Author: Thomas Sanger Page 2 of 6

Photograph of a male Anolis sagrei from Christian Cox’s website.

Squamates vary widely in the magnitude and direction of body size dimorphism, which refers to the tendency for the sexes to exhibit different body sizes. Some lineages possess male-baised dimorphism while others have female-baised. The effects of testosterone on mediating sexual size dimorphism in different squamate lineages has long been the study of the Cox lab at the University of Virginia.  Christian Cox (of no relation to his advisor) has now reported some exciting steps forward in the search for the mechanisms regulating body size dimorphism in the brown anole, Anolis sagrei. Cox is in the process of carrying out a  transcriptome-wide analysis of the genes responsible for sexual dimorphism, with particular focus on examining the genes along the insulin growth factor-growth hormone axis (IGH-GH), which is the same pathway that was reported about yesterday.  In his experiment Cox implanted testosterone pellets under the skin of juvenile male and female lizards and then looked for differences in size and gene expression. Increased levels of circulating testosterone prompted increases in body size in both males and females grew to larger sizes, indicating that females have not lost the ability the respond to testosterone. But to better understand the growth axis controlling this difference Cox took a large step forward by also comparing gene expression in the liver of experimental (implant) and control (intact) animals. As the liver is a major regulator of growth via its regulation of the IGH-GH, Cox expected that this tissue would respond to testosterone treatment. This is precisely what Cox found. Specifically, he found a number of genes that are naturally regulated in different ways in males and females and additional genes that responded to the testosterone treatments. To conclude, Cox pointed out that an important next step will be to compare castrated lizards to those intact lizards with the testosterone implant to more clearly elucidate the gene network directly responding to testosterone. But perhaps the most exciting work will come with Cox and his collaborators examining the growth mechanisms of species with male-baised and female-baised patterns of dimorphism to more thoroughly understand how evolution has reshaped these gene regulatory networks during squamate evolution.

One of the largest anoles, Anolis equestris (photo by Uwe Bartlett)

From the diminutive twig anole to the monstrous crown-griant anoles, Anolis lizards vary dramatically in their body size. Much research has focused on the patterns of body size variation among Caribbean species, how changes in body size are correlated with habitat differences among species, and rates of body size evolution upon invasion to new islands, yet an important question remains to be addressed in this body of literature, “how do anoles change body size?” S. Griffis and Dr. D. Jennings of Southern Illinois University at Edwardville are attempting to address this among Cuban anoles by searching for DNA sequence differences in known growth factories. But they are using what might be considered an unlikely model for lizard body size variation: dogs. Several years ago, Elaine Ostrander’s lab at the NIH uncovered that coding differences in the growth factors IGF were responsible for the body size variation in dogs. To a mechanist like myself, it was a surprise that this variation could be traced to coding differences in the genes, not to the levels of circulating growth factors. The authors of this poster are following Ostrander’s lead by looking for coding differences in genes involved with the IGF growth axis. But to keep their options open they are also collecting data on circulating hormone levels. When complete, if there are differences in the IGF growth axis contributing to differences in body size, Griffis and Jennings will find it.

Patterns of Convergence in the Body Shape of Squamate Reptiles

SICB 2015 is off and running and what better way to kick it of than with a lizard talk? Phillip Bergmann of Clark University filled the 8:15 time slot on day one with an intriguing evaluation of broad-scale body shape convergence among squamates. This is a perennial topic on Anole Annals due to the well-studied patterns of convergence among Anolis lizards and, indeed, Dr. Bergmann highlighted anoles early in his talk. He asked whether common functional (ecological) situations lead to body shape convergence at large scales. Rather than search for global patterns of convergence, Dr. Bermann used hypotheses specific to the transformations that occur when lineages transitioned into new habitats. As he pointed out, it is not surprising to find convergence in body shape occurring throughout squamate – after all, convergence is ubiquitous across the tree of life. He concluded his talk highlighting what he feels are some of the most pressing “Big Questions” regarding convergence which included the methods we use to detect convergence, the role of constraints in shaping convergence, and elucidating the mechanisms underlying convergence. Ultimately it was a thought-provoking talk both from the perspective of squamate organismal diversity and the topic of convergence more broadly.

I probably would have never said this a few years ago, but penises are absolutely fascinating. The phalluses of terrestrial vertebrates exhibit an incredible diversity of shapes and sizes with some possessing elaborate coils, barbs, bony spines, and multiple lobes. Many of us learn about the rapid evolution of sexual characters in our undergraduate classrooms, but until recently I, for one, did not fully appreciate the striking diversity of this organ until immersing myself in the subject area.

