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Convergence in Pinocchio Lizards Can Help Rediscover an Extinct Indonesian Agamid Species

Harpesaurus tricinctus

I believe that several ways exist to do Science in Herpetology. Proposing and experimentally verifying hypotheses, for example, that a species is new to science, is one of them. But sometimes, important discoveries come out of nowhere, at least in appearance. In fact, they come from unrelated data and observations which, in certain circumstances, come together in a mind and lead to a concrete result.

I had the chance to make several discoveries of this nature during my career. The first concerns a very abundant and widely distributed small Pacific skink that everyone thought was a single species, Emoia cyanura. By observing several thousand specimens in the laboratory, I noted certain morphological and coloring characters whose presence varied from one individual to another, but many were linked to each other. I left it at that, but once in the field in French Polynesia a few years later, very quickly I realized that all these characters made it possible to highlight two sympatric, but not quite syntopic, species that everyone was confusing until my thesis work in 1987. Immense pleasure!

The second discovery is that of an external supralabial gland whose excretion orifice is visible in all snakes of the genus Echis. Hundreds of herpetologists have counted the supralabial scales in these snakes without seeing this orifice, which is so obvious now.

The third discovery in my career as a herpetologist is that of Bocourt’s terrifying skink (Phoboscincus bocourti). It was on a small tiny island in New Caledonia that I found, in 2003, a very large lizard that had been considered extinct for almost 150 years. Never such a discovery was imaginable, even less in this small place yet already explored by herpetologists.

Harpesaurus tricinctus MNHN 0623 HOLOTYPE

Now let’s see my fourth and most recent discovery. Being responsible for the collections of squamate reptiles at the National Museum of Natural History in Paris from 1988 until 2014, I often took photographs of lizards to respond to requests from colleagues. This is how, at the beginning of my career, I was asked to photograph a very original little Asian draconine agamid due to the presence of a relatively long flexible rostrum at the front of its head. The origin of this unique specimen is obscure because only “Java” is indicated without a collector’s name. Auguste Duméril described this specimen in 1851 as a new species, Harpesaurus tricinctus. There is a vellum which represents this lizard still relatively fresh with some colors preserved. The lizard has never been found since and is considered extinct.

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In December 2016 I was looking at issue 4 of volume 47 of Herpetological Review. In this issue, an advertisement for the 11th Latin American Congress of Herpetology in Quito, Ecuador was illustrated with a photograph of a strange lizard that immediately caught my eye. I really thought I was dreaming when I saw this image. This lizard looked almost totally like H. tricinctus, yet it was a South American species! At first, I thought that Duméril was mistaken and that his Asian agamid was a South American dactyloid. Examination of complementary material in the collections quickly showed that these were indeed two distinct species from different families, one Asian (Agamidae) and the other South American, Anolis proboscis (Dactyloidae). The latter was long, mysterious and no data were published on its ecology until 2007. From that year, the species was rediscovered and its ecology described. I then thought that this extreme convergence reflected a quite comparable ecology between A. proboscis and H. tricinctus, the latter being considered extinct. The written article was submitted to the Raffles Bulletin of Zoology in 2019 but was not accepted even before its passage by the reviewers. I then contacted Wolfgang Böhme and Thore Koppetsch, who were enthusiastic to join me in completing this project. Both have completed my work and this association has allowed its publication in the famous journal Salamandra in May 2022 (available here). The well-known ecology of the first lizard should make it possible to rediscover the Asian agamid and make it a Lazarus species. Another species of the genus, Harpesaurus modiglianii, was rediscovered in Sumatra in 2020, after almost 130 years without observation. All hopes are therefore allowed to find H. tricinctus based on the ecological information obtained from its convergent dactyloid.

