Author: Jonathan Losos Page 84 of 130

Sondra Vega, a graduate student at the University of Puerto Rico, writes:

A green anole eating a piece of fruit that fell from a bird feeder. Photo from http://4.bp.blogspot.com/_I_xaKqQzxyY/R4g96myXR_I/

In lizards, omnivory/frugivory is considered unusual and particular of some species; even though dietary studies indicate that many species add plant products to their diet.  In spite of the fact that seeds and fruits have been reported in stomach content or fecal pellets of Anolis, their importance and contribution to the diet of these lizards is still unknown. At present, the general consensus is to categorize Anolis as strict insectivores.  Therefore the extent by which omnivory/frugivory plays a role in the Anolis diet still needs to be assessed.

My research aims to determine how the variations in food abundance in two forests at the northern limestone region of Puerto Rico affect the degree of omnivory/frugivory and trophic position of Anolis lizards. I am using stable isotope technology to analyze the omnivory/frugivory and trophic position of the Anolis lizard as well to quantify the importance of fruits to the diet of the omnivorous Anolis species.  Stable isotopes are a novel technique that has the potential to elucidate diets, capture interactions such as trophic omnivory, and track energy or mass flow through ecological communities.   The information will help to better comprehend the functional role of Anolis lizards in the dynamic and structure of food webs and in ecosystem function, as well as the dynamics of vegetation in tropical forests.  Although this project is focused on anole lizard species of Puerto Rico, the findings are of relevance for understanding of islands where lizards are also a dominant component of ecosystems.

Photos by Gabe Gartner and Kurt Schwenk.

This is a little far afield for anole aficionados, but recent years have seen a revolution in our picture of lizard (including snake) phylogeny. Traditionally, based on morphological analysis, lizards were thought to split into two groups, the iguanians (including anoles, other iguanids, agamids, and chameleons) and scleroglossans (everything else, including snakes). However, starting with a paper by Townsend et al. in 2004, a different picture emerged in which iguanians were nested high in lizard phylogeny, closely related to anguimorphs (such as alligator lizards, gila monsters, and monitors) and snakes. A series of subsequent studies came to essentially the same conclusion, most recently the output of the “Deep Scaly” NSF Tree of Life project which sequenced DNA from 44 genes.

Two views of lizard phylogeny. From Losos et al. (2012)

I think that most of the field had come to accept that the molecular tree was correct. But along comes a paper by the morphology team of Deep Scaly, a remarkable analysis in which 194 species were all micro-CT scanned and examined in others ways, leading to a data set of more than 600 morphological characters, 247 never previously used in phylogenetic studies. Analyzed with state-of-the-art methods, the results resoundingly support the original morphological tree and give absolutely no morphological support for the new molecular tree. The authors do an excellent job in not being strident in insisting that the morphological tree is correct, but just highlighting how very unusual morphological evolution must have been if the molecular tree is correct. Moreover, the authors note that based on analyses including the molecular data, the “Archaeopteryx” of squamates, Huehuecuetzpalli mixtecus, is placed high in the phylogeny, rather than in the basal position where morphology has long placed it. If, indeed, the molecules are right, what does that say about our ability to ever reliably place fossil species in a phylogeny?

Either the morphological or the molecular tree is incorrect, and either molecular or morphological data have been evolving in a way for which there is no good explanation. This is truly a conundrum, which was the point of a perspective piece just published by David Hillis, Harry Greene, and me. We don’t have any answers, but thought it was an interesting enough question worthy of further attention.

Earlier this year, we mentioned a paper by Juli Wade reviewing research on the green anole, which has become a model organism for integrative studies of reproductive behavior in vertebrates. One example of such research is a paper recently published in her laboratory by Cohen and Wade in which levels of testosterone were experimentally manipulated to see the effect of this hormone on gene expression in different regions of the brain. The abstract gives the details better than I could:

“Aromatase and 5alpha-reductase (5-alpha-R) catalyze the synthesis of testosterone (T) metabolites: estradiol and 5-alpha-dihydrotestosterone, respectively. These enzymes are important in controlling sexual behaviors in male and female vertebrates. To investigate factors contributing to their regulation in reptiles, male and female green anole lizards were gonadectomized during the breeding and non-breeding seasons and treated with a T-filled or blank capsule. In situ hybridization was used to examine main effects of and interactions among sex, season, and T on expression of aromatase and one isozyme of 5-alpha-R (5-alpha-R2) in three brain regions that control reproductive behaviors: the preoptic area, ventromedial nucleus of the amygdala and ventromedial hypothalamus (VMH). Patterns of mRNA generally paralleled previous evaluations of intact animals. Although no main effects of T were detected, interactions were present in the VMH. Specifically, the density of 5-alpha-R2 expressing cells was greater in T-treated than control females in this region, regardless of season. Among breeding males, blank-treated males had a denser population of 5-alpha-R2 positive cells than T-treated males. Overall, T appears to have less of a role in the regulation of these enzymes than in other vertebrate groups, which is consistent with the primary role of T (rather than its metabolites) in regulation of reproductive behaviors in lizards. However, further investigation of protein and enzyme activity levels are needed before specific conclusions can be drawn.”