Greek feet + rare mammals

Tanaqui tweaver at imbolc.ucc.ie
Sat Feb 8 07:24:49 EST 2003


Jon:
 
+ I have several friends in diferent parts of NSW
+ with platypuses (platypi ? - not it must be platypuses
+ - its greek not latin) 

Some people insist that the plural of octopus is octopodes,
which means that they've pluralised the ending of the Greek
word "foot" to mean multiple animals.

Now, one octopus has multiple feet, so I don't think that quite works.
One octopus is one marine "eight-foot", but perhaps two octopodes are
two marine quadrupeds? Two octopuses are more sensibly two of the
animals labelled "eight-foot".

***

>From language to sheer taxonomic nerdiness:

The monotremata actually constitute a mammalian sub-class (the prototheria).
There are plenty of other literal "one-holes" in nature: birds and reptiles
have a cloaca, so called the echidna and the duck-billed platypus "monotreme"
isn't actually all that helpful.

Live birth is not an essential taxonomic checkpoint for inclusion in the 
mammal class, and teats are as unnecessary as navels: suckling the young is
the relevant qualification. Getting milk out of the mammary glands onto
the underside of the mother enables the young to suckle from the fur of
the platypus, but the echidnas actually bothered with teats.

I'm afraid I go deeper into mammalian sex determination than is entirely
necessary to refute "laying eggs=/mammal" if you want to read further...

***

The monotremes are also warm-blooded, which none of the commoner one-holes
are. Laying eggs and having freaky chromosomes may make the prototheria weird,
and the late-90s analysis of the mitochondrial genome rather messed with the
divergence models, but the critters did diverge from the therian mammals 
something like 170 million years ago (going by anatomy and reproductive method,
the fossil record and molecular tracking techniques). You lucky Aussies get the
roo's share of the marsupial set of the therians as well, while the rest of us
are stuck mostly with the "placental" eutherians.

By "reproductive method" I'm referring to the complex of unpaired chromosomes
that link with the sex chromosomes in male meiosis (and you thought the poison
spurs of a platypus male were odd?). This unique behaviour means that while the
prototheria have the XX/XY "sex chromosome" complement for mammals, no-one has
yet established that the monotreme Y chromosome contains any material which 
makes the bearer a male. A monotreme homologue of the SRY gene (testes-
determinant gene in well-studied animals like mice and humans and almost
certainly in marsupials) is not to be found on the sex chromosomes of
monotremes... the homologues appear only on autosomes. It took modern 
scientific analysis of the behaviour of genes even to notice the autosome-
chromosome meiosis lineup in these guys. oh, and the sperm are weird because
they have fibrillar nuclei (look at the pretty tandem chromosomes!).

The 1967 notion of the "degraded" Y has had to be reassessed subsequent to
research on monotremes and marsupials that showed the Y was subject to addition
as well as attrition during evolution.  

(by the way, in case you thought I meant "egg-laying" by "reproductive method",
 birds have Z and W sex chromosomes, and it's the female that gets one of each,
 rather than the male. oh, and some reptile species are gender-determined in
 the egg by ambient temperature.)

The marsupial X chromosome lacks many genes present on the eutherian X, and
the similarity of the monotreme X to the marsupial X means that researchers
actually have data which enables them to make a confident choice between
the possibility that the eutherian lineage has gained autosomal material or
that the marsupial lineage has lost it... an ancestral eutherian seems to have
gained autosomal material, padding out the X in this class of mammals.

The platypus diverged from the echidnas as recently as 30-70 million years ago.
Ornithorhynchus anatinus and "Tachyglossus aculeatus" live in Australia, whilst
the "Zaglossus bruijnii" is the New Guinea echidna mentioned by Jon. It's a lot
harder to sort out the dating with so few datasets (the echidna has 9 elements
arranged end-to-end as the result of terminalisation of chiasmata in male 
meiosis, while the platypus has 8. big help, guys. on the female front, the
echidnas have teats, while the platypuses do not).

oh, and Jon? get those friends in NSW to breed their critters in captivity,
and S. Akiyama of La Trobe University in Bundoora could be spared painstaking
reconstruction of family lineages via DNA fingerprint matches enhanced by
laborious microsatellite and mitochondrial-DNA analysis. The somatic cell
genetic analysis relies on hybridisation with rodent or human cells and we're
now at the point where several genes have been cloned, but biochemists are
*way* behind the curve in mapping the monotremes.

The table I've seen for gene location gives homologues mostly for the platypus,
with a few found in the short-beaked echidna. Citation available on request,
though I can't believe any list member would want to know where e.g. RAF (the
gene for "murine leukaemia viral (v-raf) oncogene homologue") would be found
in human and platypus genetic mapping. A cancer factor for mice, you say? 

oh, and all the coding gene factors found in the echidna are in the same place
as they are in the platypus, and all but one of those common factors are on the
same chromosome to boot (the X). 

***

and coming back up from nerdiness to something you might see in the daily
papers some day -
 
who knows, some crazy biodiversity radical might some day try to release the
beasties in alien habitats, but I can't see them doing well, so I'll have to
concede the Wrongness of Scarry on geographical habitat for the platypus 
without quite endorsing the word "never" as used by Sallyo.

Tanaqui
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