Anolis proboscis. Photo by Jonathan Losos

All the data resulting from this work is available in:

Ineich I, Koppetsch T, Böhme W. 2022 – Pinocchio lizards and other lizards bearing rostral appendages − the peculiar habitus of the draconine agamid Harpesaurus tricinctus with highlights on its ecological implications and convergence with its New World equivalent, the dactyloid Anolis proboscis. Salamandra 58(2):123-138.
https://www.researchgate.net/profile/Ivan-Ineich/research

Inferring Where Anole Ranges Tend to Spread or Split

When I mentioned to Jonathan Losos that I had applied a new biogeographic model to anoles, he gave me a copy of Lizards in an Evolutionary Tree. What could I do but take it as both a generous gift and gentle suggestion? This is all to say that I’m no herpetologist. I know this, and so should you. I write humbly to the AA readers as a twig anole standing on the shoulders of crown-giants.

With that said, several close colleagues (Ignacio Quintero, Martha Muñoz, Felipe Zapata, Michael Donoghue) and I recently had a paper published that introduces a new modeling framework that allows regional features of geography to inform phylogenetic rates of dispersal, extinction, and speciation – called FIG (Feature-Informed GeoSSE). We then applied FIG to the phylogenetic and biogeographic Anolis dataset published by Steven Poe and co-authors. This piece summarizes that work and subjects its readers to a few lame jokes.

Cartoon of FIG model behavior.

The first aim of our project was to propose a new framework for modeling historical biogeography using GeoSSE, a diversification model introduced by Goldberg et al. (2011). Under GeoSSE models, all species occupy different sets of regions in a shared geographical arena. As time advances, species stochastically disperse, go extinct, and speciate. Speciation itself might occur within a region or between regions (e.g., in a manner consistent with allopatric speciation).

Thinking realistically, when and where these events happen should somehow depend on the set of regions each species occupies and the geographical features of the relevant regions. Unless instructed otherwise, however, GeoSSE estimates every possible event rate from phylogenetic and biogeographic data alone. Regional features of geography aren’t used. This leaves historical biogeographers with at least one technical problem and one missed opportunity.

First, concerning the technical problem, the number of event rates explodes with the number of regions. A geographical system with two regions (A and B) requires seven rate parameters: two within-region speciation rates for regions A and B, two extinction rates for A and B, two dispersal rates for A into B and B into A, and one between-region speciation rate for splitting the widespread ancestral species with range A+B into two daughter lineages with ranges A against B. Three regions requires 18 rates, four regions requires 45 rates, and so on. Between-region (“allopatric”) speciation rates and, to a lesser degree, the dispersal rates fuel this explosion in rate parameters. Somehow the number of rates must be reduced if we hope to estimate any of them at all.

In FIG, regional features (size, distance, barriers) can inform event rates. Different features are free to have different relationships with each process. For example, region size may increase within-region speciation rates while decreasing local extinction rates. Distance may decrease dispersal rates but increase between-region speciation rates. FIG also allows for barriers and distances to inform the rate and way in which a widespread range splits following between-region speciation.

Second, concerning the missed opportunity, biogeographers often hypothesize how geographical features should influence evolutionary outcomes – for example, that a single species should “split” in two more rapidly when its range is subdivided by a barrier – but GeoSSE has not really been used to test such hypotheses, in large part because of the technical issue mentioned above.

For hypothesis testing, FIG analyses estimate the probability that each geographical feature has an effect on a corresponding class of evolutionary rates. This is done with Bayesian reversible-jump Markov chain Monte Carlo to turn “on” and “off” different relationships between features and rates. For example, if FIG estimates that the effect of distance on dispersal was “on” in 97% of MCMC samples, you might make the bold claim that distance influences dispersal rates between regions. Or, if the effect of size on extinction was “on” in only 31% of samples, you might say no conclusive relationship or non-relationship was detected.

The other aim of our work was to see if FIG produced any biologically interesting inferences. If not, why bother? Anolis led the pack when selecting a clade to feature for empirical analysis. Anoles are well-known for their distribution throughout the Caribbean and the neotropical mainland, with some expectation of moderate, but not rampant, dispersal. Because anole phylogeny and biogeography has been studied in such detail over the decades, the clade is also ideal for test driving phylogenetic models.

