Category: New Research Page 31 of 66

When it rains, it pours. Research on the immense diversity in anole chromosomes was rampant in the 1970’s and early 1980’s, and then…nothing. Until, that is, the last two months. Not one, but two, papers appeared in Evolution, and now AA has learned of a paper on chromosomal variation in Norops clade anoles, recently published in Zoological Studies (click for a downloadable pdf). The paper, by Castiglia et al., examines karyotypes in Norops anoles and argues that karyological variation is in some cases consistent with our understanding of phylogenetic relationships within the group.

Abstract
Background: Neotropical lizards, genus Anolis (Polychrotidae), with nearly 380 species, are members of one of the most diversified genera among amniotes. Herein, we present an overview of chromosomal evolution in ‘beta’ Anolis (Norops group) as a baseline for future studies of the karyotypic evolution of anoles. We evaluated all available information concerning karyotypes of Norops, including original data on a recently described species, Anolis unilobatus. We used the phylogeny of Norops based on DNA sequence data to infer the main pattern of chromosomal evolution by means of an ancestral state analysis (ASR).

Results: We identified 11 different karyotypes, of which 9 in the species had so far been used in molecular studies. The ASR indicated that a change in the number of microchromosomes was the first evolutionary step, followed by an increase in chromosome numbers, likely due to centric fissions of macrochromosomes. The ASR also showed that in nine species, heteromorphic sex chromosomes most probably originated from six independent events.

Conclusions: We observed an overall good correspondence of some characteristics of karyotypes and species relationships. Moreover, the clade seems prone to sex chromosome diversification, and the origins of five of these heteromorphic sex chromosome variants seem to be recent as they appear at the tip nodes in the ancestral character reconstruction. Karyotypic diversification in Norops provides an opportunity to test the chromosomal speciation models and is expected to be useful in studying relationships among anole species and in identifying cryptic taxa.

Photo from http://www.frogtown.org/

For more than three decades, since the seminal work of Ray Huey, Al Bennett, and Steve Arnold, biologists have measured whole animal performance–how fast they run, how far they jump, how well they can swim–to understand how species are adapted to their environment.  Work on anoles has been a prime example of how we can study differences among individuals and species to understand how natural selection works and why species living in different environments possess different morphologies (several AA posts have discussed this sort of work [e.g., 1, 2, 3]).

But a critical assumption of all of this research is that we can get animals to perform maximally. Otherwise, it’s tough to study what causes variation in maximal capabilities if animals aren’t performing maximally. The catch is: how do you tell if an animal is going all out? Sure, it’s easy to weed out the slackers, but distinguishing a lizard giving it his all from one going at, say, 90% of max…hard to tell.

In an important and entertaining paper, Henry Astley and colleagues provide some sobering information. The short story goes as follows, and you really should watch the video below for more details and some great images: biomechanicians have studied frog jumping for decades to understand how muscles work. Bullfrogs are known not to jump very well. The maximum jump ever recorded in the lab was only 1.3 m, whereas the much smaller Cuban treefrog can bound 1.7 m. The proffered explanation was that bullfrogs live on land and in the water, and so their morphology must be a compromise.

But…the Guinness Book of World Records claims that a bullfrog–Rosie the Ribeter, to be exact–once jumped 2.18 meters at the Calaveras County Fair. That’s  68% farther than any scientist had ever recorded in the lab. Sounds like a bunch of hooey, right? Well, just to debunk this nonsense, a bunch of Brown University biologists headed to sunny California to visit the County Fair, eat some cotton candy, and check out the frogs. And, lo and behold, it’s true–bullfrogs there regularly far exceed the lab record.

The story’s a lot more complicated–it turns out that there are “pro” frog jumpers–and I won’t go into the details; the paper is well worth a read, very entertaining and sobering for lab performance types (abstract here). But the short story is this: it seems that lab studies had massively underestimated how far bullfrogs can jump, calling into question many of the conclusions that had been reached about their physiology. Moreover, records for the maximum jump distance at the fair showed a steady increase for the first 50 years before levelling off for the last 30. This suggests that the people who jump the frogs (and some families have been doing this for generations) have only gradually learned exactly what conditions and behaviors maximally stimulate the frogs. And this suggests that lab scientists, who just guess at what may work best and tinker a little bit, may not have much of a chance of hitting on the right stimuli.

There’s been lots of great press coverage, too–just google “calaveras frog astley” or something like that. But, first, watch the video and go read the paper (I can email you a copy if you can’t access it online).

httpv://www.youtube.com/watch?v=QKFpvoez7_M

Everyone’s favorite beach anole, A. onca. Photo by J. Losos

Anolis onca, the only padless anole, occurs in sandy habitats in Venezuela. Little is known about the evolutionary history of this quite distinctive species (we had a discussion of its natural history last year [1,2]).

