Author: Katie Boronow Page 1 of 2

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.

Among Anolis lizards, sexual opportunities are typically monopolized by males and female mate choice is low. One way for female anoles to gain back some control in the mating process is through their specialized sperm storage system and selective fertilization. In her talk titled “Females bite back: Sexual conflict and the evolution of venom proteins in the reproductive tract of female anole lizards,” M. Catherine Duryea described her investigation into the genetics of sperm storage in anoles.

First, Duryea asked which genes are expressed in the female reproductive tract after copulation. Duryea extracted tissue from recently mated and virgin female A. carolinensis and generated cDNA libraries. From these libraries, Duryea found that over 160,000 genes were expressed in the reproductive tract, and that 5,153 of these genes were expressed differently in mated versus virgin females. Using a gene ontology analysis, which groups genes by function, Duryea found that many of the genes that showed increased expression in mated females were related to catalytic activity, protein binding, and nucleotide binding. The Anolis genetic response to mating is similar to that reported in Drosophila, suggesting that similar processes may be occurring across distantly related lineages.

Enzymes expressed after mating in anoles may be related to enzymes in snake venom (Image: Kendall McMinimy/Getty)

Next, Duryea looked for evidence of selection in a subset of the genes identified in the previous experiment. Specifically, she focused on the serine proteases, which are known to be important in sperm storage in Drosophila. Using a BLAST search, Duryea found eight serine protease genes in her A. carolinensis data. She then sequenced the orthologous genes in A. sagrei and compared the sequences to those of A. carolinensis. One serine protease gene showed evidence of positive selection, indicated by a large number of synonymous changes shared between species. This gene displayed striking similarity to a snake venom gene. Snake venom genes have a deep origin in squamates, including in non-venomous lineages; thus, Anolis reproductive serine protease may be derived from a venom serine protease. Compared to Drosophila, in which reproductive serine proteases are derived from digestive enzymes, this would represent a novel origin of reproductive serine proteases.

While these fascinating results are an important first step towards understanding the genetic basis of sperm storage in anoles, much work remains to uncover the exact function of serine protease expression in post-copulatory processes.

Anole Annals contributor Martha Muñoz of Harvard University won the second annual Raymond B. Huey Award for her presentation discussing the role of behavior in the evolution of Anolis cybotes. The Huey Award, sponsored by the Division of Ecology and Evolution of the Society of Integrative and Comparative Biology, is given for the Best Student Presentation in the division.

Behavior is thought to play two contrasting roles during evolutionary diversification. First, behavior can expose individuals to novel environments, thereby driving physiological and morphological change. Second, behavior can be used to compensate for environmental differences, thereby impeding organismal change. In her talk, Martha described how she tested these two contradictory hypotheses in a clade of trunk-ground anoles that span a wide environmental range.

The Anolis cybotes species complex occurs in Hispaniola from sea level up to 2,500 meters in elevation. By comparing two populations of a lowland generalist, A. c. cybotes, to two independently derived high-altitude specialists, A. c. armouri and A. c. shrevei, Martha was able to detect signatures of adaptation to high elevation. First, Martha asked whether physiological evolution had occurred. She found that body temperatures in the field were not significantly different at high and low elevations, despite the fact that lizards experience air temperatures 15 degrees cooler, on average, at high elevation. In addition, there were no significant differences in preferred body temperature (measured in the lab) among the four populations, and in each case the preferred body temperature matched the field body temperature. These results clearly support a lack of change in the thermal physiology of these lizards despite occupying very different thermal environments.

Martha then tested whether behavioral inhibition was the cause of the observed stasis in thermal physiology. By recording the perch sites of lizards in the field, Martha found that low-elevation lizards perch primarily on trees while high-elevation lizards have shifted to perching primarily on rocks. To quantify how this perch shift affects a lizard’s thermal environment, Martha deployed a series of copper lizard models at each site. The copper models closely mimic the thermal properties of a live lizard, so the temperatures recorded by the models are essentially those experienced by a non-thermoregulating lizard (i.e., the operative temperature). By placing the copper models on both rocks and trees at each site, she was able to assess the thermal properties of each perch type. Martha found that at low elevation, models on both trees and rocks achieved temperatures in the lizards’ preferred temperature range, and sometimes models on rocks got dangerously hot. At high elevation, however, only models placed on rocks achieved temperatures in the preferred range, while models on trees remained too cool. These results support the hypothesis that behavioral inhibition (perch switching) is preventing evolution in thermal physiology.

In a final twist, Martha asked whether evolutionary stasis is also observed in morphology. Morphology is known to correlate with microhabitat in Anolis lizards and is rapidly evolvable, and so stasis would be a surprising result. Martha found the high-elevation populations have significantly flatter and wider heads, a common feature of rock-dwelling lizards, compared to low-elevation populations. She found no differences in limb length or lamellae number. Martha hypothesized that for head morphology, perch switching was a form of behavioral drive that promoted evolutionary change.

Martha concluded by emphasizing that niches are multidimensional, and, therefore, evolution can occur along multiple niche axes simultaneously. By examining adaptation to both the thermal niche (body temperature) and structural niche (morphology) in this study, she revealed that behavioral drive and behavioral inhibition—previously thought to be incompatible—can in fact occur simultaneously in the same organism.

Congratulations, Martha, on your award-winning talk!