Anole Annals

Hanna Wegener, a student with Jason Kolbe at the University of Rhode Island (and an Anole Annals contributor), presented a poster at JMIH on her efforts to identify the factors that drive morphological differentiation among Anolis sagrei populations found on 16 Bahamian islands near Staniel Cay. Hanna investigated morphometric, ecological, genetic, and demographic variation among these populations and, unlike many previous studies, considered variation in both males and females. Although Hanna did find significant morphometric variation among islands and between sexes, she did not find the significant correlation between morphometric variation and habitat use reported in prior work. She also did not find a significant relationship between morphometric and genetic variation.  She did, however, find that population density influences morphometric variation, with lizards living at higher population densities having significantly longer heads than those found on lower density islands. Because these lizards on densely populated islands are also more likely to exhibit evidence of injury from other anoles (e.g., loss of limbs, digits, or claws), it is possible that their longer heads may indicate a response to intra-specific competitive interactions. However,  interpretation of these results remains complicated because there is not a direct connection between injury and intra-specific competition, and the lizards on densely populated islands had longer heads, but not the wider heads that would have been expected if the goal of their morphometric shift was to increase bite force. Hanna undoubtedly has many more exciting questions to investigate with her ongoing research.

In a poster at JMIH 2014, Jonathan Clinger of Austin Peay State University found that spermiogenesis (the final step of spermatogenesis during which spermatids develop into mature spermatozoa) in Anolis sagrei is fairly similar to that previously reported in A. carolinensis.

Yesterday at JMIH, Phillip Pearson reported results from work conducted with his thesis adviser at the University of Alabama, Birmingham Daniel Warner. Pearson investigated the impact of incubation environment on the  brown anole (Anolis sagrei), and the effects of incubation in shaded versus open habitat and early versus late season in particular. Pearson reported several significant differences between the eggs (and resulting hatchlings) incubated under these two conditions. He specifically reported longer incubation intervals under early season and shaded conditions, smaller hatchling size under shaded conditions and better performance of hatchlings at 1 and 3 weeks for the eggs incubated under the late season regime. Performance of hatchlings was quantified as their speed and the number of times they stopped during a performance trial. This work is the latest in a string of interesting studies from the Warner Lab on the impact of incubation conditions on anoles. I was going to provide links to previous posts on Anole Annals about the Warner Lab‘s work, but there are so many that I’ll just suggest that you type “Warner” into the search box at the top of the page and enjoy for yourself.

I caught my first anole talk at this year’s Joint Meeting of Ichthyologists and Herpetologists in Chattanooga, Tennessee. James Stroud presented the results of work with Ken Feeley on modeling the niche of the brown anole (Anolis sagrei). Using data acquired from GBIF, Stroud showed that the environmental conditions experienced by brown anoles in their introduced range are outside of the environmental conditions experienced by brown anoles on Cuba. Stroud discussed how these data from the invasive range of the brown anole might be used to develop a more accurate model of this species’ fundamental niche. This is a work in progress.

There is considerable variation in phallus morphology among the major groups of amniotes (phallus used herein to be inclusive of both the penis and clitoris). Just for starters, while most clades – including mammals, birds, turtles, and crocodilians – have a single midline phallus, squamates have paired hemiphalluses. Although herpetologists have long appreciated morphological variation in the hemipenis for its systematic value, understanding the nuances of anatomical homology, homoplasy, and novelty at this larger scale has not been as widely addressed. Recently, the Cohn lab of the University of Florida (of which I am now a member) undertook this challenge from a developmental perspective, studying development of external genitalia in representatives of each reptilian clade: the ball python (Python regius), the pond slider (Trachemys scripta), three duck species, the American alligator (Alligator mississippiensis), and who else, but the green anole (Anolis carolinensis). A synthetic review of the complete series will have to wait for another post, but reprints of each paper are available on the lab’s website to hold over the most curious. But because of the growing interest in anole nether regions, I will briefly highlight the recent findings regarding hemiphallus development in the green anole.

Fig. 2 of Gredler et al. illustrating the development of paired genital and cloacal swellings.

The Wade lab has previously shown that both male and female green anoles develop similar hemiphalluses during the early stages of genital morphogenesis, which then later differentiate into sex-specific reproductive structures. Building upon this observation, Gredler et al. described the embryology of the green anole hemiphallus from the earliest stages of morphogenesis through sexual differentiation. Hemiphallus development begins around the time of oviposition when three sets of paired swellings appear between the cloaca and the developing hindlimb bud, reminiscent of what is observed in other amniote clades. These swellings expand and meet at the midline to form the external lips of the cloaca or remain lateral to the cloaca and mature into the hemiphalluses. Following morphogenesis, the male hemipenis continues to elongate as it forms its distinctive lobes and sulcus spermaticus while the female hemiclitores gradually regress into the cloaca. Further details of the developmental anatomy of internal reproductive structures and gene expression patterns of several key molecules associated with genital morphogenesis are described in the paper.

Fig. 4 of Gredler et al. illustrating sexual differentiation of the hemiphalluses. Red arrow highlights the formation of the sulcus spermaticus.

Although there is some variation among squamates in the relative timing of the emergence and fusion of the paired swellings associated with hemiphallus development, these results are largely consistent with classical embryological descriptions of squamate genitalia (summarized by Raynaud and Pieu in Biology of the Reptila volume 15). But the revival of this body of literature in a comparative and molecular context brings new research questions to our collective table. As discussed by Gredler et al., the seemingly modular relationship between the genital swellings, cloaca, and limb buds may be particularly interesting in the context of repeated body elongation and limb loss among squamates. Better understanding of the relationship between cloacal and phallus development may also shed new light on the mechanisms of reproductive isolation, the coevolution of male and female reproductive organs, and evolving patterns of sexual conflict. Furthermore, there remain open many mechanistic questions regarding the molecular patterning of the hemiphalluses and which processes are hormone dependent that can now be more thoroughly addressed using the newly available sex-specific molecular markers. Considering the growing literature on hemipenis variation and expanding access to genomic resources in Anolis, these may be particularly fruitful areas for future investigation.