Origin of sex chromosomes

At the heart of our cells, chromosomes are the repository of genetic information. Composed of DNA and proteins linked in pairs, they support the genes that biologically determine personality. In many species, sex is determined by the presence or absence of a particular chromosome. In mammals, and therefore in humans, the female carries a pair of X chromosomes, while the male carries an X and a Y. In other animals, where the sex chromosomes are denoted W and Z, this is the reverse. In birds or turtles, males have a Z pair, and females Z and W. In most cases, these sex chromosomes, or gonosomes, differ in that in humans, Y is much smaller than Y. X, but also poorer in genes: they say that he is degenerate. Two CNRS biologists, Thomas Lenormand of the Center for Functional and Evolutionary Ecology in Montpellier, and Denis Roset of the Roscoff Biological Station, proposed a new model to explain the formation of a degenerated chromosome.

For almost fifty years, the generally accepted theory has been based on two specific phenomena: sexual dimorphism, that is, differences between males and females, and genetic recombination. The latter is a natural mechanism by which chromosomes exchange DNA fragments. This mixing leads to greater genetic diversity in populations and promotes adaptation to the environment. The theory suggests a three-stage evolution. The protochromosome Y carries a gene that determines male sex, as well as a gene involved in sexual dimorphism, which contributes to the reproductive success of males, but not females. To maintain this beneficial association, natural selection favors “recombination arrest” so that the two genes remain passed on together. Then the lack of recombination leads to the accumulation of deleterious mutations in the “locked” region; the affected genes become non-functional, and gradually the size of the chromosome decreases. Finally, to compensate for this lack of expression, functional copies of genes located on the X chromosome are overexpressed (and one of the Xs in females is quenched): this is called “dosage compensation.”

Although this theory has long been accepted, it nevertheless suffers from a lack of empirical evidence to support it, which has gradually caused a certain amount of skepticism in part of the scientific community. Therefore, Thomas Lenormand and Denis Rose proposed a new model. They suggest that causation is inverse. In their opinion, the degeneration results from the early evolution of gene expression regulators, which are also carried by the Y chromosome. They suggest that random stops of recombination occurred along the Y protochromosome. were relatively free of deleterious, and therefore beneficial, mutations and had taken root in the population. However, due to recombination arrest, the regulators driving gene expression in these regions thus evolved independently for X and for Y. According to this scenario, Y expression quickly differed from X expression with less expression (resulting in assay compensation). Y genes are less pronounced than integrated mutations, with limited impact to ensure their spread in the population. In addition, dose compensation then causes antagonistic effects between the sexes that prevent recombination recovery. This situation inevitably leads to complete degeneration of Y, with the exception of genes that have a male-specific function. Through computer simulations, the researchers showed the effectiveness of this new model.

sex chromosomes

© Pour la Science, according to P. Muralidhar and C. Veller, Science, vol. 375, p. 616-617, 2022

How to test this scenario? The sex chromosomes are often very degenerate and of ancient origin, as in mammals. These cases do not allow us to study how the formation of gonosomes took place. However, biologists are paying attention to younger chromosome systems in which sex determination appeared quite recently, for example, in some plants. They also refer to groups of animals, such as some fish, that have recently changed their sex chromosomes. In doing so, they hope to directly observe the early stages of gonosome evolution and test the various scenarios envisaged by the old and new models.

If scientists want to find out the origin of sex chromosome features, they must better understand the functioning of genomes and, in particular, study more deeply the causes and consequences of the cessation of recombination, the most important mechanism of reproduction. . Moreover, monosomies (one less chromosome) and trisomies (one more chromosome) are usually fatal. What mechanisms ensure that a degenerated chromosome is not harmful? Therefore, it is also a matter of elucidating the little-known effects of changes in dosage and expression on health.

Origin of sex chromosomes


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