The duckbilled platypus is such an odd-looking beast that, when the first specimen made it to Europe, it was widely regarded as a fraud. And you can't exactly blame people for thinking that - they had never seen an animal anything like a platypus before. Now a study of the platypus genome, published earlier this year, has reinforced our understanding of just where this odd little animal fits in the mammalian family tree.
This animal really does look like a patchwork beast & you can see why it was first thought a fake. That duck-like bill confused people, for a start. (A platypus has its eyes, ears, & nose closed while underwater, and detects its prey using electro-sensory system in the bill.) The young develop in eggs but, like marsupials, do a lot of their development after hatching. And females feed those young on milk - but it's sucked/licked from a glandular area of the mother's skin, rather than suckled from a nipple. What's more, the males are equipped with venomous spines on their hind legs. But now that its genome has been sequenced, comparisons with other mammalian genomes can identify which features are peculiar to platypi (??platypuses??) and which are likely to be ancient mammalian characteristics.
And something else rather neat about platypuses: the way their genome is arranged. There are 52 chromosomes, mostly small with a few large ones. And - they have multiple sex chromosomes: 5 Xs and 5 Ys in males, which form chains during meiosis and segregate so that sperm are either 5X or 5Y. How strange - and how wonderful!
The research team comment : Analysis of the [platypus] genome [shows that] reptile and platypus venom proteins have been co-opted independently from the same gene families; milk protein genes are conserved despite platypuses laying egges; and immune gene family expansions are directly related to platypus biology (Warren et al. 2008).
They go on to describe their findings for a whole range of gene families, beginning with the 'odorant receptor' genes - the ones that affect the sense of smell. It turns out that platypuses have about half the odorant receptor genes of other mammals (& far more than in reptiles), but these represent the gene famliies seen in those other mammals. Many of the genes are disrupted in some way ie they are pseudogenes, & non-functional, which is predictable given the animals' reliance on electro-reception rather than scent or taste when finding prey. (In whales & dolphins, the complete set of odorant receptor genes is disabled in this way.)
What about that typical mammal feature, milk? Lactation is an ancient reproductive trait whose origin predates the origin of mammals (Warrent et al. 2008). One hypothesis for the original function of milk is that it acted to protect eggs against either drying out or bacterial infection. But milk in platypuses & echidnas, as in all other mammals, provides nutrients for the developing young (in addition to antimicrobial chemicals). And like other mammals, the platypus genome contains a cluster of genes for the milk protein casein. The casein genes seem to have evolved following duplication of one of the genes for proteins found in tooth enamel. These enamel genes are found next to the cluster of casein genes in both platypus and placental mammals. Warren et al. comment that adult platypuses, as well as echidnas, lack teeth but the conservation of these enamel protein genes is consistent with the presence of teeth and enamel in the juvenile, as well as the fossil platypuses.
Platypuses are venomous. Very few mammals produce venom; they include a few shrews, and an animal called a solenodon, which looks like a prickle-less hedgehog. The platypus is even more unusual in that males inject their venom via spurs on the hind legs, instead of by biting. The research team found that the genes encoding venom proteins had evolved from duplication of genes with quite different functions. And duplication of the same genes underlies the evolution of venom in reptiles - an example of convergent evolution in reptiles and monotremes.
The platypus immune-system genes also differ from those of other mammals, with a much greater number of what are called 'natural killer receptor genes': at least 214 in platypuses, compared to 45 in rats and 15 in humans. Platypuses and the marsupial opossums also have larger numbers of an antimicrobial peptide gene, compared to other mammals. It's suggested that these features may be related to the animals' reproductive biology. In both monotremes and marsupials the young are born at a very early stage of development and the antimicrobial peptide genes may enhance their immune systems' ability to fight off bacterial infections.
Odd little creatures, indeed :-)
W.C. Warren & 101 others (2008) Genome analysis of the platypus reveals unique signatures of evolution. Nature 453: 175-184