Lose the Y chromosome, gain a new species?

Photo by Biophoto/Photo Researchers

The human X and Y chromosomes were originally differentiated from a pair of autosomal (non-sex) chromosomes which could readily share DNA. Once these two separate chromosomes emerged, they each had an additional region of DNA for the expression of sex; after the initial divergence, some X-degenerate genes were retained on the Y chromosome, even though these genes were probably unnecessary. As the chromosomes evolved, it became evident that the Y chromosome had a much faster evolutionary rate than the X chromosome. This may have facilitated the development of mutations within the Y chromosome, causing the somewhat rapid deletion of genes from non-recombining regions; there are now under 200 working genes left on the chromosome. The X chromosome, however, retained the majority of its genes through a slower evolutionary rate similar to that of the original autosomal pair, and functions with approximately 1100 working genes.

It is speculated that most genes were lost from the Y chromosome because they were deemed unnecessary for the survival of the species, though in a study done on the evolution and survival of sex chromosomes, it was found that many of the genes on the Y chromosome have functions that are completely unique to the function of genes on the X chromosome. This suggests that certain genes carried only on the Y chromosome are important and should be retained, which is why degeneration of the male chromosome is providing interesting theories as to what will happen if the chromosome disappears completely.

One such theory is that an entirely new species will evolve with the demise of the Y chromosome, though if this actually eventuated, it is speculated that it would take from 6 - 15 million years. This is not to say that the male population will become extinct; the missing genes that are vital for making humans male that are usually found on the Y chromosome could be expressed on the X chromosome instead. It is thought that non-sex chromosomes would start functioning as sex chromosomes for males, though this could potentially give rise to DNA mutations resulting in a variety of structural, physiological or behavioural changes that could significantly alter the way in which the theoretical new species functions.

The possibility of a new species seems far-fetched, though there is no way to correctly predict what would happen to the human race given the loss of one sex chromosome. It is thought that the male species would have to develop in some way to deal with the loss, the consequences of which will be unknown until the need for such evolution arises, if indeed it ever does.

Amy Pammenter (42002198)

Journal:

Wilson, A.M., Makova, K.D. (2009) Evolution and Survival on Eutherian Sex Chromosomes. PLoS Genetics. 5:1 - 11

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