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Platypussies
Ah, the noble Platypus, such a lovely creature.



In the news this week was the fact that the entire genome of the Platypus has now been sequenced.
Unfortunately most news sources failed to report much of substance except to say: gee whizz, this is one odd creature. Well we already knew that in the 18th century (I assume the Aborigines knew that 42.000 years before but they probably don't really count) and spectacular headlines about the Platypus having venom like a reptile* notwithstanding there is a lot more to report that is of interest.

Here are the best reports I've found (if you can stand some duplication).
www.sciencedaily.com
www.npr.org
www.nature.com

Some of what's reported isn't really new though. An article I found in 'Comparative Biochemistry and Physiology - Part A: Molecular & Integrative Physiology' called "The monotreme genome: a patchwork of reptile, mammal and unique features?" (Frank Grutzner et al) from 2003 already told us about the oddity of monotreme sex chromosomes, and it's probably a lot weirder than the articles linked to above make out.

As most of my readers won't have access to the fabled 'Comparative Biochemistry and Physiology Part A' (how can you get through the day without it?) I'll post a quick summary.
Almost every eukaryote (animals, plants, funghi) living being has its chromosomes paired. Humans have 46 chromosomes, in 23 pairs (2n =46). Maize has 20 in 10 pairs (2n=20); Chimpanzees have 48, in 24 pairs (2n=48).
Sidenote: sometime before the emergence of Homo sapiens we fused two chromosomes together.
Read PZ on: Basics: How can chromosome numbers change? and Luskin's ludicrous genetics.


The odd thing seems to be that the Monotremata (the Platypus and the two species of Echidna) seem to have some chromosomes that are not paired at all. There's no simple XY system to represent male or XX for females. (Would this be noted as 2n=46+6 or simply 2n=52?) Screenshot from Grutzner et al. Weird shit, especially when you take into account the fact that during cell division and gamete forming somehow every cell must end up with an equal and viable number of chromosomes. There's a lot of zig-zagging involved here and I won't go into it in detail except to say it all seems to be able to be worked out by temporarily fusing some chromosomes together during celldivision and then dividing the resulting zig-zag in an upper and lower half. A few plantspecies seem to do the same thing so it's not entirely unique but still odd.

Incidentally the article also tells some very interesting things I didn't know before about "regular" mammals. The Y-chromosomal gene that regulates the formation of testis is a mutation of the X-chromosomal SRY gene that regulates brain development. (Insert joke here about men thinking with their private parts).
Also food for thought is a section on X-inactivation which seems logical when you think about it. If females have 2 X chromosomes how come they're not inundated with the products of these (duplicate) genes? Simply inactivate most of the copies and you're good to go.
Interesting stuff. The humble duckbilled platypus can teach us a lot it seems and with the unraveling of its genome we can look forward to more and deeper understanding of what makes a mammal.


*) An example of convergent evolution, where remote species develop the same trait separately, that is: the genes for this venom were not inherited from a common ancestor to both mammals and reptiles. (See also the parallel evolution of the eye in molluscs like the octopus and the eyes of chordates like a salmon or a cat.)

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