We all know that some of our favorite anole species are abundant in urban settings, yet many others are not. Why is this? Do species have to evolve and adapt to city living? Maybe not. In what may be a surprising preliminary analysis, Kristin Winchell over on her blog Adaptability suggests that Caribbean anoles ancestrally had what it takes to live in human settings, and not being able to do so is an evolutionarily derived trait. Sounds crazy at first, right? Until you remember that anoles colonized these islands over water, and so to be successful, had to be flexible and able to cope with whatever life through at them–including, apparently, concrete sidewalks, trashcans, cars, and cats. Check out the details on Kristin’s post.
One of the major experimental advances in recent decades has been the battery of methods capable of functionally validating hypotheses regarding the molecular networks that regulate biological processes. For biologists, these emerging methods allow us to move beyond descriptive and correlational studies to new dimensions where we can experimentally validate our observations. Until recently these technologies were, by and large, reserved for the most well developed laboratory model systems (e.g., mouse, chicken, zebrafish, Drosophila), systems that rarely have direct utility to ecologists and evolutionary biologists. But the topography of biology is changing. These methods are rapidly becoming more easily applied to non-model systems, such as our favorite genus Anolis. In an upcoming paper from the Menke Lab, the tools of functional genomics are applied to anole limb development, taking another step towards making Anolis a truly integrative model system.
In situ hybridization showing expression of of early limb genes in A. sagrei.
Park et al. describe a micromass culture system to explore the molecular regulation of anole limb morphogenesis. In their protocol, Park et al. collect cells from early limb buds of A. sagrei, dissociate the cells from one another, and then add them to a dish as a small (i.e., micro) bolus (i.e., mass) of cells with the appropriate growth media. Even when removed from the embryo, these cells maintain the characteristics of limb cells, developing cartilage after about two weeks and maintaining their molecular signature for at least eight days. This small mass of cells can be grown for up to 30 days and, therefore, provide a useful template for experimental manipulations. More details of this protocol are described in great detail in the paper. Compared to other technologies which require far greater investment, their protocol should be accessible to anyone with access to a tissue-culture laboratory.
Anolis is an emerging model of limb development, but previous studies have focused on describing morphometric patterns of limb growth, not the molecular regulation of limb development. In fact, there have been no studies systematically dissecting the molecular regulation of limb development in any squamate species despite broad interest in this topic in the laboratory mouse and chick systems for 40 years. To study the molecular mechanisms regulating limb morphogenesis, Park et al. forced the expression of the gene Pitx1 – a hindlimb-specific molecule in mouse and chick – in micromass cultures derived from both forelimb and hindlimbs. This experiment verified that one step of the limb regulatory network, the relationship between Pitx1 and Hoxc11, is likely conserved among amniote lineages. While at this time this may have been a proof of principal experiment, this protocol may have future implications for both developmental and evolutionary research in Anolis. For example, multiple transgenes can be readily cloned and incorporated into the micromass cultures. In addition, micromass cultures derived from species with distinct limb morphologies may also open to door to finding pathways that are regulated in novel ways across Anolis lizards.
Binding domains of Pitx1 in the intergenic region of Hoxc11. Note conservation of binding region throughout mammals (shaded arrows), but lack of conservation among amniotes (white arrows).
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.
A while back, we noted that “apparently no one has posted a picture of an anole sitting on an orchid on the internet.” Recently, alert reader Tsjok De Clercq has discovered that this is no longer true. He has pointed us to an image on The Orchid Source that shows a festive anole (A. sagrei) on what appears to be a houseplant. Of greater interest is the post on Ricardo’s Blog, Orchids, Parrots, Fish and People describing a Puerto Rican crested anole found in nature on a red orchid, which seemed to be a complete fail in the remaining cryptic department. Thanks for the tip, Tsjok!
The tweet-o-sphere is full of news that there was some sort of anole question on Jeopardy late last week, and that apparently none of the contestants got the answer. But I can’t find any specific online. Does anyone have the inside skinny?
Although sexual dimorphism is found in many animal species, the mechanisms by which it evolves remains a hot topic. Selection may favor different phenotypes in the two sexes, but sharing a genome may put constraints on if and how sexual dimorphism might evolve. Many anoles have sexual dimorphism, of course, but the degree to which they are dimorphic varies quite dramatically. Robert Cox studied how between-sex genetic correlations in Anolis sagrei, a very dimorphic species, might degrade over ontogeny to result in divergent male and female phenotypes.
Anolis sagrei displays marked sexual dimorphism. (photo from Bob Cox’s website)
Using a large breeding colony of brown anoles from the Bahamas, Cox found that between-sex genetic correlations were lowest for traits that are the most dimorphic, like body size. Even more interestingly, the correlations change as the individuals get older. Whereas juvenile anoles have high between-sex genetic correlations for most traits, those correlations decrease around sexual maturation, most strongly in those traits that are dimorphic. This suggests that the pronounced divergence in phenotype seen in adults is associated with a degradation of the between-sex genetic correlations for those traits. Cox is currently exploring what mechanisms lead to this degradation, and is especially interested in whether testosterone is a major player.
