Good Biology

Sex chromosomes are cool. Because they're cool, I've written about them before. It's cool to trace the origins of sex chromosomes. It's cool to study how they evolve. And it's cool to compare similarities and differences of sex chromosomes within and between taxa. In organisms that use sex chromosomes to determine sex (eg, mammals, Drosophila, and birds), there is a big honking chromosome that looks like most autosomes and a piddly little chromosome that doesn't even recombine. In some organisms, males have one copy of the dinky chromosome (which we call a Y chromosome) and one copy of the normal looking chromosome (the X chromosome), while the females have two copies of the X chromosome. In other taxa, females have one dinky chromosome (which we call a W chromosome) and one copy of the normal looking chromosome (the Z chromosome), while males have two copies of the Z chromosome.

And then there are organisms that spend the majority of their life history in a haploid state -- only a single copy of the genome in each cell. If they have sex chromosomes, we can guess that they won't have a piddly little Y or W because they'd be shit out of luck missing the necessary genes from the complementary X or Z. An analysis of a liverwort genome tested this hypothesis. The researchers found that the liverwort Y chromosome has maintained a bunch of necessary genes because many of the Y chromosome bearing individuals are missing the X chromosome to do the dirty work that a degraded Y couldn't do. But that X chromosome is missing from the haploids containing only a Y, so the Y doesn't lose genes.

The title of this post is in reference my last post which bitched out human geneticists for neglecting the Drosophila literature. This paper does a good job of referring to research on the evolution of Drosophila sex chromosomes, which differ dramatically from the human and liverwort XY system. In the most XY systems, the Y chromosome is a degraded version of the X. In Drosophila, Y chromosomes have evolved multiple times, and each Y chromosome has arisen from an autosome, not an X chromosome. Anyway, much propers to these guys for understanding research done on non-human model organisms.