Geographical distribution of Anolis.

We adapted the Poe et al. (2019) dataset into a nine-region biogeographical system with five continental and four insular regions of varying sizes and distances. There are ~380 anoles in this dataset, with nearly as many insular as continental species, but nearly [*] all widespread anoles are restricted to adjacent continental regions. This is exactly the type of pattern you’d expect to see if geographical distance and oceanic barriers restricted anole movement. That is, widespread fragmented ranges should be difficult to maintain and should therefore be rare.

Biogeographic rates. Left: extinction (nodes) and dispersal (edges) rates. Right: within-region speciation (nodes) and between-region speciation (edges) rates.

Reassuringly, FIG inferred that dispersal was limited by distance, especially over water. Similarly, between-region speciation rates split ranges the fastest when the inhabited regions were far apart and/or separated by water. Region size and island-status did not have a predictable effect (or non-effect) on extinction and within-region speciation rate. The network diagrams above summarize different regional rates of evolutionary change.

Dispersal rates, between-region speciation rates, and distances.

We also were interested in a critical distance that we defined – the maximum range cohesion distance – beyond which widespread species tended to split into two rather than remain as one. The idea is that if dispersal rates are low, interpopulation migration rates should be low, and therefore rates of allopatric speciation should be high. If we assume that FIG’s dispersal rates approximate the rates of pulsed migration needed to maintain gene flow between regions, we predicted the corresponding distance at which organismal movement became too slow to maintain range cohesion. So how far is too far?

Our scrappy little estimator predicted that range cohesion is almost certainly degraded beyond ~470km over land and ~160km over water. As it turns out, six pairs of continental regions had average distances closer than 470km, where we find all widespread continental anoles today. We don’t expect that these distances perfectly describe the limits of range cohesion for all anole species in all contexts, but a cursory review of the anole population genetic literature convinced us that our estimates were at least reasonable.

Extinction rates, within-region speciation rates, and sizes.

We didn’t recover an analogous relationship for the ratio of extinction and within-region speciation rates with region size. But neither does FIG reject those relationships. Still, recalling the histogram of regional species richness above, it seems unlikely that size alone dictates richness in anoles across continental and insular regions – as they’re defined here – since Amazonia has so few species relative to much smaller regions, like the island of Hispaniola. More work on this issue is needed.

In its conclusion, our paper reflects on how phylogenetic models of biogeography treat allopatric speciation. With anoles under FIG, for example, the consequence of dispersal into a new region depends on the geographical context. If dispersal is between adjacent continental regions, dispersal tends to result in range expansion. But, if dispersal is between distant regions or involves insular regions, it tends to result in cladogenesis that’s consistent with allopatric speciation. In large part, this inference was made possible because regional features inform evolutionary rates in FIG, letting us predict where anoles tend to “split or spread.” We think this historical view of allopatric speciation in a phylogenetic context will be worth exploring further.

Not to pander, but Anolis has played and continues to play such an instrumental role in the development of biologically meaningful models of ecology and evolution. Many of the statistical phylogenetic models that I’ve looked to for inspiration in my own research over the years were introduced by anolologists. Maybe this is a second meaning for model clade? Rambling aside, our project was a true collaboration among some of the most creative organismal and mathematical biologists I’ve known. Our hope is that our small contribution lives up to the high standard of modeling anole diversity.

*–An innocent question from an outsider: is A. sagrei really a single species? Pretty wild.

Paper:
MJ Landis, I Quintero, MM Muñoz, F Zapata, MJ Donoghue. 2022. Phylogenetic inference of where species spread or split across barriers. Proceedings of the National Academy of Sciences 119: e2116948119. doi:10.1073/pnas.2116948119

Code:
https://github.com/mlandis/fig_model

Lab site:
http://landislab.org

Dorsal Pattern Formation in Anolis Lizards

Picture of a female A. sagrei with the diamond pattern.

