The evolution of sex chromosomes

by Matthew Cobb

Sex is an odd business. In some animals, like us, sex is determined by which combination of a pair of chromosomes the individual carries. Males are XY, females are XX. In birds (and butterflies, for some reason) things are the other way round – males are ZZ, females are ZW.

Sex determination by specific chromosomes is not the rule, not was it the ancestral state – in both plants and mammals it appears to be a relatively recent invention. Zsex can also be determined by overall chromosome number (eg ants and bees), and in some reptiles, like crocodiles, sex is determined by the temperature at which eggs are incubated. Some species are hermaphrodite, while others can change their sex in response to the social or environmental changes or the action of a parasite.

In those species that do have chromosomally-based sex determination (like us), there’s nothing particularly special about the sex chromosomes – they were originally just like the other chromsomes (“autosomes”). But as time goes on, the chromosome that cannot exchange genetic material (the Y chromosome in humans, or the W chromosome in birds) gradually loses its genes. In humans the Y chromosome used to have over 1,000 genes. Now it just has a few dozen. How does the corresponding X chromosome cope with the declining number of genes in its opposite number?

In the fruitfly species Drosophila miranda, a new X chromosome has recently been formed (the “neo-X”). A recent study by Doris Bachtrog and colleagues from Berkely, published in PLoS Biology, has looked at the genes on the neo-X and compared them with those on the ancestral X chromosome. They found clear signs that the genes on the neo-X had recently been the subject of intense selection, as they adapted to their interaction with the Y chromosome. The authors conclude:

“Thus, newly formed X chromosomes are not passive players in the evolutionary process of sex chromosome differentiation, but respond adaptively to both their sex-biased transmission and to Y chromosome degeneration, possibly through demasculinization of their gene content and the evolution of dosage compensation.”

As well as providing a fascinating example of how genes and chromosomes interact to form individuals, this kind of genetic study poses a massive problem for all those who refuse to accept the facts of evolution. What other explanation is there, but that genes, and populations, evolve over time? The only other interpretation is that these signatures of selection were put in the fly’s DNA by the Creator as a whim, a joke, or a way of testing our faith…

This is my last post as the vacation blogger – Jerry is back on dry land tomorrow. Thanks for the comments, and thanks for reading!

Citation: Bachtrog D, Jensen JD, Zhang Z (2009) Accelerated Adaptive Evolution on a Newly Formed X Chromosome. PLoS Biol 7(4): e1000082. Open Access here.