The first study to sequence the chicken Z chromosome revealed surprising facts about its mirror image in humans – the X chromosome.
For a century, scientists have assumed that the mammalian X sex chromosome has evolved very little in comparison to the Y chromosome. But a study in Nature today shows that the X chromosome evolved dramatically and in parallel to the chicken’s Z.
In mammals, females have a matching pair of sex chromosomes (XX) while males carry two different ones (XY). In chickens the situation is reversed: males have a matched pair of Z chromosomes and females have a ZW set. Scientists have long thought that most evolutionary change occurred in the chromosome found in only one of the sexes, the W and the Y.
“The X has been viewed as the unchanging foil against which we measure change in the Y,” says author David Page from the Whitehead Institute in Cambridge, Massachusetts.
Before ancient birds and mammals parted company around 300 million years ago, “when we were reptiles of some sort”, Page says, “we had no sex chromosomes”. Both the X and Y and the Z and W evolved from two separate pairs of identical non-sex chromosomes – or autosomes.
Now, after sequencing the chicken Z chromosome, Page’s team found some similarities with its mirror image, the X chromosome.
The Z chromosome has the lowest gene density of any chicken chromosome – less than half the average of that found on non-sex chromosomes. The researchers also found a large number of genes on the Z chromosome that were amplified and expressed predominantly in the testis.
These features reminded the team of the X chromosome – which also has relatively few genes along its length and contains a number of amplified genes expressed in the testis. This latter similarity was surprising because it seems to say that the evolutionary forces at work in sperm production seem to be trumping the differences between bird and mammal sex chromosomes, Page says.
“We hadn’t been looking for a parallel,” Page says, “but now there are two independent experiments of nature yielding the same outcome.” In both the mammalian and avian lineages, different portions of the ancestral genome were remodelled substantially to become specialized sex chromosomes.
To show that these traits were acquired, the authors compared the gene content of the Z and the X chromosomes to corresponding genes in fish species that are surrogates for ancestral gene content. These two features now present in chromosomes shared by both sexes were not present on the proto-sex chromosome. Like the convergent evolution of wings in bats and birds, the evolution of Z and X chromosomes ended up with two of the same traits despite having evolved independently.
The authors show that the X and Z today are very different chromosomes than they would be had they remained autosomes – challenging the assumption that sex-specific chromosomes were the sites of most evolutionary modification, an unchallenged assumption with its roots in a 1914 study on sex chromosomes in fruit flies. “The X was thought to be a pristine unchanging chromosome,” Page says. “We’ve almost treated the X as a living fossil, but now we show that it’s very much an evolving entity itself.”
According to molecular biologist Scott Hawley of the Stowers Institute for Medical Research in Kansas City, Missouri, this paper is rather “a quantal leap” in our view of the X and Z chromosomes. “The prevailing view was that both chromosomes were all but innocent bystanders in the process of sex chromosome evolution,” he adds. “This paper fundamentally changes that.”
Image: Portrait of a Rooster by Sarah.McDougall via flickr under creative commons