Readers of this blog will be well aware of the conspicuous variation in dorsal color patterns of Anolis lizards – think of the spots of A. sabanus, or the bands of A. transversalis (both worthy a google search if you happen to be unfamiliar with them). Such patterns are particularly interesting when several patterns exist within a single population – a polymorphism. This is the case in A. sagrei where females have either a chevron-like pattern, same as all males of this species, or a diamond-like pattern on their backs.

In 2016, I visited some A. sagrei populations in Florida and became interested in this female-limited color pattern polymorphism. Being a developmental biologist, what puzzled me most was not if selection can maintain multiple patterns, but how the two patterns actually develop.

On the one hand, the difference between the two patterns ought to have a simple genetic basis. Diamond- and chevron-females mix within the same population and the two morphs are discrete (although there is a continuous variation within both morphs, and the diamonds sometimes look more like a band [1]). On the other hand, the developmental biology of pattern formation is far from trivial. Here is a little teaser of the complexity: in vertebrates, pigment cells are derived from neural crest cells that originate along the dorsal midline of the early embryo. Once detached from the crest of the neural tube, pigment cell precursors migrate towards their final destination in the epidermis, where they differentiate into pigment cells – xanthophores, melanophores and iridophores. What these cells do and how they organize themselves are what gives rise to patterns. And this does not seem to be very simple at all. So how can we reconcile a simple genetic basis with the complex developmental biology of pattern formation of diamonds and chevrons?

The two female morphs and a male of A. sagrei.

Simply out of curiosity, we set out to solve this puzzle. Thanks to our anole breeding group at Lund University, we already had three generations of A. sagrei of both morphs that we could use to address the pattern of inheritance of diamonds and chevrons, and to identify the underlying gene(s). Our intuition about the simple genetic architecture of the polymorphism held true, and we identified a single Mendelian locus that perfectly segregated with the morph patterns.

Surprisingly though, the locus was not on a sex chromosome, so this could not explain why only females are polymorphic. Instead, the identity of one of the two genes located at the Mendelian locus pointed to a solution: the estrogen receptor 1 gene is crucial for the development of female traits and therefore expressed at higher levels in females compared to males. Because of the close physical proximity of the two genes at the Mendelian locus, also the second gene shows a female-biased expression pattern. So while the estrogen receptor 1 gene does not explain the pattern differences, it explains why the polymorphism is only present in females.

But what is this second gene at the Mendelian locus, and how does it explain the difference in patterning? It is a gene encoding the coiled-coil domain-containing protein 170, or CCDC170. Not exactly a usual suspect of color pattern formation. In fact, not much is known about this gene, but the cancer literature had demonstrated that it codes for a structural protein that regulates the migratory capacity of cells (mutations in CCDC170 can make cancer cells migratory and are associated with an aggressive type of breast cancer). The proteins encoded by the diamond and chevron alleles were predicted to form CCDC170 proteins with structural differences. This is likely to affect the function of the protein – perhaps by influencing migratory behaviors of pigment cell precursors.

Testing this hypothesis is easier said than done. As a first proof-of-principle, we used an in silico modelling approach to test if tinkering with the migratory capacity of cells can switch a system from generating chevron-like to diamond-like patterns. Miguel Brun-Usan is a magician with cell-based computer modelling and his trick is to create computer models that are complex enough to capture real biological phenomena, yet simple enough to allow us to trace and understand what is going on. To get to the bottom of the diamond/chevron morph differences, he implemented cells on a growing epithelium with a Turing-type mechanism consisting of a gene regulatory network (GRN) with up to five genes. By running a number of experiments (yes, computer scientists also run experiments, I learned), Miguel could demonstrate that some GRNs generated back patterns that very much resembled those of real lizards. Moreover, he found that modifying a single gene in the GRN that regulates the migratory capacity of cells is sufficient to switch the system from generating chevrons (if migration is switched ON) to diamonds (if migration is switched OFF).

