Category: New Research Page 50 of 66

Anolis carolinensis is increasingly used as a genetic model organism, but we know suprisingly little about the distribution of geographic genetic variation in this species across its native range.  At this year’s evolution meetings, Marc Tollis presented his recently published work on phylogeography of Anolis carolinensis.  His work provides basic information on geographic genetic diversity within A. carolinensis, and permits tests of hypotheses about the contribution of riverine barriers, sea-level changes, and southern refugia to this diversity.  Tollis sampled 190 anoles from 9 states and obtained sequence data from mtDNA and 10 novel nuclear loci (4 introns and 6 anonymous loci).  Using phylogenetic analyses and the Bayesian clustering algorithms in Structurama, Tollis identifies four major clades that appear to have diverged from one another around 2 million years ago: North Carolina, Gulf-Atlantic, Suwannee, and Everglades.  Although these populations appear to have experienced range expansions, Tollis rejects the southern refugium hypothesis because expansion events predate the inter-glacial, genetic diversity is no greater in the south, and there is no consistent pattern of northern genotypes nested within southern genotypes.  Instead, Tollis’s data points to a rapid and recent westward expansion.    Tollis’s work also rejects the hypothesis that rivers are important barriers to Anolis carolinensis dispersal, a result that he suggests is not surprising given the group’s well-established overwater dispersal capabilities.  Because this phylogeographic work on Anolis carolinensis rejects both the riverine barrier and refugium hypotheses, it appears that the distribution of genetic diversity is somewhat unique and not widely shared with other taxa distributed across the same region.  Phylogeographic analyses of A. carolinensis are long overdue and Tollis’s presentation and associated publication are a most welcome contribution to the field.

Tollis M, Ausubel G, Ghimire D, & Boissinot S (2012). Multi-Locus Phylogeographic and Population Genetic Analysis of Anolis carolinensis: Historical Demography of a Genomic Model Species. PloS one, 7 (6) PMID: 22685573

As regular readers of this site will know, anoles are remarkable for the repeated, independent evolution of ecomorphs on the four islands of the Greater Antilles. Each ecomorph is defined by a suite of ecological and morphological traits that appear to be shaped by natural selection.

In a recent paper, Kolbe et al. ask whether those suites of morphological traits are actually suites. In other words: is convergence in form across islands reached by evolving the same sets of characters in a similar manner? Do all trunk-ground ecomorphs, for example, achieve relatively long limbs by growing both the femur and the humerus (i.e. those traits covary together)? Or do some trunk-ground anoles achieve long limbs by only growing the tibia and the radius while others grow the femur and radius etc.?

Covariance ellipses for 8 species for five trait sets. Find the ellipse for Anolis distichus in the first column. It suggests that A. distichus will have a short humerus when it has a short femur and a long humerus when it has a long femur and this covariance is fairly tight (an oblong ellipse). For lamella# and femur length, however, there isn’t a tight relationship (a circular ellipse) and it’s hard to predict lamella# from femur length. Note the similar shape of the covariance ellipses for the three trunk-ground anoles, A. gundlachi, A. sagrei, and A. cybotes. These suggest convergent evolution of trait sets in that ecomorph.

Understanding whether and how different sets of traits vary together can give a good understanding of how natural selection and evolutionary history combine to explain the convergent evolution of Anolis ecomorphs.

The authors ask several questions in this paper.

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The first poster session at Evolution 2012 got off to a great start last weekend with Rosario Castañeda’s poster on ecomorphological evolution in mainland Anolis.  Currently a post-doc in the Losos Lab at Harvard, where she’s working to bring Anolis to the Encylopedia of Life, Rosario’s Ph.D. thesis with Kevin de Queiroz and associated publication investigated phylogenetics and ecomorphological diversification of the Dactyloa clade of Anolis.  For those who aren’t already familiar with the Dactyloa clade, this group of impressive anoles can be found in the Lesser Antilles and South America.  In her poster, Rosario presents results obtained by combining her multi-locus phylogeny with morphometric data.

Using UPGMA analyses of principal component axes extracted from morphological measurements of 50 species of Dactyloa and 28 species from the Greater Antilles, Rosario initially reported recovering nine distinct morphological clusters (four of which include only one species).  Rosario further reported that these phenotypic clusters do not correspond with monophyletic groups on her phylogenetic tree, suggesting that each cluster did not simply evolve a single time.  Finally, Rosario used distances among species in morphometric space to show that fifteen species in the Dactyloa clade are similar to one of the replicated Greater Antillean anole ecomorphs.  She specifically reported that species from the Dactyloa clade can be assigned to trunk-crown, trunk-ground, and twig ecomorphs.

The Evolution meetings are now ended, but the fond memories linger on. Such as Yoel Stuart reporting the results of his study of character displacement in Mosquito Lagoon, Florida. Dredge spoil islands were created about 50 years ago when the area was dredged, producing big piles of sand which were subsequently colonized by plants and, eventually, green anoles. Within the last 10 years, many islands were invaded by brown anoles, but some remained sagrei free. Yoel set out to compare the green anoles on islands with and without brown anoles.

