7.17 Platypus genome as distinctive as its owner
http://arstechnica.com/journals/science.ars/2008/05/07/platypus-genome-as-distinctive-as-its-owner
By John Timmer | Published:
May 07, 2008 - 12:01PM CT
The platypus has such a bizarre
mix of features that the first samples sent back to Europe were examined for
signs of fraud—biologists suspected a trickster had stitched together pieces of
different animals. It and its lone fellow Monotreme, the echidna, have the fur
and temperature regulation of a mammal, but lay eggs through a single
reproductive/excretory orifice like reptiles. Adding to the confusion, the
well-defined fossil series that details the evolution of mammals from therapsid
reptiles trails off around the time that monotremes split off from the rest of
mammals about 170 million years ago. Now, a few of the mysteries have been
cleared up, thanks to the completion of the platypus genome, published today in
Nature.
The paper starts off with a
nice description of the agglomeration of features distinct to the monotremes.
Although they lay eggs, the young that hatch are not fully developed, being
more akin to those of marsupials at birth. The young require several months of
nursing; platypuses produce milk, but provide it to their young without the
benefit of nipples. Male platypuses have testes that remain internal; they also
produce venom, but inject it via a hindlimb claw. Platypuses are also unique
among mammals in that they have an electrosensory system.
That weird collection of
features has been known to extend to the genome level, and the completed
sequence greatly expands on this. The platypus has a whopping 52 chromosomes,
many of them quite small, a configuration that looks like those of birds and
reptiles. Its sex chromosomes are inherited like the mammalian X/Y system, but
share sequence homology to the birds' Z/W chromosomes and, in a uniquely
platypusian twist on things, the animals run around with ten sex chromosomes
(males have five each of the X and Y). Incidentally, birds, being derived from
dinosaurs, are used as a reptilian reference genome in the absence of having
sequenced anything that's commonly thought of as a modern reptile.
Platypuses have an extensive
collection of non-coding RNAs. Many of their microRNAs are shared with birds
and mammals, and only small fractions are shared with only one or the other of
these groups. But roughly half of the microRNAs found were specific to
monotremes, and many of these appear to be involved in reproduction in some
way. All eukaryotes also make a class of non-coding RNA called snoRNA that
helps in the processing of ribosomal RNAs. In the platypus, however, snoRNAs
have somehow latched on to a reverse-transcription based transposase, and have
hopped all over the genome, resulting in a gross overabundance of these
sequences. Actual transposons account for roughly half the genome, and it
appears that some of these may still be active.
At the gene level, the
platypus appears to have about 18,500 protein coding regions, roughly in line
with the count in other mammals, but a bit higher than the chicken; 82 percent
of these appear in one of the other species of mammals or birds. The genes
involved in fertilization are a mix of those found only in mammals and those
found only in birds and fish. The platypus also has only a single gene for egg
yolk protein, in contrast to the three in the chicken genome. Like the rest of the
mammals, the genes for platypus milk proteins are clustered together and reside
next to the tooth enamel genes that they arose from via duplication.
The genome also sheds light
on some of the unique platypus features. Like other mammals, the platypus has
expanded their sensory abilities by duplicating many of the genes involved in
smell. Oddly, in the platypus, the family undergoing the largest duplication is
exclusively used for pheromone sensing in the mice. The authors suggest that
these were adapted for use by the electrosensing system of the animal. The
platypus' venom appears to be derived by duplication of many of the same genes
that have given rise to components of reptilian venom. Most of these genes have
been duplicated anyway on the mammalian lineage, but a separate duplication
event created a side branch of genes that were exapted into the venom system.
It's easy to think of this
mix of mammalian and reptilian/avian characteristics in the genome as simply
reflecting the biology of the platypus itself. But the genome goes a long way
towards clarifying what systems are truly inherited from a common ancestor (sex
chromosomes, for example), which date from the ancestors of all mammals (milk
proteins), and which are uniquely montreme (venom genes). In that sense, it
tells us a lot more about what's uniquely mammalian and, in that respect, it
tells us much about ourselves.
Nature,
2008. DOI: 10.1038/nature06936.
