Anole Annals

Zachary Morris presents his research entitled “The role of craniofacial growth zones in shaping crocodylian snouts.”

Evolution of the crocodylian skull is driven by developmental changes. While embryos share many similarities, at some point within development they diverge into unique ecomorphs. Prior studies in American alligators (Alligator mississippiensis) showed that the snout is a source for early facial proliferation, wherein later stages lack growth plates that resemble post-hatching anole lizards. Snouts of crocodilians are described as moderate, blunt, and slender. Snout structure is related to dietary and ecological differences; for example, long, slender snouted crocodiles such as the Tomistoma (Tomistoma schlegelii) feed largely on fish. Heterochrony of slender-snouted crocodiles is responsible for continued elongation of the embryonic snout.

Zachary Morris of Harvard University following his great talk at SICB 2019.

To further understand forces behind these developmental changes, Zachary Morris, a current PhD student of Dr. Stephanie Pierce at Harvard University, began by asking the questions “When does this difference (in skulls) become apparent?” and “What are the cellular dynamics of snout elongation?” He presented his work on Saturday at the 2019 SICB conference and shared his experimental approach to answering such questions. To do this, he traveled to Imperial College London to work with Dr. Arkhat Abzhanov.

In answering his initial question, Morris incubated A. mississippiensis, dwarf crocodile (Osteolaemus tetraspis), and T. schlegelii embryos to developmental stages 14 and 17. These time-points were selected based on Ferguson staging. He could then examine snout/head length ratios to determine when skull differences, such as elongation, became apparent. At stage 14, no differences among blunt, moderate, or slender-snouted crocodilians were visible. However, at stage 17, he found that slender elongation began. To investigate his latter question, he followed the same procedure, but was able to calculate cell proliferation rate for tissue regions utilizing injection of EdU in ovo. He found that while elongation patterns such as facial shape were apparent during stage 17, early cell proliferation rate at the same stage was not apparent. His findings suggest that blunt species types (such as O. tetraspis) have decreased cell proliferation along the tip of the snout in comparison to slender species types (such as T. schlegelii).

In the future, Morris hopes to investigate whether cell proliferation at the tip of the snout is maintained in Tomistoma and if there is greater proliferation at lateral edges of facial structures in broader-snouted crocodilians. While these modern-day dinosaurs derived species-specific morphological differences from actual dinosaurs, the evolutionary processes by which these occurred remains the target of Zachary Morris’ interesting and exciting research.

We tend to think of exercise as a human activity–training our bodies in specific way to accomplish tasks, maintain strength and endurance, and live a healthier lifestyle. Wild animals exercise just as often if not more than humans, but the benefits to animal exercise have been somewhat contentious in biology. The responses to organismal exercise are very conserved with respect to evolutionary change and speciation. Those responses include an increase in performance capacity (such as speed or endurance), but often times trades-off with some other physiological trait, such as immunocompetence. With this trade-off looming, one outstanding question remains: does the exercise response enhance reproductive success and survival? Can we call this response “adaptive”?

To test this idea, Jerry Husak from the University of St. Thomas and his co-author, Simon Lailvaux at the University of New Orleans, measured the exercise response in 90 green anoles (Anolis carolinensis). They had 30 control lizards, 30 lizards trained with speed trials, and 30 lizards trained with endurance trials. They found that overall, exercise enhanced performance for all lizards relative to the controls, but training decreased the ability of a lizard to fight off an infection. They found that lizards that had been trained did not exhibit increases in movement rates, and also found that over time, trained lizards exhibited decreased survival relative to the controls! Is exercise actually bad for lizards?! Green anoles were twice as likely to survive when they were not trained, and differences in lizard body condition might be intimately linked with their probability of survival. Husak and Lailvaux are going to continue to test the idea that the exercise response confers some benefit to lizards outside of performance. Stay tuned!

Members of the same species share a common genome, the same set of genes and regulatory networks that build the proteins responsible for phenotypic diversity. However, that means that both of the sexes share a common genome, and that is an issue when males of a species are sensitive to their own environmental stimuli, and when females are required to invest a lot of their energy towards reproduction and rearing offspring. Species are often sexually dimorphic as well, where either the male or the female will be larger than their inter-sex counterpart, and this has to do with what ecological tasks each sex is responsible for. Do you have to defend a territory? You better be big! Do you want to create a lot of high-quality offspring? You better not invest that much energy into your own growth! It’s an interesting question in biology today: what happens genetically when a sexually-dimorphic species develops to adulthood?

To answer this question Albert Chung, a graduate student at Georgia Southern University, and his colleagues Robert Cox and Christian Cox designed an experiment to quantify changes in gene regulation in brown anoles throughout ontogeny and quantify how changes in gene regulation produce sexual dimorphism. They quantified gene expression at four different time points until adulthood across three different tissue types (brain, liver, and muscle) using RNA sequencing (RNAseq). Chung and colleagues found that sex-biased gene expression exhibits age specificity, with different age classes exhibiting different patterns of sexual dimorphism in gene expression. They also found that the number of sex-biased genes increases throughout development, important for a species to be able to develop both a larger sex and a smaller sex. In addition, sex-biased gene expression also varies among the different tissue types, with the liver exhibiting an increase in sex-biased genes throughout development, potentially to increase growth in male brown anoles compared to females! Chung spoke in the Raymond Huey Best Student Paper Award session for the Division of Ecology and Evolution and delivered a fantastic presentation. We look forward to learning more about the development of sexual dimorphism (especially in anoles!) from Albert and his co-authors.

