Thursday, 24 December 2015

12th genome of Christmas: The platypus

In 1799 George Shaw, the head of the Natural History Museum in London, received a bizarre pelt from a Captain in Australia: a duck bill attached to what felt like mole skin. Shaw examined the specimen and wrote up a description of it in a scientific journal,  but he couldn't help confessing that it was "impossible not to entertain some doubts as to the genuine nature of the animal, and to surmise that there might have been practised some arts of deception in its structure." Hoaxes were rife at the time, with Chinese traders stitching together parts of different animals - part bird, part mammal - to make artful concoctions that would trick European visitors. Georgian London was becoming rather skeptical of these increasingly fantastical pieces of taxidermy.

But the duckbilled platypus is no hoax. It is one of the last extant remnants of the egg-laying mammals, monotremes (along with the far more commonplace, but less exotic, Echidnas - hedgehog-like mammals). Duckbilled platypuses have a bizarre set of features: they are the only mammal with a specialised venom organ (one or two shrews have developed poisonous saliva; the duckbill has a specialised claw), with venom strong enough to incapacitate a human. They hunt in muddy streams and have a sixth sense: electrosensation, as in fish, thought to be sensed via their leathery bill. And of course the female lays leathery eggs, out of which hatch tiny little (and super-cute) “hatchlings”, which will first feed from milk patches on the female.

So in the middle of the first decade of this century, when genome sequencing was becoming marginally more routine, it seemed obvious that at least one monotreme should be on the list. The duckbill simply had to take a star turn. Echidnas are also cute, but, frankly... far less weird.

And the genome did not disappoint. One complicating factor was platypus sex chromosomes. Even before the genome was sequenced, it was clear that platypus sex was no simple affair. Like all other mammals, platypuses have sex chromosomes, but there are 10 of them in five pairs, rather than the usual two sex chromosomes in one pair. This could lead to 25 possible sexes, but it doesn’t seem like there's much diversity in platypuses. As it turns out, at the key point in meiosis (the process of making sperm and eggs) the five X chromosomes all line up together with the five Y chromosomes in a spectacular act of chromosomal ballet, and divide as one, such that each sperm either gets five X chromosomes or five Y chromosomes, but a mixture, say 3X and 2Y in one direction, and 2X and 3Y in the other. This means that each sperm was either all X or all Y.

The genome sequence was even more surprising. Birds also have genetic sex determination (in contrast, many reptiles and fish do not). However, the avian system is on different part of the genome (there is no standard way of doing sex chromosomes). It's the other way around from mammals; females have the different chromosomes (ZW) whereas the males are the homogeneous set (ZZ). (An oddity in the naming system for sex chromosomes is that they are always called either XY or ZW, even though the X chromosome in mammals has absolutely no relationship to X in, say, fruitflies. It’s just a convention.) The bizarre split-five-ways sex chromosome in platypus is mainly similar to the Z chromosome (which is similar to the human chromosome 9). It's as if the platypus, genetically, is some mixture of bird and mammal: a bird-like sex determination, but flipped the other way around like mammals.

The platypus genome also definitively put milk as the major mammalian innovation, before bearing live young. The thin milk produced by platypuses was unclear in its origin (it is also quite hard to study this, as the female is protective of her hatchlings) but the milk caesin gene is clearly in the same location in platypus as placental mammals.

For me, working on the platypus genome drove home both the diversity of life (egg laying, milk producing, weird sex chromosome mammalian poisoner, anyone?) and its arbitrary nature. Perhaps there is an alternative planet Earth where egg-laying mammals are the dominant species, and the live-young-bearing placental mammals were the oddities. If we had these bizarre sex chromosomes, I am sure there would be all manner of speculation about how this system was somehow linked to our intelligence or dominance.

But on this Earth, platypuses are the strangers, and serve every day as a reminder that biology is far, far more imaginative than we are. And we are all the richer for that.