Web Bit 16-1: Battle of the Sex Chromosomes
By Mary K. Miller
It's not easy being Y. Compared to its much larger and functionally more complex sex-chromosome partner, the X, this tiny, much-maligned chromosome has the reputation of being a one-trick pony. Its primary claim to fame is a gene called SRY. This gene switches on during development to start a cascade of hormonal events that instructs the embryo to grow testicles. In short, the Y is what makes embryos with one of these chromosomes (along with one X chromosome) grow up to be adult males. Embryos with two X chromosomes in their cells, rather than one X and one Y, develop into females.
But recent research is finding that the Y chromosome is more interesting and important than first believed. In 1997, 12 new genes were discovered on Y, including five that are known as "housekeeping genes," critical for everyday cellular function. Other newly discovered Y genes are linked to male fertility and affect sperm creation and motility. Of all the chromosomes in the human genome, the Y is the most "coherent," with all its genes closely related in proximity and function. The rest of the genetic territory is not so well organized. For instance, 15 different genes have been implicated in the development of asthma so far. These genes are scattered all over the map, including locations on Chromosomes 1, 2, 5, 6, 8, 11, 12, 13, and 14.
One enduring question: why is Y so small? The X chromosome contains over 10,000 genes (10% of the human genome), but Y only has about 54 genes identified thus far. X and Y weren't always so mismatched. Early in the evolution of functionally discrete sex chromosomes in mammals and birds, about 240 to 320 million years ago, X and Y were identical. This isn't so strange; all other chromosomes come in matched pairs. The chromosome pairs line up during the production of sex cells (eggs and sperm) and swap genes. This process, called recombination, shuffles the genetic deck and helps weed out mutated or harmful genes before they can be passed along to the next generation.
But the X and Y only line up at their tips and swap very little genetic material. For good reason: if the SRY gene shuffled over to the X chromosome it would wreak genetic disaster with all future generations male regardless of what combination of sex chromosomes they inherit. At three different points during the evolution of Y, whole sections of the chromosome flipped over, preventing it from lining up with the X and swapping genes back and forth. That gave the Y chromosome an opportunity to concentrate and protect genes that were helpful to male reproductive success but not necessarily beneficial to the female of the species. Over time, the Y discarded nearly all the genes that weren't crucial to male fitness and shrank to its current size.
This protective strategy is necessary because the Y chromosome is outgunned and outnumbered in an ancient conflict known as antagonistic coevolution. For every Y chromosome in the species gene pool, there are three X chromosomes. Genes on the Y that aren't good for the species as a whole would be under threat of annihilation if they were subject to normal genetic recombination.
Consider the bright plumage of many birds. Colorful males are more successful at mating, but they're also more conspicuous to prey, a genetic tradeoff. But pretty plumage doesn't contribute to the female's reproductive success (she get mates anyway) and may adversely affect her mortality. Natural selection would soon weed out those bright plumage genes if they were located on any chromosome but the protected Y. In the battle of the sex chromosomes, retreat and isolation proved to be the best strategy for the wily Y chromosome.