The Genetics of Platypus and Avian Sex (from Fail Quotes)

[evensgrey]

Indeed. The question is not, "Which came first, the chicken or the eggSHELL?"

On the question of chicken or egg first, it's quite clear that the egg came first, since all vertebrates use eggs of some form, and amniotic eggs precede chickens by rather a lot, going back to the basal form of the clade that includes all birds, reptiles, and mammals (although, properly speaking, there's no clade that includes all the things we commonly think of as 'reptiles' that doesn't also include all birds and mammals).

A much more interesting question would have been how the platypus, which is in the mammal clade, got a large chunk of the genetics of bird sex determination, which isn't in the other two subclades of mammals, and, as far as I know, isn't found in other clades that should be more closely related to birds than any mammal clade (like crocodilians).

[MrBogosity]

That isn't true. Birds use ZW chromosomes to determine sex. Platypodes use XY chromosomes, they just use TEN of them! Male platypodes are XYXYXYXYXY and females are XXXXXXXXXX.

[Ibrahim90]

That isn't true. Birds use ZW chromosomes to determine sex. Platypodes use XY chromosomes, they just use TEN of them! Male platypodes are XYXYXYXYXY and females are XXXXXXXXXX.

what he said. though I must add that in the ZW system, that males are ZZ, females are ZW-the exact opposite of the XY scheme (XX=female, XY=male).  this is because the ovum determines sex, not the sperm, as is typical in the XY system.

and yes, WW exists as an abnormality-they're females, and often the ones that are products of Parthenogenesis (in case anyone doesn't know: virgin births). they are otherwise normal.

to add: Komodo dragons and related reptiles also use ZW chromosomes for sex determination. though it isn't necessarily the case that the ZW birds used was original to their reptilian ancestry; ZW is just a term used for organism whose females are heterogematic and males Homogematic. ZW systems of sex-determination afterall is also found in some arthropods, some fish, etc.

though I have to admit, with some laughter: could the platypuses have possibly been any clearer in how male or female a platypus is? talk about overkill 

oh, and here's a new fail quote (sorta):

[yt]NZlGPhntPaU[/yt]

EDIT: sorry, I just couldn't help but post on biology: the paleontologist (oddly enough) within me XD

[evensgrey]

That isn't true. Birds use ZW chromosomes to determine sex. Platypodes use XY chromosomes, they just use TEN of them! Male platypodes are XYXYXYXYXY and females are XXXXXXXXXX.

Having an anomalously avian-like chunk of sex-determining genetics isn't the same thing as having a non-mammalian sex chromosome structure.

Crocodilians, which are much more closely related to birds than mammals are, don't have sex chromosomes at all, instead having sex determined by incubation temperature rather than an unrepressable factor that determines the organism will be one gender rather than the other.

What I'm saying about these chunk of genetics is that, while it still looks like a (remarkably messy and complicated) mammalian sex determining system, it has a chunk that looks much more like a bird than other mammal clades.  That would probably be a VERY old section that birds and platypus happened to both inherit relatively unchanged, while it got changed much more in other mammal clades.  Since the monotremes are a very early branch from the other two clades of modern mammals, having some exotic genetics that resembles a completely different clade isn't something completely unexpected.  There was lots of time for that chunk to be changed or removed in other lineages.

[Ibrahim90]

Having an anomalously avian-like chunk of sex-determining genetics isn't the same thing as having a non-mammalian sex chromosome structure.

if so, then you have to be more specific in what you are describing: which part of the sex gene are you referring to?

because now you've made me curious 

[evensgrey]

if so, then you have to be more specific in what you are describing: which part of the sex gene are you referring to?

because now you've made me curious 

There's a paper at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2413164/ that describes homologous regions of the sex chromosomes of different vertebrate groups and autosomes in other groups (with the aim of identifying what genome regions each sex chromosome evolved from) which finds a match between one part of the platypus sex chromosome group (and also describes the quite peculiar way that the platypus cells arrange their sex chromosomes so that they can pull of a successful meiosis) and some bird sec determining genetics.

A very interesting finding is that platypus X chromosomes and human X chromosomes do not seem to be homologous to each other, but are homlogous to autosomes in the other species, indicating a different origin of the sex determining system in monotremes and eutherians.  This means four known evolutions of sex chromosomes in vertebrates, and this one is downright weird and doesn't seem to have any similar systems elsewhere.

Like I said, given the fact monotremes are a very ancient divergence from the lineage that leads to us, all manner of strange things are likely to be in there.

[Ibrahim90]

There's a paper at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2413164/ that describes homologous regions of the sex chromosomes of different vertebrate groups and autosomes in other groups (with the aim of identifying what genome regions each sex chromosome evolved from) which finds a match between one part of the platypus sex chromosome group (and also describes the quite peculiar way that the platypus cells arrange their sex chromosomes so that they can pull of a successful meiosis) and some bird sec determining genetics.

A very interesting finding is that platypus X chromosomes and human X chromosomes do not seem to be homologous to each other, but are homlogous to autosomes in the other species, indicating a different origin of the sex determining system in monotremes and eutherians.  This means four known evolutions of sex chromosomes in vertebrates, and this one is downright weird and doesn't seem to have any similar systems elsewhere.

Like I said, given the fact monotremes are a very ancient divergence from the lineage that leads to us, all manner of strange things are likely to be in there.

sorry I couldn't answer sooner, but I will say thanks. I'll be reading this paper ASAP.