A brief graphical abstract of our study.

So it seems that what reconciles the simple genetic basis of the polymorphism with the complex process of pattern formation is that the underlying developmental system exhibits a high degree of controllability. Simply modifying migratory capacities of cells by tinkering with one gene in the core GRN leads to changes in collective cell migration, visible as a distinct and new dorsal color pattern. Such high controllability could explain the evolvability of dorsal color patterns and result in high turn-over rates between patterns, as beautifully demonstrated by geckos [2]. I am excited to continue this research to see if the model can be substantiated mechanistically. I would also like to test if convergent evolution of diamond patterning in different Anolis species is underpinned by convergent developmental and genetic mechanisms. If you are interested in joining our research group, please get in touch (nathalie.feiner@biol.lu.se; www.feiner-uller-group.se).

You can read the full article here:

Feiner, N., M. Brun-Usan, P. Andrade, R. Pranter, S. Park, D. B. Menke, A. J. Geneva, and T. Uller. 2022. A single locus regulates a female-limited color pattern polymorphism in a reptile.
Science Advances 8:10 DOI: 10.1126/sciadv.abm2387

Other references:

  • Moon, R.M., and Kamath, A. (2019). Re-examining escape behaviour and habitat use as correlates of dorsal pattern variation in female brown anole lizards, Anolis sagrei (Squamata: Dactyloidae). Biol J Linn Soc 126, 783-795.
  • Allen, W.L., Moreno, N., Gamble, T., and Chiari, Y. (2020). Ecological, behavioral, and phylogenetic influences on the evolution of dorsal color pattern in geckos. Evolution 74, 1033-1047.

Anoles Are Powerful Educators, Use ’em!

Did you ever read those choose-your-own-adventure books as a kid? I had a whole collection. What if lectures were like that too? Check this one out on anoles (above).

This lecture came about from the need to update a lecture on ecological competition for a second year undergraduate course. In the past, someone might have handed me a textbook and I would have quickly shelved it, never having opened its cover. As a student I hated textbooks and things really haven’t changed for me now as an educator. The real challenge isn’t the content, it’s presenting that content effectively. We’re now on the other side of “the great digital shift of 2020,” but this challenge of engagement remains the same, if not more so. Does this choose-your-own-adventure lecture offer the solution?

Let’s step back for a moment so I can first make the case for anoles…

Anoles first came into view for me way back in my first year of graduate school. Not in real life, of course — there were no anoles at any of my field sites in Sydney. Instead, I happened across a remarkable paper appearing in one of the weekly tabloids. It recounted how researchers had returned to some tiny islands in the Bahamas where a bunch of lizards had been introduced a decade or so before. I couldn’t make head nor tail of the PCA plots or Tables. But the scatterplot later in the paper was clear to even a dunce like me. These lizards had adapted their limbs over a matter of years (years!) to cope with living on spindly bushes. Evolution happening in real-time? Holy cow, this was revolutionary for me. Why am I only seeing this now?

I’d never really thought about adaptation outside of centuries or millions of years. But then my undergraduate experience was the usual, tired textbook fodder of ecology and evolution that never came to life, regardless of how glossy the graphics might have been. My undergraduate experience was mostly about memorising facts and figures, and there was a great mental chasm between those and the real world around me. What I actually saw in nature were animals doing weird and crazy things, so I ultimately gravitated towards animal behaviour for my PhD. But when I discovered this paper, I had just finished reading Richard Dawkin’s “selfish gene” and Dan Dennett’s “Darwin’s dangerous idea,” and I was now fascinated by evolution.

