The platypus genome has been sequenced. Some of the cooler details:
- Sex determination. It has been known that platypuses have not one but five (!) pairs of sex chromosomes. Male platypuses exhibit an XYXYXYXYXY genotype. And those sex chromosomes bear some connection to the ZW sex determination system found in birds. But what the newly sequenced genome reveals is that the platypus's male-determining gene, the monotreme equivalent of primates' SRY gene, is not located on any of those five pairs of sex chromosomes. Instead, it resides on an altogether different chromosome, one with no apparent connection with sex determination.
- The beginnings of a transition from oviparous to viviparous reproduction. Whereas some birds (such as chickens) have as many as three genes affecting the production of egg yolk protein, platypuses have just one. The platypus genome evidently exhibits some sort of transition from egg-laying, which requires the delivery of nutrition during incubation, toward providing more nutrition after hatching.
- Platypus milk. And that nutrition takes the form of milk, which female platypuses deliver without the benefit of nipples. Secreted from "milk patches" on the abdomen, platypus milk appears to be a modified version of a moisturizing fluid originally developed for keeping eggs from drying out during incubation. At least five distinct genes direct the progressively growth of nutritional complexity in platypus milk.
- Detecting chemicals under water. Platypuses have an impressive arsenal of "vomeronasal" genes that help them detect pheromones under water. Platypuses can thereby detect mates and prey even as they close their eyes and nostrils while diving.
- Venom. Male platypuses deliver venom from spurs their rear legs. The only mammal to make venom, platypuses generate chemicals that are very similar to some snake venoms. Apparently these similarities are the product of evolutionary convergence rather than genetic descent. Although platypuses and snakes built venoms from the same starter molecule in their immune systems, they evolved venom independently and by different genetic routes.