Re: [asa] Platypus- a transitional creature?

On Thu, May 8, 2008 at 11:07 AM, David Campbell <pleuronaia@gmail.com>
wrote:

> > A question- young earth creationists constantly say there are no
> > transitional life forms. On the surface, doesn't the platypus easily
> defeat
> > that argument?
>
> Although the platypus retains some intermediate features between a
> reptilian grade organism and more standard mammals, it also has plenty
> of specialized adaptations for its particular lifestyle, so it is not
> as good of an example of a transitional form as many fossils.
> Nevertheless, it is a fairly good example and one that many people
> have heard of, limiting the opportunities for misrepresentation (not
> that there aren't published YEC attempts to deny that the platypus is
> a transitional form).
>
>
> > "Neither fish nor fowl: Platypus genome decoded"
> > Excerpt:
> > According to a study released Wednesday, the egg-laying critter is a
> genetic
> > potpourri -- part bird, part reptile and part lactating mammal.
>
> Neither this nor the AP story that it seems to derive from gave
> information about locating the actual study, and a bit of searching
> failed to turn it up, so I can't comment in detail on exactly where
> the error of "part bird" came in. Probably from the duck-like beak,
> though the story citing the venomous spur of male platypus as a
> reptile-like feature didn't know what it was talking about, either.

No kidding. Here is the actual conclusion from the paper. It's very much a
mammal. See what I bolded below.

The egg-laying platypus is a remarkable species with many biological
> features unique among mammals. Our sequencing of the platypus genome now
> enables us to compare its sequence characteristics and organization with
> those of birds and therian mammals in order to address the questions of
> platypus biology and to date the emergence of mammalian traits. We report
> here that sequence characteristics of the platypus genome show features of
> reptiles as well as mammals.
>
> Platypus contains a largely standard repertoire of non-protein-coding,
> ncRNAs, except for the snoRNAs, which exhibit a marked expansion associated
> with at least one retrotransposed subfamily. Some of these retrotransposed
> snoRNAs are expressed and thus may have functional roles. The platypus has
> fully elaborated piRNA and miRNA pathways, the latter including many
> monotreme-specific miRNAs and miRNAs that are shared with either mammals or
> chickens. Many functional assessments of these novel miRNAs remain to be
> carried out and will surely add to our knowledge of mammalian miRNA
> evolution.
>
> The 18,527 protein-coding genes predicted from the platypus assembly *fall
> within the range for therian genomes*. Of particular interest are families
> of genes involved in biology that links monotremes to reptiles, such as
> egg-laying, vision and envenomation, as well as mammal-specific characters
> such as lactation, characters shared with marsupials such as antibacterial
> proteins, and platypus-specific characters such as venom delivery and
> underwater foraging. For instance, anatomical adaptations for chemoreception
> during underwater foraging are reflected in an unusually large repertoire of
> vomeronasal type 1 receptor genes. However, the repertoire of milk protein
> genes is typically mammalian, and the arrangement of milk protein genes
> seems to have been preserved since the last common ancestor of monotremes
> and therian mammals.
>
> Since its initial description, the platypus has stood out as a species with
> a blend of reptilian and mammalian features, which is a characteristic that
> penetrates to the level of the genome sequence. The density and distribution
> of repetitive sequence, for example, reflects this fact. The high frequency
> of interspersed repeats in the platypus genome, although typical for
> mammalian genomes, is in contrast with the observed mean microsatellite
> coverage, which appears more reptilian. Additionally, the correlation of
> parent-of-origin-specific expression patterns in regions of reduced
> interspersed repeats in the platypus suggests that the evolution of
> imprinting in therians is linked to the accumulation of repetitive elements.
> We find that the mixture of reptilian, mammalian and unique characteristics
> of the platypus genome provides many clues to the function and evolution of
> all mammalian genomes. The wealth of new findings and confirmation of
> existing knowledge immediately evident from the release of these data
> promise that the availability of the platypus genome sequence will provide
> the critically needed background to inspire rapid advances in other
> investigations of mammalian biology and evolution.

 David C continues:

>
>
> Among living animals, birds are most closely related to crocodiles and
> turtles. In turn, those (plus dinosaurs and assorted other extinct
> things) are related to tuataras, snakes, and lizards. The entire
> reptile+bird group in turn is related to the mammals. Platypus and
> echidna are the earliest-diverging branch of mammals to still be
> alive. Thus, the connection of bird and platypus is through the
> reptiles. "Part bird, part reptile" is misleading, and I would be
> very surprised at any feature that is more like birds than any
> reptile.

To add a little more detail here (and this is from the paper and not the
review articles which is kinda strange): Synapsids (mammal-like dinosaurs)
and Sauropsids (bird-like dinosaurs) split 315 Mya, Protherian and Therian
Mammals split 166 Mya, Therian Mammals split into Marsupials and Eutherian
Mammals 148 Mya. Platypus is a monotreme which a kind of Protherian Mammal.
Another monotreme is echnida. This part bird and part reptile is nonsense.
Archosaurs (birds) and Lepidosaurs (reptiles) split from Sauropsids (no date
given in the paper). Platypus is in a completely different branch than
either of these. The bill of a platypus is a classic example of analogy
rather than homology.

David C. continues:

>
> The fact that platypus and certain lizards and snakes both produce
> venom is no more significant phylogenetically than the fact that wasps
> and conoidean snails also produce venom.

 The close similarity between
> Gila monster venom and snake venom, on the other hand, is quite
> interesting evolutionarily.
>

You nailed it again. Note my emphasis. This is why I went round and round
with Dick about phenotypes and genotypes.

Only a handful of mammals are venomous, but the male platypus is unique
> among them in delivering its poison not via a bite but from hind-leg spurs.
> Despite the obvious difficulties in obtaining samples, it is now known that
> platypus venom is a cocktail of at least 19 different substances46<http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html#B46>including defensin-like peptides (vDLPs), C-type natriuretic peptide (vCNP)
> and nerve growth factor (vNGF). When analysed phylogenetically and mapped to
> the platypus genome assembly, these sequences are revealed to have arisen
> from local duplications of genes possessing very different functions (Fig.
> 4<http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html#f4>).
> Notably, duplications in each of the [image: beta]-defensin, C-type
> natriuretic peptide and nerve growth factor gene families have also occurred
> independently in reptiles during the evolution of their venom47<http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html#B47>
> .* Convergent evolution has thus clearly occurred during the independent
> evolution of reptilian and monotreme venom*48<http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html#B48>
> .
>

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Received on Thu May 8 14:10:50 2008