Walt Meshaka has just published a fabulous new monograph on the introduced reptiles and amphibians of Florida in Herpetological Conservation and Biology. Check it out here. It includes the latest word on the eight introduced anoles of that fine state.
I just finished attending a workshop in Kesthely, Hungary on Niche evolution and speciation – two of my favorite topics. Sadly there was no Anolis news to report from any of the excellent talks, but the work I presented is related to one of the anole projects I’m planning for my postdoc.
Speciational evolution is, as the name implies, evolutionary change that occurs rapidly when one species is being split into two; this means the amount of evolution in a lineage should depend on the number of times speciation has happened in its history. This contrasts with the standard Brownian motion model of gradual evolution where the amount of evolution depends on the length of time that has passed. 
Speciational evolution might occur for a number of reasons (for example, due to genetic drift in small geographically isolated populations), but when one trait shows speciational evolution and another does not, we may be able to infer something about the process of speciation. For example, speciation may involve divergence in habitat (the ‘beta-niche’), or in traits that affect local resource use within a habitat (the ‘alpha-niche’).
Yes, it’s true. A “third eye” does exist, not only in the ancient Hindu literature and the new age imagination, but in birds, amphibians, reptiles, fish, lampreys, and hagfishes. We’re talking about the pineal gland, a small organ located on top of the brain, just underneath the surface of the skull. Although it doesn’t have visual capabilities in the image-forming sense, it is intrinsically photosensitive, responding to light signals without any help from the lateral eyes. (Mammals, including humans, have a pineal gland too…but it has lost the ability to detect light).
You can see the parietal eye on top of this anole’s head (it’s the tiny circle in the middle). The pineal gland can’t be seen externally, but it’s just posterior to the parietal eye and right underneath the surface of the skull. Photo credit: TheAlphaWolf, License:Creative Commons Attribution-Share Alike 3.0 Unported
Anoles, and some other lizards, actually have two “third eyes,” one being the pineal gland, and the other being the parietal eye, which can be seen in the picture above.
Photo by David E. Scott/Savannah River Ecology Laboratory, Aiken, S.C.
We’ll try to keep this post updated with links to coverage of the anole genome paper (please use the comments to tell us about new articles as they appear!):
Commentaries: Science 2.0, Why Evolution is True, Nature, National Geographic, Dust Tracks, myFDL (are you a septic of evolution?)
Press Release and Summaries: Broad Institute Press Release, Bloomberg, Harvard Gazette, Redorbit, International Business Times (and some amusing chatter about this article), TruthDive, io9, R&D Daily, GenomeWeb Daily
Image copyright Andrew M. Shedlock.
The anole genome paper is out in Nature today (although links on Nature’s own page only take you to a list of authors at the present time, I’m assuming this glitch will be fixed shortly). Nature also published a brief commentary highlighting some of the most interesting discoveries from this work. For more coverage of work related to the genome, check out this post and stay tuned to Anole Annals – we’ll have a bunch more genome posts over the next few days.
As the publication of the anole genome approaches, one might ask: “Just how was Anolis carolinensis selected to be the first non-avian reptile to have its genome sequenced?” Turns out that it’s a long and convoluted story, and this is one man’s first-hand account.
To set the stage, we have to go back to the early days of genome sequencing, all the way back to 2005. This was a time when to sequence a genome was a really big, time-consuming, extremely expensive affair (the human genome had cost ca. $2 billion; by 2005, the price had dropped to ca. $20 million per genome). Such a big deal, in fact, that there was an NIH committee that decided which species would be sequenced, and assigned them to one of the three genome sequencing centers (Baylor University, Washington University in Saint Louis and the Broad Institute in Cambridge) that had been created as part of the human genome sequencing initiative. The first few species selected were chosen exclusively with regard to their potential relevance to human health. They were the laboratory model systems, the workhorses of biomedical research, such as the mouse, chimp, Xenopus, chicken, Drosophila and C. elegans.
By 2005, a couple of mammals had been sequenced and representatives of all classes of vertebrates except one: reptiles.
Interested in transposable elements in the Anolis genome? You should be!
As DNA sequences that can move about the genome, transposable elements – or TEs – are also called “jumping genes”. These are some of the most important components of genomes, accounting for much of the variation in genome size and structure across vertebrates. The activity of TEs add to the genetic variation of populations in neutral, deleterious, and sometimes adaptive ways. In the human genome, TEs can insert into genes and cause numerous genetic diseases such as muscular dystrophy (Cannilan and Batzer 2006).