Many biologists study the penis under the umbrellas of different research disciplines, but relatively little work has been performed to explain its anatomical diversity. For example, how many times has a penis/phallus evolved among terrestrial vertebrates? This may seem like a trivial question, but the diversity in form, function, and physiology in the adult phallus actually makes this question difficult to address. Historically there has been much conjecture, but little data to support whether the mammalian penis, squamate hemipenes, and phallus of turtles, crocodilians, and basal birds share a single evolutionary origin or are independently derived. But where comparative anatomy has struggled, comparative developmental biology has recently forged ahead. Within the last several months two independent groups have published a total of seven new research articles that help us resolve the question of phallus homology.

Figure 1 from Gredler et al. 2014 illustrating phallus diversity among amniotes. The darker lines illustrate the sulcus spermaticus which goes on to become internalized in mammals as the urethra.

I previously wrote about a series of five papers published from the Cohn lab (University of Florida) describing the embryology and gene expression patterns for the developing phallus. Since then this group has published a sixth paper synthesizing this wealth of information, using it to lay out a number of outstanding questions regarding phallus development and evolution. More recently, the Tabin lab (Harvard University) published a paper comparing the cellular-level origins of the genitalia in the laboratory mouse, green anole, house snake, chick, and python. I have had the distinct pleasure of working with both groups as their “anole guy.” Although these studies vary widely in their experimental and comparative breadth, together they have shed much needed light on the evolution of vertebrate genitalia. Here my goal is to discuss how this new wave of research changes what we now know, what we don’t know, and what we think we know regarding the evolution of external genitalia among vertebrates. Take a look at the original research papers for details of the developmental analyses, which represent many technical steps forward in our use of anoles as a laboratory model system and intellectual advancements in our understanding of genital development.

During the gradual transition of life onto land, vertebrates evolved the amniotic egg to facilitate their departure from moist environments.

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A recent paper by Matt Helmus, Luke Mahler, and Jonathan Losos highlighted the ways in which globalization has influenced the distribution of Caribbean Anolis lizards. At the heart of this research was the relationship between commercial shipping traffic and lizard biogeography. Two more recent observations can now extend these findings well beyond the Caribbean, much, much farther north.

First, Twitter user Vlen Puppehface recently posted a story about a stow away green anole that he found on pallet in Edmonton, Canada. This was a shipment of oil field supplies that originated in Houston, TX and that took ten days to reach its destination. This male green anole survived the trip all the way to Canada and is now housed in a new terrarium. The tweet originally posted October 16th and since then the anole has shed and appears to have adapted well to its new home.

The Canadien green anole.

Photo by Randi Duun

In separate case of stowaway lizards, another anole survived a transatlantic journey to Denmark in a shipment of bananas and was discovered incapacitated on the floor of the stockroom where incoming bananas are fumigated. The photo is too small for me to be certain, but this appears to be an Anolis cybotes female. According to the original post by Randi Duun in the “Anoles” Facebook group, the shipment originated in Colombia, Costa Rica, or the Dominican Republic so this would be consistent with an A. cybotes hitchhiker. It would be interesting to know how long a shipment like this takes, but I bet that it is longer than ten days port-to-port. Regardless, just like the globetrotting green anole, this anole is healthy following its journey, housed in a terrarium and enjoying Danish mealworms.

In contrast to the research described by Helmus et al, it is probably safe to assume that despite the perseverance of these anoles, and any others that make their way towards the arctic circle in subsequent shipments, escapees will not be establishing viable introduced populations.

Regular readers of Anole Annals may remember the “Find the Anole” series that has been popular over the last few years. It has been a while since we enjoyed such fun times, so I wanted to breathe new life into this classic challenge.

Earlier today I visited Dinosaur World in Plant City, Fl. and enjoyed the contrast between Mesozoic and Cenozoic  reptile diversity. It was very exciting. Below are two images from their grounds for your enjoyment. Can you find and identify the anoles in these photos? A far bigger challenge may be to identify the dinosaurs illustrated by these statues.

Find and identify the anole.

On a separate note, if you are ever passing through central Florida with your families, stop by Dinosaur World. The interpreters were quite good with our kids, there are over 200 life-sized (and colorful) dinosaur statues, they clearly state that the earth is 4.5 billion years old, and there are no humans riding dinosaurs. I was pleasantly surprised by all of this in this part of the country. Its worth a few hours of your time!

Find the anole and identify the species.