Wednesday, September 19, 2012

Snakes, An Origin Story

Quite honestly, I should have been reading a plant paper for my upcoming lab meeting. But then I stumbled across a really cool snake paper and, well, that won out. I regret nothing. As with most people, I will most likely read the plant paper right before the meeting anyway.

A paper published online today in Biology Letters takes a look at the phylogeny of squamate reptiles (lizards and snakes). This group of reptiles is one of the most diverse and well-known vertebrate groups including approximately 9000 species among 61 families. As with many groups, taxonomists and geneticists are trying to reconcile morphology and molecular analysis. This paper is taking the molecular approach, specifically looking at sister groups and interrelationships of major snake clades and iguanian families. Being a well-studied group, molecular analyses have been conducted in the past. These studies have suggested that squamate molecular phylogeny results differ quite a bit from morphological ones. This study takes one gigantic step forward, increasing the sampling of taxa dramatically and doubling the number of genes studied.

The researchers sampled 161 squamate species and 10 outgroup taxa, including mammals (Homo, Mus, Tachyglossus), crocodilians (Alligator, Crocodylus), birds (Dromaius, Gallus), turtles (Chelydra, Podocnemis) and a rhyncocephalian (Sphenodon). Then they sequenced portions of 44 nuclear genes, targeting single-copy genes evolving at appropriate rates. The nucleotide sequences were then translated into amino acids to aid alignment. This alignment consisted of 33,717 base pairs! I’m not going to go into all of the bootstrapping, likelihoods, and Bayesian analyses that were used (even the word Bayesian makes my brain shut down in protest). But suffice it to say that the different analysis techniques that were used yielded similar phylogenies, providing strong support for the relationships found.

The results of this molecular analysis were found to be consistent with other, recent, similar studies. However, there were some interesting relationships discovered. The first of these was that dibamid (legless lizards found in tropical forests) and gekkotans (geckos and the limbless Pygopodidae) are together the sister group to all other squamates. They also found strong support for paraphyly of scolecophidian snakes (blind snakes). Scolecophidians have reduced eyes and are specialized burrowers. Considering these traits, the paraphyly of this group suggests that it is the ancestral form, that other snakes may have been burrowers ancestrally. This makes sense if you compare the morphology of snakes to other burrowing species such as limb-reduced lizards. They both have short tails and elongate trunks. Very good for tunneling their way through the earth.

Overall, a really interesting study that was huge in its scope. I look forward to more of these kinds of studies in the future.

ResearchBlogging.orgJohn J. Wiens, Carl R. Hutter, Daniel G. Mulcahy, Brice P. Noonan, Ted M. Townsend, Jack W. Sites Jr., & Tod W. Reeder (2012). Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species Biology Letters, 4 (11) DOI: 10.1098/rsbl.2012.0703

If you are a non-scientist and I used too many biology-jargony words for you or you just need a refresher on phylogeny, then I recommend looking through these sites:
Fullerton’s Biology 261 course page on Interpreting Cladograms
Berkeley’s Understanding Evolution page on reading phylogenetic trees

Here is some more information on blind snakes:
ScienceBlogs article Scolecophidians: seriously strange serpents

ScienceShot article: Snakes' Slitherin' Subterranean Kin

(images from  Encyclopedia of Life and Neoseeker, respectively)

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