Tropical ectotherms such as anoles are considered to be especially vulnerable to climate change. Given that tropical lizards already function near their upper tolerances, even a modest increase in ambient temperature can have disproportionately large negative fitness consequences. Most models that predict how climate warming will impact tropical ectotherms rely on ambient temperature. Michael Logan, a graduate student at Dartmouth College, presented a study suggesting that temperature alone is insufficient to predict the impacts of environmental warming on organismal fitness. He points out that other abiotic factors, such as humidity and wind speed, may be equally important in determining whether and how organisms will be impacted by climate warming.
For this study, Michael explored how daily variation in temperature, humidity, and wind speed interact to determine the abundance of two species of anole, Anolis allisoni and A. lemurinus, from the Bay Islands of Honduras (see map above). He deployed sensors that recorded temperature, humidity, and wind speed in a forest site, where A. lemurinus is found, and an open-habitat site, where A. allisoni is found.
Contrary to expectations, he found that environmental temperature alone is a poor predictor of lizard abundance in the open habitat. Rather, wind speed constrained lizard activity in the open habitat more than any other environmental factor. Further, environmental temperature predicted lizard abundance only when wind speed was low. Michael posits that there might be a trade-off between thermoregulation and evaporative water loss on windy days, such that the ability to achieve high body temperatures through basking may be counterbalanced by the ability to maintain water balance. Michael found that in the closed forest habitat, the variance in environmental temperature and the degree to which the temperature varied from the lizard’s optimal range were important predictors of A. lemurinus abundance. These results suggest that this species might thermoregulate more than was previously thought, as forest anoles are generally considered to be thermoconformers.
Together, Michael’s results suggest that factors besides temperature are important determinants of lizard abundance, and that they should be more explicitly considered in predictive models for the biological impacts of climate warming.
Both images from ganeshdhane’s flickr page: http://www.flickr.com/photos/ganeshdhane/
A non-anole regular on Anole Annals (e.g., 1, 2, 3) made an appearance at SICB this year. Not the species itself, but a fascinating presentation by Ambika Kamath on population variation in dewlap dimorphism in Sitana ponticeriana. Kamath presented information on display behavior for three color variants of Sitana: uncolored, colored, and intermediate. She wondered whether the three geographically separated variants display differently and whether the dewlap variation might be due to environment or sexual selection.
Coloured-fanned, intermediate-fanned, and white-fanned male Sitana ponticeriana. Photographs by Shrikant Ranade, Jahnavi Pai, and Jitendra Katre respectively.
By studying eight populations of this species, Kamath found that the three variants did indeed display differently. The colored variants had long displays with remarkable head turns and twists (wow, there was some amazing video!). The uncolored variants had body position changes, but no head turns and twists. Finally, the intermediate variants simply had short displays with no head turns or body position changes. Multivariate analysis of behavior clearly separated the populations based on color variant. Also, they flick that throatfan VERY quickly!
Based on the available data, it seems unlikely that environmental variation in habitat type or vegetation explains the variants, but sexual selection does appear possible. Colored dewlaps are associated with male-biased sexual dimorphism, whereas the uncolored variants have no dimorphism or female-larger dimorphism. Further, scaling of dewlap area to body size revealed that the colored and intermediate variants have evolved large dewlaps in different ways. This also supports Kamath’s proposal that there are multiple origins of large dewlaps and colorful dewlaps within the distribution of this widespread species. Future research will no doubt be of interest to us at Anole Annals and beyond!
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.
Though we now understand that post-copulatory sexual selection (such as sperm competition and cryptic female choice) can be as important in determining variance in reproductive success as pre-copulatory sexual selection, and though we recognize that the expression of traits subject to pre-copulatory sexual selection is often condition-dependent, it turns out that we know almost nothing about the condition-dependence of traits under post-copulatory sexual selection.
Sperm of Anolis sagrei. Picture by Ariel Kahrl.
In a session devoted to post-copulatory sexual selection, University of Virginia graduate student Ariel Kahrl described her research on the condition-dependence of sperm characteristics in Anolis sagrei. By feeding size-matched male lizards differentially for a period of four months, Kahrl not only generated differences in the body condition of these males, but also ensured that their sperm had developed under her imposed dietary regime. Kahrl predicted that male body condition would affect sperm morphology and sperm count. Pairs of males reared under different dietary conditions were also mated to a single female (making sure to control for mating order by using a reciprocal mating design), thus putting the sperm of two males with different body conditions into direct competition. Kahrl predicted that the fertilization success of males would depend on sperm morphology and count.
Not surprisingly, males with higher body condition had higher fertilization success. It turns out that variation in fertilization success may be influenced by a tradeoff between sperm mid-piece size and sperm number. This situation is interesting, because one can reasonably predict that males on either end of the tradeoff could have high reproductive success—having many sperm per ejaculate could increase the odds of fertilization, akin to purchasing multiple lottery tickets, but having sperm with larger mid-pieces, and thus potentially more mitochondria, perhaps might provide sperm with the burst of energy necessary to win the fertilization race.
In fact, Kahrl found that males with high sperm counts but small midpieces achieve high reproductive success. Intriguingly, she also found that high-condition males had sperm with less variable morphology than low-condition males, and hypothesizes that the dimensions of these uniform sperm match the dimensions of the tubules in females where sperm is stored. Kahrl’s results link pre-copulatory to post-copulatory sexual selection through condition-dependence, and represent a sizeable piece in the puzzle of how sexual selection works in Anolis lizards.