the genes of ordinary chromosomes and less changed. To test this hypothesis, David Page and colleagues compared the X chromosome genes of humans and mice — species that diverged over 80 million years ago.
To begin with, scientists have clarified the sequence of the human X chromosome, which was previously not established in sufficient detail, and then made a comparison. In fact, it turned out that of the approximately 800 genes found on the X chromosome of mice and humans, the majority are the same. These stable genes are active in both species and in both sexes, although they work only in one copy (in females, recall, the second X chromosome is “disabled”, and in males there is a Y chromosome instead). Recessive mutations on the X chromosome cause known diseases - such as hemophilia or Duchenne myopathy.
On the other hand, scientists have isolated 144 genes of the human X chromosome, which are not present in mice, and in mice - 197 genes, which we do not have. Surprisingly, out of 144 unique human X-genes, 107 are in more than one copy - many of their duplicates are located on the same chromosome, which allows them to evolve not slower, but even faster than “ordinary” genes.
To find out the role that these genes play, the researchers checked in which tissues of male and female organisms they are active. To their surprise, it turned out that many of them in women generally remain idle, but are activated in male testicles, especially in the epithelial tissue of the tubules, from which sperm are produced.
“It seems that the X chromosome leads a double life, ” the authors note. In fact, it turns out that one part of its genes - stable - is really slowly evolving, while the other, on the contrary, is highly variable and, moreover, is associated with the male rather than the female reproductive system.
According to ScienceNOW