Credit: Wikipedia
From yolk to milk
Like other mammals, around 2 per cent of the platypus genome codes for proteins – 18, 527 of them. Some provide tantalizing traces of the journey from reptile to mammal, particularly the ones that produce egg yolk and milk.
The yolk of a reptile's egg is all the nourishment the developing foetus will ever get. But a mammal nourishes its developing young continuously, first with the placenta and then with milk, which allows for the development of the big mammalian brain. But how do you go from being an egg-laying reptile to an animal with a placenta and milk production?
The platypus doesn't give all the answers but it does give us a clue. It seems the first step is to get the milk production happening; then you can afford to ease off on yolk production. Essentially the foetus gets weened from yolk to milk. That's what you see in the platypus genome. All the casein milk protein genes that other mammals have are already present. They are nestled right next to the genes for making tooth enamel proteins.
Most likely the milk protein genes evolved from those tooth enamel genes. A copying error created a second but faulty set of tooth genes that eventually turned out to be useful as milk proteins. On the other hand the platypus egg yolk genes, the vitellogenins, are on the way out. While chickens have three copies, the platypus only has one.
Venom genesis
Another feature the platypus shares with its reptilian ancestor is a potent venom it produces from a spur on its hind legs. Kathy Belov and colleagues at the University of Sydney accidentally identified the genes that produce the venom.
Her main interest had been to find out how the platypus protects its young from the grotty environment they live in. When born, they have no functioning immune system – no spleen, no thymus, no T or B cells, no antibody-making cells. The belief was that mother's milk carried some anti-microbial compounds. Belov found the antimicrobial gene cathelicidin, seven copies of it in fact. Humans only carry one.
As many studies of animal genomes are showing, duplicating genes is a quick way to evolve new traits. But Belov's code scan also picked up a gene that vaguely resembles another anti-microbial gene known as defensin. It turned out this gene was not employed against microbes but against larger adversaries: it was the major component of the venom.
Reptile venom, it turns out has also evolved from the defensin gene. But according to Belov's analysis, the co-opting of defensin in platypus and in reptile venom were independent events.
fascinating
so what happends if there arent a complete transfer of 5 chromosomes?
like 6 x's and 4 y's ?
is the 5 transferred as a package?
is information on the y equivalent chomosomes dominant over each other?
are they duplicates or are they different?
so many more questions
Eliot
platypus sex chromosomes
Hi Eliot,
good questions. You would not get offspring if the platypus had not worked out to sort out the ten sex chromosomes and we showed that it works reliably (Grutzner et al. 2004, Nature). How they do it? The all stick together in a chain of chromosomes X1Y1X2Y2....X5Y5, then all Ys wander off in one direction and all X in the other (in a way it is as a package), how they do it? We don't know,
The chromosomes are not duplicates they all carry different genes.
Cheers, Frank.
Platypus genome
Excellent article, marred only by the common practice of journalists to confuse 'code' with 'information'. With minor and rare exceptions, the genetic code is universal, being the same for cabbages, mushrooms, jellyfish and camels. Just as it was possible to transmit an infinite number of messages using the Morse code, the genetic code makes possible a virtually infinite variety of proteins.
Martin Hanson, retired science teacher