Anole Annals

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.