And here was some character named Jonathan Losos, along with his mates Ken Warheit and Tom Schoener, reporting in a glossy magazine called ‘Nature’ years before (in 1997 no less) that evolution happens now, not in the past… Now! If only I had been exposed to this and other stuff like it as an undergraduate. [NB: Jonathan gives a great backstory in his book about how this study almost never left the bottom drawer].

These days I am towards the other end of the student-teacher continuum and I make a point of not teaching from a textbook. First, they are WAY too expensive for students. Second, they are out of date by the time they are published. Third, if classic works are covered (like those on anoles), the format of a textbook makes even the most exciting example remote and dull. My approach has always been to go directly to the source. And anoles offer such a rich collection of content for educators.

But what of this new “choose-your-own-adventure” style format? What is really being achieved here? My sales pitch to you is that it prompts student engagement at strategic points. By doing so, it maintains an active connection between the student and the content. In other words, it should stop students cognitively dropping out while writing copious amounts of notes that they will only ever read just before the exam and promptly forget soon afterwards. By forcing students to direct their own learning experience, they are being subtly pushed to reflect on the content explicitly and intuitively, and they might not even realise it. The hope is they not only grasp the concepts being presented more effectively, but retain (and apply) that comprehension outside the bounds of the course and into the future. And it’s fun too.

Convinced?

The danger is the format could just be a gimmick that’s great as a one-off, but quickly becomes annoying or distracting. The analogy I think of here is the transition from slides to powerpoint in my early conference days at the start of the 2000s. For the ancients among you who remember that time, you might recall having to sit through a plague of animated slide transitions with cheesy swirly sounds as presenters explored the seemingly infinite number of options on offer. Oh, the liberation of going digital! Then most of us eventually realised how annoying and distracting it all was and went back to simpler presentations. Perhaps “choose-your-own-adventure” lectures are the same? Would you have an entire course with choose-your-own-adventure lectures?

Huge thanks to Mike Kasumovic and Arludo for both the hideous yellow shirt and putting the lecture together for me. I use a lot of Arludo’s interactive digital games in my teaching as well – they’re free, the students love them, and they have clear educational outcomes. Evolution, ecology or behaviour, whatever you need, they’ll have something you can engage your students with. Do check them out.

More Evidence Of Seed Dispersal In Anoles

Anolis porcatus is the most recent Caribbean anole to have been documented to consume and disperse seeds. Natural history research by Armas (2022) observes A. porcatus feeding on West Indian holly, and finds successful germination of the fecal pellets. Credit Thomas Brown (Wikimedia Commons).

Recent research on Anolis lizards has suggested their omnivorous tendencies might aid in the dispersal of seeds. Most recently, this was discussed by Giery et al. (2017), who reported Cuban Knight Anoles (A. equestris) to consume and disperse royal palms. This year, Armas (2022) reports Anolis porcatus consumes and disperses West Indian holly. Check it out!

New literature alert!

Consumption and Dispersal of West Indian Holly (Turnera ulmifolia, Turneraceae) Seeds by Cuban Green Anoles, Anolis porcatus (Squamata: Dactyloidae)

In Reptiles & Amphibians

Armas (2022)

Literature Cited:

de Armas, L. F. (2022). Consumption and Dispersal of West Indian Holly (Turnera ulmifolia, Turneraceae) Seeds by Cuban Green Anoles, Anolis porcatus (Squamata: Dactyloidae). Reptiles & Amphibians, 29(1), 115-116.

Giery, S. T., Vezzani, E., Zona, S., & Stroud, J. T. (2017). Frugivory and seed dispersal by the invasive knight anole (Anolis equestris) in Florida, USA. Food Webs11, 13-16.

How Many Anoles Are There in Captivity (Pets, Zoos, Labs) Worldwide?

Photo from http://www.petworldshop.com/

Nigel Rothfels, a historian of animals and culture at the University of Wisconsin-Milwaukee, asks:

Given the previous AA post on anoles in the pet trade, the amount of in-country breeding there must be of anoles, the general life-span of anoles, and the general growth in pet-keeping since Covid, what is your highly educated guess on the number of anoles currently being kept in captivity world-wide (as pets, for educational supply companies, in labs, or zoos).  With 350,000/year being collected in just Louisiana in 2006, it makes me think that something like 3-5 million might still be an underestimate.