Now a recent paper appears in the journal Saber  in which a team of Venezuelan scientists led by Alejandra Tejada used starch gel electrophoresis methods to measure the degree of genetic differentiation among populations. The paper can be downloaded, albeit a bit slowly, and is in Spanish, but here’s the English summary:

Anolis onca is a lizard species located in the Araya peninsula, in northern Venezuela. Populations of this species may have been isolated in the late Cretaceous and later recombined during the Quaternary through a new isthmus by sedimentary processes. To test this assumption, in five populations of A. onca, starch gel electrophoresis was used to estimate genetic variability within populations, interpopulation differentiation (FST), and gene flow (Nem). Additionally, under the premise of genetic differentiation between subpopulations under the isolation by distance (IBD) model, we conducted a phylogenetic analysis for five subpopulations of this lizard. Increases of genetic distance values (D) between subpopulations arranged consecutively between the Chacopata and Guayacán locations and a clear structuration as estimated by the FST parameter, evidence isolation by distance as indicated by the IBD model. However, Nem values did not conform to this model, suggesting that the subpopulations, although actually connected, may have been shaped by independent evolutionary processes. The two clades resulting from the phylogenetic analysis do not group populations closer geographically since clade B (Chacopata+Istmo Sur) lies in areas geologically ancient whereas clade A [(Istmo Centro+Istmo Norte)+Guayacán)] occupies areas of recent sedimentary origin. It is thus reasonable to infer that other factors besides the geographical distance between subpopulations may have also conditioned the structure found.

Anolis sagrei has impressive sexual size dimorphism, but what causes it? (Photo by Bob Reed)

Sexual dimorphism is always a hot topic at SICB, and this year it was no exception for anoles (1, 2). Christian Cox, a postdoc in the laboratory of Bob Cox (no relation) at the University of Virginia, sought to explain how testosterone might lead to phenotypic divergence in a number of sexually dimorphic traits. As many of us are aware, sexual dimorphism varies widely among lizard species, and evolutionary shifts to and away from dimorphism are common, including in anoles. Testosterone has been shown to be an important regulator of growth in several lizard species, so Cox experimentally tested this effect in Anolis sagrei.

Both males and females were given a testosterone or blank implant and allowed to grow to maturation. One group was manipulated as juveniles, just as phenotypic divergence was beginning, and the other group was manipulated as subadults after divergence. Testosterone addition increased growth in body size and mass, increased metabolic rate, increased dewlap size, and changed dewlap coloration in both sexes and both juveniles and subadults. Fat storage was reduced as expected, in both sexes and age classes. These results are intriguing, because a sex difference in testosterone production may play a role in the degradation of between-sex genetic correlations. The next question is how that happens, as both sexes produce testosterone, just to different extents.

Numerous variables can affect an organism’s survival, including its age and sex, the demographics of the population in which it resides, and environmental conditions like climate, and habitat. However, the relative importance of these factors is poorly understood. Dan Warner described his investigation into factors affecting natural selection in wild populations of anoles in his talk titled, “Spatial and temporal variation in phenotypic selection in the lizard Anolis sagrei.”

Warner measured directional selection on A. sagrei on six islands in the Matanzas National Estuarine Reserve in Florida. These islands were intentionally founded with populations having unequal adult sex ratios. Half of the islands were founded with more males than females (male-biased), and the other islands received more females than males (female-biased). This manipulation was done to strengthen the effects of male-male competition on the male-biased islands. Warner measured survival selection on adult and juvenile body size by marking and recapturing individuals over the last three years.

Warner found a lot of variation in the strength of directional selection on adult and juvenile body size both across islands and within each island in different years. However, there was no relationship between the strength of selection on each island and either habitat structure (represented by canopy openness) or island size. Thus, the probability of survival at a particular body size does not seem to depend on environment.

However, population demographics did seem to affect survival at different body sizes. There was a negative correlation between the strength of selection on body size and the density of adult lizards, indicating that smaller body sizes are favored at high population densities (and vice versa). This trend was observed in both adults and juveniles, but was more pronounced in juveniles. Warner hypothesized that it was the density of adult males in particular, rather than the total density of adults, that was driving the observed trend. To test this idea, he tested for a correlation between the strength of selection on juvenile body size and the adult sex ratio. He found a negative correlation, indicating that large juveniles are favored in more female-biased populations while small juveniles do better in male-biased populations. One possible explanation is that on islands with male-biased sex ratios, large juveniles are more likely to come into contact with territorial adult males, are more likely to be perceived by these males as a possible competitor, and are therefore more likely to be harassed by these males. The presence of adult males might even reduce recruitment, as evidenced by slower population growth rates on male-biased versus female-biased islands.

These results suggest that patterns of natural selection on individuals can depend on characteristics of the population. Only with long-term field studies such as this one can we begin to unravel the many factors affecting selection in wild populations.