Tuesday, 22 May 2012

The Disappearing Y Chromosome

The size of the human Y chromosome is only a fraction of what it used to be. Once it was homologous to the X chromosome, but it now contains only 45 out of the 1700 genes it used to have[1]. Based on previous studies and assuming a linear rate of decay, it is expected that the human Y chromosome will disappear in roughly 4.6 million years[1].

Does this mean the end of males? Not quite.

A degenerating sex chromosome seems to be the norm of genetically determined sex[2]. In fact, organisms exist where the male chromosome is completely lacking, supporting the theory of the disappearing Y chromosome[2]. However, a study conducted by Hughes et al. provides new evidence on why the Y chromosome to stay for much, much longer.

The male specific region of the Y chromosome (MSY) is not capable of genetic recombination with the X chromosome. Without genetic recombination, there are fewer mechanisms for genetic repair, leading to higher rates of mutations, deletions and insertions, all this, pointing to the inevitable degeneration of the Y chromosome[2].

Hughes et al. argues that the human Y chromosome has reached stability, where it will not degrade further, by sequencing the genomes of the rhesus macaque and comparing it to that of the chimpanzee and human genome. The study focused on the MSY, identifying five distinct regions or “strata” based on its degree of similarity to the X chromosome.

Comparison between the macaque MSY and the human MSY revealed that they share the same 18 ancestral genes in strata 1 to 4, indicating that their last common ancestor shared the same genes and that there was no subsequent loss of these genes after the human and macaque lineages diverged 25 million years ago[3]. In strata 5 all species showed significant gene loss, losing 5 ancestral genes (4 for macaque) over the past 30 million years[3]. However, the same genes are found on the human and chimpanzee MSY, indicating that this region has stabilised prior to the human/chimpanzee split[3].

The findings of this study contradict the linear model for the degeneration of the Y chromosome as they fail to explain the relative stability of the human MSY genes over the past 25 million years. The results suggest an exponential decay with a baseline constant which extends the lifespan of the human Y chromosome indefinitely into the future[3].

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