Cover Girl… or Boy?

I just learned that our friendly Half Moon Bay coastal garter snake photo made the cover of PLoS Biology today.

Are there any herpetologists who can summarize why this article also made the Editor’s Pick? Here’s the Full article.

Part that caught my eye:

“What makes males males, and females females? For the majority of animals, the answer lies in the sex chromosomes, which come in two distinct types, with the combination in which they are doled out determining the gender of the offspring.

Amazingly, considering the crucial role of sexual reproduction, sex chromosomes have evolved on multiple occasions. Genomic studies show that in each case the two sex chromosomes were once also a matched pair of autosomes, one of which has since degenerated over evolutionary time, with intriguing variations on the theme: In most mammals, presence of two undegraded (X) chromosomes produces a female (XX), while presence of a degenerate (Y) chromosome indicates a male (XY); in birds and reptiles, the matched set (ZZ) produces a male, while the unmatched (ZW) set produces a female. The degeneration process seems to start with a loss of the ability of the chromosome pair to exchange genes with each other (meiotic recombination), and proceeds with gradual reduction in expression, mutation accumulation, and eventual loss of genes on the Y or W. Similarities and differences between XY and ZW systems offer rich opportunity to explore the origins of sex chromosomes as well as the implications for how the traits they carry are expressed and shared from one generation to the next.

Comparing the three species’ genomes [see graphic below] at a molecular level, the researchers showed that Z genes evolve faster than those located on autosomes—in other words, evolution is male-driven in snakes, as has been previously observed in birds.”

So, snake evolution is male-driven. Hmm. In humans by contrast, are we the least bit surprised that progress is female driven?

Matt Ridley summarizes the inter-gene warfare going on within our bodies, especially between the X and Y sex chromosomes, with the typical British wit. The X chromosome spends two thirds of its time in women, and one third in men. X is evolving much more rapidly in its ability to attack Y than vice versa, and Y has withered over time, shutting down most of its gene targets (most of it is now non-coding DNA that serves no purpose at all). But Y has one very interesting and important gene: the SRY gene on the Y chromosome triggers the masculinization of the embryo. It is one of the fastest evolving genes, with 10x the normal variation between species. The reason: the X chromosome tries to shut down the SRY gene, distorting the sex ratio in favor of females. When a SRY mutation evades attack, it spreads like wildfire in the male population and becomes the new standard throughout all members of the species.

This seemed pretty odd and inefficient to me at first, but then I thought of Dawkins: evolution is the survival of the fittest genes. Genes compete; we are just the vehicles of genetic expression.