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| The Platypus |
The platypus, Ornithorhynchus
anatinus, displays unique features that have puzzled evolutionary
scientists ever since the first specimen was sent to Britain in 1799.
Possessing a combination of reptilian, mammalian and avian characteristics, the platypus and other monotremes are now considered to represent a separate
evolutionary branch that arose between primitive reptile-like mammals
(therians) and marsupial and eutherian (placental) mammals.
Recent
biochemical findings, along with structural and physiological comparisons, have
supported this idea. Some recent reports, however, appear to refute the
interpretation that the platypus merely shares a common ancestor with all three
vertebrate groups. Their inference, instead, is that there is a closer
relationship between these organisms.
Similarities Between the Platypus and Reptiles
One of the
reasons the platypus has been linked to reptiles, for instance, includes its
ability to secrete reptilian-style venom from the spurs on its legs. Molecular
studies have shown that this venom, consisting of b-defensin proteins, has a
similar composition to snake venom. However, Dr. P.Z. Myers, of the University
of Minnesota, argues that this similarity may simply be the result of
convergent evolution rather than an indication of a close relationship between monotremes
and reptiles.
Similarities Between the Platypus and Birds
The yolk-rich
eggs laid by the platypus have also been used to suggest they are somehow part
bird in nature. Myers and other scientists such as Dr. Ewan Birney
, however, suggest that this may instead be due to a common ancestry between
birds and monotremes rather than a close genetic linkage between the two
groups. This may also offer an explanation for recently observed similarities
between the sex chromosomes of monotremes and those of birds.
An example of
this similarity can be found on the fifth X chromosome of the platypus, which
contains the bird sex-determination gene DMRT1. The first X chromosome,
however, has been found to resemble ancient therian chromosomes. These
findings, along with the discovery of mammalian–like milk protein genes in the
platypus, help to reinforce the notion that the platypus, although definitely a
mammal, perhaps diverged from birds, reptiles and early therian mammals before
marsupials and placentals evolved.
The Most Primitive Mammalian Order
Indeed, the
fact that the platypus possesses ten sex chromosomes, as opposed to only two in
other mammals, suggests its unique evolutionary status. The carbohydrate content
in platypus and echidna milk also differs when compared to that of other
mammals. Unlike marsupial and placental milk, which predominantly contain
either lactose or galactose, monotreme milk has a high fucose content.
These
distinctive features, combined with molecular studies made by researchers at
the University of Nijmegen in 2005, support the ‘Therian hypothesis’, which
maintains that monotremes diverged before the separation of marsupials and
placentals. Previous studies of mitochondrial genes by Janke, Xu and Arnason in
1997 had in fact temporarily revived the ‘Marsupionta’ hypothesis, first
proposed by William King Gregory in 1947, which held that monotremes and
marsupials diverged after placental mammals.
Support for the Therian Hypothesis of Platypus Evolution
Analysis of
nuclear genes by the University of Nijmegen scientists, however, has led them
to estimate that monotremes diverged from therians between 231 and 217 million
years ago, while marsupials separated from placentals as recently as 193-186
million years ago.
Fossil
evidence does not clearly support this order of divergence, as the oldest
monotreme fossil, Taenolophis trusleru, is estimated to be only 123
million years old, while an opalised tooth of Steropodon, found at Lightning
Ridge, New South Wales, has been dated at 110 million years. On the other hand,
Sinodelphys szalayi , the earliest known marsupial fossil, has been
estimated to be approximately 125 million years old.
Morphological
studies, however, do suggest the monotremes are a more primitive group than
marsupials and placentals. Reasons for this conclusion include observed
differences in mammary glands, dentition and inner ear architecture among the
three groups. Perhaps the discovery of older fossil monotremes may help to
reinforce this in the future.
References
•
AFP, 2008, 'Neither Fish nor Fowl: Platypus Genome Decoded',
afp.google.com
•
Janke, A., Arnason, U., 1997,'The complete mitochondrial genome of the
wallaroo (Macropus robustus) and the phylogenetic relationship among
Monotremata, Marsupialia, and Eutheria', Proc. Natl. Acad. Sci. USA
•
Medical Research Council, 2008, 'Platypus Genome Unravelled', mrc.ac.uk
•
Myers, P.Z., 2008, 'Interpreting Shared Characateristics: The Platypus
Genome', University of Minnesota, nature.com
•
University of California,2011, 'Monotremata: Fossil Record',
ucmp.berkeley.edu
•
Van Rheede, et.al, 2005, 'The Platypus in its Place: Nuclear Genes and
Indels Confirm the Sister Group Relation of Monotremes and Therians',
University of Nijmegen, oxfordjournals.org
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