First, though, he demonstrated the islands with and without brown anoles didn’t differ consistently in any environmental parameter. Thus, nature has set up a very good experiment.

Yoel found that green anoles perch higher in the presence of brown anoles, presumably a result of interspecific interactions. Moreover, on brown anole islands, green anoles have better developed toepads. A common garden experiment reveals that these differences are  not the result of plasticity. Hence, morphological differences have evolved in a very short time as a result of a habitat shift caused by the presence of another species–an excellent example or rapid evolutionary change and character displacement in action.

Many studies find that two populations are extremely genetically differentiated and assume that they are reproductively isolated. Last night, Anthony Geneva reported results of a study that goes the next step, actually testing for the form of reproductive isolation. His focus was on two parapatric members of the Anolis distichus group in Hispaniola that differ in dewlap color and genetically differentiated (see previous talk in this meeting  by Julienne Ng). By bringing individuals into the laboratory and conducting a massive breeding experiment, he tested whether they would mate and produce offspring and, if so, whether the offspring were viable. This is an enormous undertaking–something like this has never been done on anoles.

After one generation of the two generation experiment, some results are already clear. Members of the  interspecific crosses (based on genetic differentiation, they have been named as different species) will mate–no pre-mating isolation, apparently, despite the different dewlap colors; or at least, not complete isolation. However, the number of inviable eggs is greater in the hybrid crosses. No signs yet of Haldane’s rule of any asymmetric degree of postmating isolation, but more work is yet to come.

Recent years have seen great enthusiasm for the idea that populations experiencing different selective pressures will diverge genetically, perhaps to the point of speciation. Ian Wang examined 17 species of Anolis lizards to determine the extent to which genetic differences between populations were a function of ecological differences in the environments they occupy versus geographic differences. Across all 17 species, geography explained twice as much of the variation as did ecological differences, although patterns varied from one species to another. These results suggest that although adaptation to different environment plays some role in driving genetic differentiation, other factors are equally or more important in most cases.

Previous work by Cox and Calsbeek has shown that ovariectomized lizards grow faster and survive longer than lizards with intact ovaries. Ovariectomized lizards also develop larger fat bodies, and a reasonable explanation is that it is the greater fat that these lizard accumulate that allows them to survive better over the winter. To test this hypothesis, the authors experimentally removed fat bodies from some lizards and not others. They found that this treatment had no effect on survival, thus disproving the hypothesis. In other words, removal of the ovaries both increases fat body buildup and survival, but the two phenomena are not related, a nice demonstration of the importance of experimental manipulation to understand disentangle correlation from causation and elucidate physiological mechanisms.

Anolis punctatus. Photo from http://www.flickr.com/photos/32688820@N02/3121948727/sizes/m/in/photostream/

Anolis punctatus is one of the coolest looking anoles of South America, which is saying a lot. It is widely distributed throughout South American rainforest habitats, but has been relatively little studied. Last night Ivan Prates exhibited a poster reporting the results of a phylogeographic analysis of the species from Amazonian and Atlantic forests. The study is impressive in its scope and sampling, and finds a high degree of genetic divergence throughout the species’ range, paralleling results for another Amazonian species group, A. chrysolepis and relatives. In addition, the Atlantic forest populations are nested within Amazonian populations, suggesting that dispersal occurred from the Amazon to the Atlantic. Molecular calibration puts the date of the dispersal at ca. 3 million years ago, which would correspond with vegetation reconstructions that suggest the forests were connected at that time.

In addition, the study contained samples of the extremely little known horned anole of the Amazon, A. phyllorhinus, which places this species as the close relative of A. punctatus, and hence distantly related to the Ecuadorian horned anole, A. proboscis.

Anolis marmoratus from Guadeloupe. Photo from http://www.karibische-anolis.de/

It was a colorful morning here in Ottawa. First, Julienne Ng reported on her work on the causes and consequences of dewlap color evolution in Anolis distichus in Hispaniola. This species is renowned for the variety of dewlap colors–primarily whites, yellows, and oranges, but also red–displayed by populations throughout the island, and a phylogeographic analysis indicates that different dewlap colors have evolved multiple times. Julienne demonstrated that a correlation exists between environmental variables (e.g., precipitation) and dewlap color and brightness; these variables explained much more of the variation than did geographic distance separating populations or the degree of genetic differentiation. She then asked whether differences in dewlap color serve to reproductively isolate populations. She tested this hypothesis by sampling four transects across areas whether dewlap color changes over a short distance. She found that in one transect, the two populations differing in dewlap color were highly differentiated genetically; in the other three cases, by contrast, the populations were not at all differentiated.  This finding is potentially important, as dewlap color is often used to describe different species; the results indicate that populations with different dewlap colors may not be strongly isolated genetically.

Later in the morning, Chris Schneider reported on studies of the genetic determinants of color in the wildly variable Guadeloupean species, Anolis marmoratus. This species exhibits so much variation that 12 subspecies have been described from Guadeloupe and nearby islands. By illumina sequencing, Schneider has found 250 fixed differences between populations differing in color–one with red heads, the other with blue. Preliminary analysis suggests that at least 60 protein-coding genes are involved. This work is a promising first step in identifying the genes underlying color differences in anoles.