We published a review in last month’s issue of Mobile Genetic Elements (Tollis & Boissinot 2011) describing the diversity and abundance of TEs found so far in the Anolis genome, and how they impact our understanding of genome evolution in reptiles and mammals. The Anolis genome contains an extraordinary diversity of TEs, including DNA transposons (“cut and paste” elements) and long terminal repeat (LTR) and non-LTR retrotransposons (“copy and paste” elements). Even though there are many different kinds of TEs in Anolis, within most TE families there are low copy numbers relative to the human genome, suggesting that purifying selection keeps tight control.
Fig. 1: Figures illustrating Cuban origins of A. carolinensis from our 2005 paper in Molecular Ecology. Green shading indicates the range and phylogenetic position of A. carolinensis, blue shading indicates Cuban populations related to A. carolinensis. The arrows indicate possible dispersals from Cuba, some of which are supported by phylogenies (including the dispersal from Cuba to the continental United States indicated by the bold arrow).
With all this discussion of the green anole’s genome, it seems like a good time to remind everyone of how Anolis carolinesis came to be the model organism that it is today. The simple answer, of course, is that A. carolinensis is the only species of anole endemic to the continental United States. As such, its always been the anole species most accessible to the broadest range of researchers. The deeper answer – and the focus of this post – concerns how A. carolinesis happened to become the continental United States’s only native anole in the first place.
For information on why the anole genome is useful for evolutionary studies, go here.
For information on how the genome is already being used in research, try here, here, here, here and here.
For the history of discovery and study of anoles, go here.
For the evolutionary history of the green anole, check this one out.
For a great story, don’t miss this one.
For great pictures of anoles and their dewlaps, try here, here, and here (among others).
For many other topics in anole ecology, behavior, and diversity, try looking up terms in the blog’s search window.
One of these species has had its genome sequenced, and the other has independently evolved to look very similar and live in the same environment. The anole genome will make anoles an even more powerful group in which to study evolutionary convergence. Photos by Melissa Losos (left) and Pete Humphrey (right).
When the genome of Anolis carolinensis is finally published, most attention will focus on how this genome, the first reptile to be sequenced (not including birds), differs from other vertebrate genomes, and what these differences may tell us about genome evolution. No doubt this will be interesting, but the real value of this genome–in my unbiased opinion–resides in the questions we finally will be able to address about the evolutionary process, particularly in one model system of evolutionary study, Anolis lizards. Chris Schneider published a perceptive article, “Exploiting genomic resources in studies of speciation and adaptive radiation of lizards in the genus Anolis,” on this topic three years ago, and I will briefly expand on his points here.
An anole genome will be useful for evolutionary studies in two ways. First, a long-standing question in evolutionary biology concerns the genetic basis of convergent evolution (i.e., when two or more evolutionary lineages independently evolve similar features). Do convergent phenotypes arise by convergent evolution of the same genetic changes, or do different lineages utilize different mutations to produce the same phenotype? In other words, does convergence at the phenotypic level result from convergent change at the genetic level, or can different genetic changes produce the same phenotypic response? In the last few years, molecular evolutionary biologists have produced a wealth of studies investigating whether convergent changes in coat color in rodents, eye and spine loss in fish, bristle loss in fruit flies and many other changes are the result of changes in the same gene, even some times by the very same genetic mutation. Underlying these questions are more fundamental questions about constraints and the predictability of evolution (these topics have been reviewed a number of times in the last couple of years, most recently in a paper by me, in a paper which refers to other recent reviews).
The anole ecomorphs, habitat specialists behaviorally and morphologically adapted to use different parts of the environment. The same set of ecomorphs (with several exceptions) have evolved independently on each island in the Greater Antilles. Figure from "Lizards in an Evolutionary Tree," based on earlier figures in Ernest Williams' work.
Anolis lizards are, of course, the poster child for evolutionary studies of convergent evolution. Indeed, convergence has run rampant in this clade. AA has prattled on endlessly about the famous anole ecomorphs, a set of habitat specialist types that have evolved repeatedly on each island in the Greater Antilles to occupy different habitat niches. This convergence is usually studied in terms of limb length, tail length, and toepad dimensions: arboreal species have big toepads, twig species short legs, grass species long tails, and so on, with these traits independently evolving many times. But the ecomorphs are convergent in many other traits that have received less attention: head and pelvis dimensions, sexual dimorphism in both size and shape, territorial and foraging behavior, to name a few, and the more closely we look, the more convergent traits we find. And, further, anole convergence is not entirely an ecomorph phenomenon; some traits vary within an ecomorph class, but are convergent among species in different ecomorph classes, for example, thermal physiology and dewlap color.
In other words, there’s more convergence in Anolis than you can shake a stick at, and the availability of the anole genome sequence will provide the tools to investigate its underlying genetic basis.
Powered by WordPress & Theme by Anders Norén