 

Anyone want to venture an estimate?

The New Yorker Features an Anole Cartoon

SICB 2022: Ecological and Genetic Basis of a Sexual Signal

This year at SICB, I had the great opportunity to talk about part of my work as a postdoctoral researcher in the lab of Dr. Michael Logan at the University of Nevada, Reno. In collaboration with John David Curlis (University of Michigan), Christian Cox (Florida International University), W. Owen McMillan (Smithsonian Tropical Research Institute), and Carlos Arias (STRI), we have been studying the Panamanian slender anole Anolis apletophallus, which has a dewlap polymorphism: males either have a solid orange dewlap (solid morph) or a white dewlap with an orange spot (bicolor morph). Preliminary results from John David Curlis’ PhD dissertation research suggests that, in our mainland study population, the frequencies of these morphs change in conjunction with understory light levels—the solid morph is more frequently observed in brighter areas where more light reaches the understory, whereas  the opposite is true for the bicolor dewlap, which is more frequently observed in darker areas of the forest. Thus, it seems possible that selection is maintaining this polymorphism following the predictions of the sensory drive hypothesis, which states that sexual signals should have characteristics that make them the most transmissible given the physical characteristics of the local habitat.

As part of an effort to understand how this trait is evolving in the wild, I set out to understand the genetic basis of this dewlap polymorphism. To do this, my collaborators and I first assembled the full slender anole genome which we then used as a reference for a pooled population sequencing (Pool-Seq) approach using half individuals with solid dewlaps and half individuals with bicolor dewlaps to identify the genomic region underlying this dewlap polymorphism.

Our genome assembly showed pretty good results (Scaffold N50 154,613,287). The Pool-Seq results presented a clear peak of differentiation between solid and bicolor morph groups that corresponded to a region on Scaffold 3. We have a promising candidate gene within this region that may underly the dewlap polymorphism, but will continue to explore these data further to understand the genetic basis of this charismatic trait.

Making the Fancy Feet of Anoles and Geckos

A mourning gecko (Lepidodactylus lugubris) climbing vertically on glass with the help of its impressive toe pads.

I think most people visiting Anole Annals could argue that the adhesive digits of anoles are some of the most fascinating aspects of their biology (or maybe I’m just biased). Digital adhesion is accomplished through toe pads: a collection a broad, modified plantar scales which bear thousands upon thousands of microscopic, hair-like structures (i.e. setae). Through frictional and van der Waals forces, these collections of setae allow toe pad-bearing lizards to easily access vertical surfaces and exploit habitats many lizards cannot. Shockingly, adhesive toe pads have independently evolved several times across lizard evolutionary history (at least 16 times by recent estimates) — once in the common ancestor of anoles, once in a clade of southeast Asian skinks, and 14 times in geckos. Both within and between the different evolutionary origins of toe pads, there is substantial variation in toe pad size, shape, number of scansors/lamellae, and position of the adhesive apparatus.

In our recent study, my collaborators and I took the first steps to characterize how embryonic development is modified to achieve this incredible diversity. Using embryonic material my coauthor Thom Sanger collected as a postdoctoral researcher in Marty Cohn’s lab, in addition to embryonic material I collected over the course of my Ph.D. training in Tony Gamble‘s lab, we aimed to compare embryonic digit development of ancestrally non-padded lizards with that of anoles and padded geckos. We used a model clade approach to broadly sample anoles and geckos, although some species breed more easily in the lab and have more embryological resources than others. All together, we sampled a range of toe pad morphologies in both clades (trunk-ground and trunk-crown Anolis ecomorphs and leaf-toed and basal pads in geckos). To help polarize the developmental changes leading to the origin of toe pads, we also included two ancestrally padless species in our comparisons. After the collection of these diverse embryos, we used scanning electron microscopy (SEM) to characterize scale morphology of the digits throughout embryonic development.

