The Making of Sex Chromosomes

Many animals use a special pair of chromosomes to determine their sex during development, but how did this system evolve? Where did these chromosomes come from and how do they change over time? Questions like these are at the heart of a study conducted by researchers at UC Berkeley, in California, and published earlier this year in PLOS Biology.

In animals with an XY sex determination system (including humans, other mammals, and some fish and insects), individuals with two X chromosomes develop as females, while those with an X and a Y turn into males. By contrast, in the ZW system used by birds and some reptiles, it's the females who have two different chromosomes (a Z and a W), while males have a matched pair (two Zs). Some of the genes on the odd, unmatched chromosome (the Y or W) act as sex-determining factors, but what about the rest of the chromosome? In some cases, the chromosomes stop lining up with their partner (X or Z) to exchange DNA via recombination; as a result, they degenerate over time. That's not always the case, though; sometimes, the Y and W chromosomes show more variation. While some W chromosomes have faded to fractions of their former selves, others have remained intact and look similar to their Z counterparts. Still others lie somewhere between these two extremes, with W and Z chromosomes that look somewhat different from each other. We don't really understand the basis of this variability, but the researchers took advantage of it to investigate the ZW-baesd sex determination system in several groups of snakes.

Snakes all have ZW sex chromosomes, but the W chromosome has degenerated to a different extent in various species. Beatriz Vicoso and her colleagues sequenced the entire genome of three snake species: boa constrictors, which have W and Z chromosomes that look quite similar under a microscope; pygmy rattlesnakes, in which the two are distinctly different; and common garter snakes, which are somewhere in the middle. They compared these sequences with the genome of a closely related lizard species and found that the sex chromosomes were similar to the sixth chromosome pair in the lizard. This matches well with earlier studies, which had suggested that the ZW chromosomes might be derived versions of the sixth pair. The researchers also confirmed that the Z and W chromosomes are similar in boas and different in rattlesnakes. Although the Z and W chromosomes still look somewhat similar under a microscope, the sequence data showed that, at a genetic level, they were as different as in the rattlesnake. The team also discovered that the W chromosomes of these two species are more similar to one another than to their corresponding Z chromosomes, implying that the Z/W split happened before the two species parted ways.

Finally, by sequencing RNA from the three species to measure how strongly genes were expressed, they showed that in species where the Z and W chromosomes have diverged, genes on the Z chromosome are expressed roughly half as strongly in females as in males. This isn't inherently surprising, since the females, being ZW, only have one Z chromosome compared to the males' ZZ pair; it is remarkable, however, in light of the fact that the same isn't found in most animals with an XY sex determination system. In general, animals with an XY system have evolved a mechanism to compensate for the difference in X dosage. Birds, which have a ZW system, don't compensate, and it seems that snakes don't either. Why these systems have evolved differently remains a mystery; it may have to do with how quickly the Y and W chromosomes degenerate, differences betweem males and females in terms of life history and selection pressures, or other factors. All we can say for certain is that it will take more research to unravel the basis of these differences.

Vicoso's study certainly hasn't answered all the questions posed at the beginning of this post, but it has gotten us closer to working them out. Its findings have enriched our understanding of sex determination systems, and the similarities between the independently-evolved ZW chromosomes in birds and reptiles have sharpened our questions, as have their differences from the XY chromosomes. A fuller picture of the evolution of these systems will only emerge through continuing studies combining the power of high-througput modern sequencing technologies with analyses informed by a knowledge of the life history and constraints of these many different kinds of animals.

References
Vicoso B, Emerson JJ, Zektser Y, Mahajan S, Bachtrog D (2013) Comparative Sex Chromosome Genomics in Snakes: Differentiation, Evolutionary Strata, and Lack of Global Dosage Compensation. PLoS Biology 11(8): e1001643. doi:10.1371/journal.pbio.1001643

Hoff M (2013) Slithering Toward Clarity: Snakes Shed New Light on the Evolution and Function of Sex Chromosomes. PLoS Biology 11(8): e1001644. doi:10.1371/journal.pbio.1001644

Image Credits
The Boa constrictor image is by user Tod Baker on Wikimedia Commons.