For the platypus, weirdness extends all the way to its DNA. With a "duck bill" and eggs instead of live young, this species belongs to the oldest group of mammals, the monotremes, which date back 210 million years. And the platypus genome, oddly enough, contains more than the usual two sex chromosomes. Now researchers have found even more strangeness in how the sex chromosomes behave during the cell cycle. The findings help explain how the platypus manages to keep its reproduction from going awry.
Many organisms have two sex chromosomes. Women, for example, have two X chromosomes, and men have one X and one Y. But in the platypus, males have five X and five Y chromosomes, while females have 10 Xs. If the male platypus's X and Y chromosomes randomly segregated into sperm, this would greatly complicate sex determination. Frank Grützner, a geneticist at the Australian National University in Canberra, and his colleagues took a closer look by "painting" individual chromosomes with specific dyes and following them through all the stages of cell division.
Grützner and his colleagues watched the 10 sex chromosomes first link up into a chain--an unusual configuration for mammals--in precursor sperm. The chain consisted of alternating X and Y chromosomes. During the key step in sperm formation--a division that results in two cells, each with half the original number of chromosomes--the X and Y chromosomes peeled off from the chain one by one and headed into separate cells, all segregating faithfully with their own kind. This ensures that half the sperm each have five X chromosomes; the other half have five Y chromosomes, Grützner and his colleagues report online 25 October in Nature. (The system is simpler in females, who contribute only X chromosomes to the next generation.)The team found another oddity in the platypus genome: The sequences of the various X chromosomes differed in length and makeup. The genes in the largest X chromosome look mostly like their counterparts in humans. In contrast, the sequence of the last X in the chain looks a little like bird DNA, specifically the bird Z chromosome. Both the Z chromosome and this X chromosome seem to have a gene called DMRT1, which in birds is likely the sex-determining gene; that is, the gene that helps steer a developing bird embryo toward one gender or the other. The researchers don't yet know if the platypus DMRT1 protein serves the same purpose. The platypus comes from a very ancient line of mammals; thus the ancestor to all mammals may have had a Z-like sex chromosome at one time. "This suggests that the mammal and bird sex chromosome system is not as different as we have always thought," says Marilyn Renfree, a zoologist at the University of Melbourne, Australia. But other researchers question the link between mammalian and bird sex-determining systems, saying the connection is unclear from the present data. Even so, says Harvard's David Haig, "this is beautiful work; it appears to have resolved the long-standing question of the nature of the monotreme sex-chromosome system."