By comparing embryonic material of anoles and geckos, we essentially span the diversity of squamates in a single comparison.

Because of the ~200 million year divergence between anoles and geckos and dramatic differences in adult morphology, we anticipated that we would see stark differences in the developmental origins of toe pads in these species. To our surprise, we found striking similarities in toe pad development between all of the pad-bearing species we examined. We found that toe pads develop after digit webbing recesses. In all pad-bearing species, ridges that become the adhesive scansors and lamellae first form in the distal half of the digit. Throughout development, new ridges begin forming in the proximal direction while the previous ridges begin to grow laterally. Elaborations and derivations in toe pad form, such as bifurcation, occur in the latter-half of embryonic development. The presumably ancestral pattern of plantar scale development we observed in our leopard gecko and fence lizard embryos (both species lacking adhesive digits) demonstrated that scale ridges form all at once along the length of the digit. These differences are similar to those documented between developing non-padded gecko tails and padded tails of crested geckos. This means that anoles and geckos have converged on a similar developmental process! We suggest that toe pads are initially formed through a major repatterning of digital development and then variation is achieved through relatively minor “tinkering,” through either timing or location of developmental patterns.

Scanning electron micrographs (SEMs) of embryonic lizard digit development, progressing from early development (left) to late development (right). The pad-bearing brown anole (Anolis sagrei) and mourning gecko (Lepidodactylus lugubris) have converged on scansor ridges forming in a distal-to-proximal direction, while the paddles leopard gecko (Eublepharis macularius) has scale rows forming all at once along the length of the digit. Lizard photos courtesy of Dr. Stuart Nielsen.

This is by no means the end of this story. We’ve just scratched the surface and there are a several directions to head in. A logical next step is to characterize histological organization through toe pad development. From there, characterizing the genes involved in toe pad morphogenesis, in tandem with the possibilities of new gene editing technologies, would allow us to test mechanisms of toe pad formation and how variation is generated. And, of course, characterizing toe pad development in other species (such as the secondarily padless Anolis onca) may elucidate further conservation or derivation from the trends we found. This is an exciting time to be a toe pad biologist!

SICB 2022: Repeatability and Correlation in Thermal Traits

As we know, anoles are ectotherms which could spell trouble under a changing climate. By closely relying on the temperature of their environment to regulate all sorts of physiological processes, anoles may be at risk when the environmental temperature shifts due to climate change. Because of this, many studies have measured traits which describe a lizard’s thermal physiology, and two popular traits are the lizard’s preferred temperature (Tpref) and heat tolerance. But few studies have looked at a) how repeatable these traits are in any individual and b) whether the two are correlated!

Shannan Yates stands next to her SICB poster.

Shannan with her poster at SICB 2022 in Phoenix.

Shannan Yates, a graduate student at Tulane University in Dr. Alex Gunderson’s lab endeavored to do just this, and presented the results at SICB this year with a very compelling poster. Shannan hypothesized that if Tpref was measured twice in the same individual, that these temperatures should be repeatable. Secondly, she hypothesized that preferred temperature would be correlated with heat tolerance, as these traits are expected to be phenotypically linked, and individuals with high heat tolerances should prefer higher temperatures.

Interestingly, though, Shannan found quite the opposite! Tpref was neither repeatable nor correlated with heat tolerance! This has important implications for many studies which attempt to quantify these traits in Anolis lizards. Shannan concluded that either thermal preference is flexible, or that the current methods used to study Tpref may affect our measurements. Also, thermal preference may not actually be correlated with thermal tolerance at an individual level, or our current methods are indeed obscuring a relationship.

Shannan’s poster is up on SICB+, so be sure to check out the data for yourself, and check out Shannan on twitter here

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