Chromosome X story: The evolution of a sex chromosome

Forget about the Y chromosome - it's time to talk about the X chromosome.

In the first evolutionary study of chromosomal sex determination at the University of California, Barkeley, biologist Doris Bachtrog and colleagues showed that the history of X chromosomes is as interesting as that of infection. Y chromosome has long been interested in research, and also from that gives important suggestions to determine the origin and benefits of sexual reproduction.

Bachtrog, assistant professor of integrated biology, member of Berkeley University's Research Center on Evolutionary Genetic Theory, said 'Contrary to the traditional view that X chromosomes are just one nucleus' In fact, X plays an active role in the evolution of sex chromosomal differentiation. '

Bachtrog and Dr. Jeffrey D. Jensen of UC Berkeley and Dr. Zhi Zhang, formerly of UC San Diego University, now at the University of Munich, described their findings in this week's edition of PLoS. Biology.

'In the draft, we first describe another aspect of sex chromosomal evolution: X chromosome has undergone periods of profound change in evolution to create parts. new genes dominate the process of sex in many species, including our species, ' she said.

Not all plants and animals rely on genes to determine whether an embryo will develop into a male or female individual. For example, many reptiles rely on environmental factors such as temperature to form the sex of the embryo.

But in species with a sex chromosome pair - from fruit flies to mammals and some plant species - two X chromosomes inherited from the mother's body look almost like the Chromosome often, Bachtrog said. However, the Y chromosome inherited from the father's body later combined with an X chromosome, in fact, is a small form of shrinkage of the X chromosome when many genes have been lost because they are not associated with chromatin. The other X.

In mammals, this happened about 150 million years ago, while in the Drosophila melanogaster, a species commonly used in many experiments, the sex chromosomes began to divide into two types. about 100 million years ago. In both humans and fruit flies, the Y chromosome has shrunk from a few thousand genes to a few dozen genes.

The question is why, and how does the Y chromosome lose genes when it stops interacting with the X chromosome. Scientists have found that pairs of males in males are pairs. The only chromosome does not separate and recombine every time a cell divides, so it cannot gain the advantage that harmful gene mutations will be removed. While the XX pair in the individual split and recombined, the only way for the XY pair to eliminate harmful mutations in a gene is to stop the activity or erase the entire gene. Over millions of years, the genes have been stopped, and the Y chromosome shrinks.

'Without recombination, natural selection will be less effective in eliminating adverse genes,' Bachtrog said. 'Y is asexual chromosome, and it has to pay for that by losing genes.' Bachtrog has spent most of his career studying the degeneration of the Y chromosome, decided to focus on studying the X chromosome from a few years ago, and began studying chromosome pairs. can only appear, in the process of adapting to their new role. The documents she wrote mostly around the study of three sex chromosomes in a rare western fruit fly called Drosophila Miranda, a darker relative of D. melanogaster. (Many organisms have more than one pair of sex chromosomes; for example, the platypus has 5 pairs, all pairs are inherited together.)

The new X chromosome (above) and the new Y chromosome of the Drosophila Miranda fruit fly, show how the Y chromosome has shrunk through the process of losing the gene.The X chromosome remains the same size as the normal chromosomes in flies, although its genes also undergo adaptation to the degeneration of the Y chromosome. (Photo: Doris Bachtrog / University studying UC Berkeley)

One of the D. miranda sex chromosomes transmitted from the original sex chromosome appeared in Drosophila nearly 100 million years ago, while the second chromosome formed about 10 million years ago. , and the third chromosome began to appear 1 million years ago. The previous two chromosomes look quite similar, Bachtrog said: The Y chromosome in each pair has lost its genes and changed its shape, while the two X chromosomes are exactly the same.

The third, the youngest, is completely different. The Y chromosome has not yet shrunk, although it contains a lot (about half of all genes) that are not functional and will eventually disappear. However, according to the researchers, the X chromosome, often called the new X-chromosome, is undergoing rapid changes, which is 10 times greater than the adaptation to normal chromosomes. .

With the adaptive phrase, Bachtrog wanted to refer to the genetic sequences in the X chromosome being adjusted when the last random mutations fixed some beneficial changes to match the increasingly incompatible Y chromosome. prefer. Approximately 10 to 15 percent of the genes of the new X-chromosome show adaptive signs, while this number in chromosome genes is usually 1 to 3 percent.

'After all, this is not surprising,' Bachtrog said. 'The new X-chromosome is more difficult than normal chromosomes because the Y chromosome paired with it is degenerating. Y's genes no longer produce proteins, so X-new must compensate by regulating its genes. We found many genes on the X chromosome related to this compensatory process. '

For example, in humans, all genes on the X chromosome work twice to compensate for the Y chromosome missing genes. The female body adapts to this by completely stopping an X chromosome to avoid producing too much protein, Bachtrog said.

Another change in the new X-ongoing, according to Bachtrog, is the elimination of genes harmful to the female. Recent biologists have recognized that some genes have opposite effects in males and females, and evolution is a struggle between harmful genes removed in males. to be kept by the individual and vice versa.

'The sex chromosome is where the opposite genes are reproducing, these genes can benefit this world but are harmful to the other,' she said. Y chromosomes always form males, so genes on Y will not affect females.

'In contrast, the X chromosome has good genes for females but is not good for males,' Bachtrog said, adding that the X chromosome also loses genes that are only used in an individual. male.

To better understand the evolution of the X chromosome, Bachtrog said she was looking for more fruit flies with different five-year-old sex chromosomes than the species being studied 'to better understand the problem. Sex chromosome evolution takes place in practice. ' She said the evidence of adaptation to becoming a sex chromosome can be best seen after 1 to 10 million years. Bachtrog is also completing the genetic sequence for D. Miranda, which is not one of 12 species of Drosophila that has been noticed by the genetic sequence community. She hopes the fly will become a research sample system like D. melanogaster fruit flies .

'Now, finally, we have been able to study sample systems like D. Miranda flies that we dared not think a few years ago,' she said. Bachtrog predicts that 'comparisons of the entire genome will revolutionize evolutionary biology, ecology and many other areas.'

This research was supported by the National Institutes of Health Research, Alfred P. Sloan Society of Molecular Biology and Computational Research, and the Foundation of Lucile Packard and David.

References:
Bachtrog D, Jensen JD, Zhang Z. Evolution adapted in a newly formed X chromosome.PLoS Biology, 2009;7 (4): e82 DOI: 10.1371 / journal.pbio.1000082