Re: [asa] Platypus- a transitional creature?

On May 8, 2008, at 9:33 PM, Jon Tandy wrote:

>
>
> From: asa-owner@lists.calvin.edu [mailto:asa-owner@lists.calvin.edu]
> On Behalf Of Rich Blinne
> Sent: Thursday, May 08, 2008 1:09 PM
> To: David Campbell
> Cc: Dehler, Bernie; asa@calvin.edu
> Subject: Re: [asa] Platypus- a transitional creature?
>
> 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.
> (*snip*)
> 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.(*snip*)
>
> Notably, duplications in each of the -defensin, C-type natriuretic
> peptide and nerve growth factor gene families have also occurred
> independently in reptiles during the evolution of their venom47.
> Convergent evolution has thus clearly occurred during the
> independent evolution of reptilian and monotreme venom48.
>
> I ask:
> Since the platypus is part of the protherian mammals (higher up the
> tree than the bird/reptile splits from the Sauropsids), why then the
> pervasiveness of reptilian features "that penetrates to the level of
> the genome sequence"? In several defenses of evolution I've read
> recently, the argument is made that if a fossil or creature were
> discovered which had characteristics that didn't fit with the
> predicted phylogenic sequence (i.e. characteristics from a different
> branch, instead of diverging from a common branching point), it
> would mean trouble for the evolutionary theory. Yet, this article
> speaks of "convergent evolution" to justify the fact that this
> protherian mammal has evolved the same features that reptiles have
> apparently evolved independently.
>
> Now, I'm certainly willing to give the experts due reverence in
> their fields of expertise, but isn't this perhaps a little begging
> the question? We are told that the phylogenic tree is descriptive
> of common ancestry, but when features appear on two different
> branches that don't follow the path of common ancestry, we are told
> that "convergent evolution" can occur? Before I jump to
> conclusions, isn't this what the article is saying?
>
> On the other hand, if modern evolutionary biology can tell us with a
> straight face that convergent evolution is a possibility, then isn't
> it a little disingenuous for them to turn around and claim that
> consistent phylogenic lineages is sure evidence that the
> evolutionary model is correct? I'll grant the possibility that a
> gene might mutate the same way twice in two different lineages, or a
> duplicate feature might emerge regardless of the genetic similarity,
> but what are the odds?
>
> Jon Tandy
>

They aren't begging the question. First the full answer can be found
here: http://www.genome.org/cgi/content/abstract/gr.7149808v1

The short answer is that the different genes mapped to four different
synteny groups. This is done by phylogenetic analysis. Declaring
convergent evolution is very much like declaring homo sapiens and
Neanderthals are different species in my earlier post. The genes for
the venom are similar but not the same and they are in different areas
of the genome and come from different common ancestors. (See
supplementary figures 4 and 5 from the paper.) Here's how the paper
describes it.

> The venom of some sauropsid reptiles contains crotamine and
> crotamine-like peptides (vCLPs) (Torres and Kuchel 2004; Fry et al.
> 2006), which have a similar disulfide bonding pattern and structure
> to the beta-defensins (Zhao et al. 2001). Our phylogenetic analyses
> show that these genes appear to have evolved independently from
> genes belonging to the same ancestral beta-defensin synteny group as
> the platypus OvDLPs (Fig. 5): Lizard vCLPs and chicken beta-
> defensins 1, 2, and 11 share a common ancestor, and snake venom
> crotamines and mouse beta-defensin 51 share a common ancestor. The
> platypus OvDLPs share a common ancestor with mouse beta-defensin 33.
>
> The independent evolution of both reptile venom crotamines and vCLPs
> and platypus OvDLPs are not the only ex-
> amples of venom evolving from antimicrobial peptides. Other groups
> of antimicrobial proteins, including gamma-thionins and whey acidic
> protein (WAP) proteins, appear to be similar to animal peptide
> toxins (Kaplan et al. 2007). Similar selective pressures on
> antimicrobial peptides (which evolve to target and kill a wide range
> of rapidly evolving microbes) and venom peptides (which have evolved
> to specifically cover a wide range of prey) suggest an evolutionary
> connection between antimicrobial and venom peptides (Kaplan et al.
> 2007).
>
> In the platypus, antimicrobial peptides are not the only molecules
> that have given rise to venom peptides. We have also characterized
> the genes of the only other two sequenced venom components, OvCNP
> and OvNGF, which have homologs in other species that function in
> nontoxin roles (Supplemental material 3). Homologous proteins too
> can be found in reptile venom, and phylogenetic analysis suggests
> that these venom components may also have originated independently
> in reptiles and platypuses (Supplemental material 3). Clearly,
> recruitment of genes involved in nonvenom cellular pathways is a
> common feature in the evolution of venom, when it is considered
> that at least 18 molecules in reptile venom have evolved in this
> way, including acetylcholinesterase, CRISP (cysteine-rich secretory
> protein), CVF (cobra venom factor), and Factor X (Fry 2005).
>
> We have identified genes in the platypus for beta-defensins and the
> earliest known alpha-defensins. We have shown that venom
> molecules have evolved independently in the platypus and sauropsid
> reptiles, and we propose that there are specific protein motifs that
> are preferentially selected for evolution to toxin molecules (Fry
> 2005). Independent origin of venom molecules appears to coincide
> with independent origin of venom glands: Snake venom glands evolved
> from modified salivary glands, whereas platypus venom glands evolved
> from modified sweat glands (Temple-Smith 1973).

Rich Blinne
Member ASA

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Received on Fri May 9 00:39:53 2008