A few weeks ago PNAS published a paper on the evolution of snake sex chromosomes. The authors compare snake sex chromosome evolution with that of mammals and birds. Given my passing interest in sex chromosome evolution, I decided to check it out.
Snakes use sex chromosomes to determine the sex of their progeny. Sex in other reptiles, such as crocodiles, is determined by the temperature at which the eggs are incubated. There are two main types of sex chromosome systems in vertebrates: XY and ZW. Most mammals, some fish, some reptiles, and some amphibians use the XY system -- males are heterogametic (XY) and females are homogametic (XX). Snakes, birds, and some other vertebrates use the ZW system -- males are homogametic (ZZ) and females are heterogametic (ZW).
The chromosomes found only in the heterogametic sex (the Y and the W) are degraded versions of the homogametic chromosomes (the X and the Z, respectively). It is thought that the ancestral state of sex determination in vertebrates was through environmental cues. The chromosomal sex determination systems are thought to be independently derived mechanisms (although see here for an alternative hypothesis). We can also trace the origins of the sex chromosomes back to an autosomal (non-sex chromosome) state.
Eric Vallender and Bruce Lahn offer a commentary on the PNAS paper, which contains the following figure illustrating the hypothesized origins of the eutherian mammal, snake, and bird sex chromosomes:
The mammalian, snake, and bird sex chromosomes all descended from different autosomal ancestors -- the sex chromosomes in one taxon are not homologous to those from the other taxa. Here is how Vallender and Lahn describe the origins of sex chromosomes:
One popular model of sex-chromosome evolution postulates that the sex-determining locus first arises when an autosomal gene involved in environmental sex determination acquires a new mutation that consistently gives rise to either male (in the case of the XX:XY system) or female (in the case of the ZZ:ZW system) development.
This model seems to be consistent with vertebrate sex chromosome evolution, but it fails to explain the origins of Drosophila sex chromosomes. In mammals, sex is determined by genes on the Y chromosome. That means individuals with a single X chromosome (X0 or Turner's Syndrome) are morphologically female, although sterile. Triplo X females (XXX) are phenotypically normal, while XXY males may suffer from Klinefelter's syndrome.
In Drosophila, however, sex is determined by the ratio of X chromosomes to autosomes. That's because the Y chromosome is not a degraded X chromosome, and it does not possess the genes for male sex determination (see here for some work on Y chromosome evolution in Drosophila). Long story short, a new Y chromosome may evolve when one of the autosomes fuses to the X chromosome. The old Y will fuse to an autosome (the order of these fusions can be reversed) and the homolog of the fused autosome will become a new Y. The new Y will degrade over time to look like a typical Y chromosome.
Because the Y chromosomes don't determine sex in Drosophila, the implications of missing or extra chromosomes are different than in mammals. As I mentioned above, sex is determined by the ratio of X chromosomes to autosomes. If you have at least two X chromosomes, then you are female. One X chromosome makes you a male. That means that XO flies are males -- although they are often sterile because the Y chromosome carries genes necessary for gametogenesis in many species. It also means that XXY flies are female because they have two X chromosomes. More on sex determination in Drosophila can be found in Scott Gilbert's excellent developmental biology textbook.
Anyway, my reason for pointing out the dramatic difference between the mammalian XY system and that of Drosophila is to show how even sex determination systems that appear to be similar upon first glance are quite different upon closer inspection. The XY systems in mammals and Drosophila not only have unique historical origins, but they also have dramatically different mechanistic origins.
Given that sex determination of insects and arachnids must have evolved independetly of any vertebrate system, you'd expect that to be distinct. What about our local weirdo, the platypus?
The local weirdo is weirder that all the rest.
Platypus have 10 sex chromosomes - 5 different Xs and 5 different Ys. At meiosis, they pair up head-to-tail in a long daisy chain, and alternate chromosomes in the chain go this way and that.
The current model for their evolution is one of repeated translocation between sex chromosomes and autosomes, gradually recruiting more and more pairs of chromosomes into the chain.
The real blow-your-mind bit is that one end of the chain appears to be homologous to the bird Z/W pair, while the other end had some homology to the mammalian X/Y pair.
So - does this show a transitional form between one set of sex chromosomes and another? Did the common ancestor of platypuses and eutherians have an XY, a ZW, or environmental sex determination? How about the forms further back, on the branches between birds and platypuses - and *their* common ancestor?
The platypus is awesome, for the reasons Peter gives. And, yes, it's pretty interesting that one end is homologous to the chicken Z and the other is homologous to the human X. I'm not sure whether the snake Z falls in the chain or not.
As I mentioned here, the chicken homologue to the mammalian X and the mammalian homologue to the chicken Z have a signature that makes some people believe that the platypus may have the ancestral state. In short, both the mammal X and chicken Z lack genes invovled in cancer because selection against these genes is greater on these chromosomes (they're haploid in males, so males would be at a high risk for cancer). Interestingly, the autosomal homologue of the sex chromosome in the other taxon also show a deficiency of cancer related genes.