Ectotherms such as lizards often take extreme care when it comes to laying their eggs. Eggs are incredibly sensitive! When female lizards don’t retain their eggs and give live births (we call this viviparous), female lizards will seek out the perfect temperature and moisture conditions for her eggs to develop and hatch. Oftentimes, when researchers use anoles in the laboratory for a variety of experiments, they incubate eggs at the temperature that these eggs experience in their natural environment. However, many studies only use a constant, mean temperature, rather than using a more realistic diel cycle of fluctuating temperatures over time. However, does that matter at all? It’s an important question for a variety of anole biologists that raise eggs to hatching: does the thermal regime of your incubator matter when it comes to offspring phenotype?

Josh Hall, a Ph.D. student at Auburn University under Dan Warner, set out to address just that question! He raised brown anole eggs in one of four distinct thermal treatments: a constant temperature, a repeated sinusoidal (sine wave) fluctuating temperature, a fluctuating temperature parameterized by the mean daily fluctuations these lizards experience in the field, and natural environmental conditions. He did this for both a cool developmental temperature (reflecting eggs developing earlier in the reproductive season) and a warmer developmental temperature (reflecting eggs developing later in the reproductive season). Even Josh didn’t recommend doing so many treatments! However, the amount of work that went into these treatments is impressive.

Hall and Warner found that natural temperature treatments increased the developmental rate of brown anole eggs only at cooler temperatures, and actually decreased the developmental rate at warmer temperatures. They didn’t find any other effects of any treatment on egg phenotypes. They also found that natural temperature treatments increased the endurance of hatchlings compared to the constant temperature treatment  at cooler temperatures. I was really struck by this study: it seems really obvious that eggs would do better in the lab when developed at temperatures that they experience in the field, but an experiment like this is so rarely performed because it requires so much intensive care and work! Can’t wait to hear more about how different developmental conditions affect anoles from Hall and Warner!

An anole toepad imaged with a scanning electron microscope.

In the endless comparison between the adhesive systems of geckos and anoles, today we learned a bit more about anole toe pads. University of Akron grad student Austin Garner, from the Peter Niewiarowski and Ali Dhinojwala labs, presented a poster on setal morphology across the toe of Anolis (Deiroptyx) equestris or the Cuban Knight anole. Like a lot of studies on toe pads, the inspiration for this work can be traced back to a previous study by Tony Russell (Johnson and Russell. 2009. Journal of Anatomy). In their 2009 study, Johnson and Russell found that the more distal lamellae of Rhoptropus geckos were more narrow and that their distal setae were longer, lower in diameter, and more densely packed within and across lamellae. This study came to shape our knowledge of within-toe gecko setal morphology.

Today Garner presented preliminary data evaluating the same patterns in an anole. Although their numbers are very preliminary (N=2), it suggests some interesting deviations from Rhoptropus. The more distal lamellae of equestris are narrower, similar to Rhoptropus. Distal setae are also more densely packed, but setae seem to be the longest and have the widest diameter in the middle of the lamellae, and in the middle of the toe, which is very different than what was observed in Rhoptropus. 

If this pattern holds as Garner et al. increase their sample size, it will have interesting implications for how we think about toe detachment in geckos and anoles and well as how the two groups navigate rough and smooth surfaces. I am really excited to see what patterns emerge from the study and so stay tuned to see how their results shake out!

Animals have to perform a lot of complex tasks within their environment in order to reproduce and survive. To perform these tasks, animals often rely on their ability to move throughout their environment, and animals that do this often are often better fit within their environment. That’s why exercise is so important, ladies and gentlemen! Frequent exercise will increase your ability to run fast or run far, but it often comes at a cost. For one, increased exercise response is met with a reduction in your ability to fight an infection (i.e. immunocompetence) or reproduce.

To further understand the effects of exercise on animals in general, Kara Reardon, a student of Jerry Husak’s at the University of St. Thomas, devised an experiment to understand how increases in cellular mitochondria (the powerhouse of the cell!) influence performance after endurance training. They provided green anoles (Anolis carolinensis) with pyrroloquinoline quinone (PQQ) a training supplement to artificially increase their mitochondria production and found that PQQ (and a higher level of mitochondria) didn’t necessarily influence endurance capacity directly, but found that it lowered metabolic rates in their lizards. They also found that muscle metabolism was not affected by training, but that exercise overall increased the performance capacities of green anoles. Next up, they are going to quantify the genes involved in their observed endurance enhancements in these anoles. Great stuff from Reardon and Husak!