Anole Annals

Dorsal Pattern Formation in Anolis Lizards

On April 18, 2022

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!

By Terry Ord

On March 31, 2022

In All Posts, Anole Videos, Education and Anoles

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

By Aryeh Miller

On March 28, 2022

In All Posts

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 Webs, 11, 13-16.

How Do Anole Species Tell Each Other Apart?

By Terry Ord

On March 26, 2022

In New Research, Research Methods

When it comes to finding a mate or defending a territory, animals need to recognise members of their own species. The reasons are intuitive: you only want to mate with your own species to ensure viable offspring, and you should only invest the effort in being territorial when confronted by rivals from your own species. There are exceptions and these are interesting—hybridization or territorial competition between species—but generally animals need a system for species recognition.

The large, often spectacularly coloured throat fan or dewlap of anoles seems like an obvious way to evaluate species identity. Taxonomists have historically thought so, too. Each species appears to display a dewlap that’s unique in colour and pattern. But there are various Anole Annals posts highlighting this is not always the case. Instead, the colour of the dewlap is often an adaptation to the light environment for enhancing the detection of territorial displays.

So what about those territorial displays? Might anoles use the complex movements of the head-bob and push-up display to figure out species identity?

Classic work by Charles Carpenter and Tom Jenssen revealed how often the head-bob movements of lizards, and anoles in particular, seemed specific to each species. Pioneering experiments using video playback by Joe Macedonia in the ’90s has also provided evidence that anoles are able to distinguish displaying rivals of their own species from those of other species. But what is it about the pattern of movements used in the head-bob and push-up display, or even how the dewlap is extended and retracted, that conveys species identity? Is there one feature that varies the most among species that anoles commonly rely on to identify species?

Display-Action-Pattern graphs (above) showing the complexity of movements used by Puerto Rican anoles for territorial advertisement displays

These are hard questions to answer. Anole displays are complex, using many different types of movements, so there’s a huge number of possibilities. One approach would be to isolate and manipulate each type of movement and use video or robot playbacks to ask the anoles themselves. But doing that would take an entire career. There are a seemingly infinite number of combinations to consider. In fact, it would be impossible without a way to narrow things down.

Claire Nelson is a creative (and courageous!) graduate student who had an eye for solving the challenge. She figured it was possible to leverage the large archive of footage I’d accumulated over many, many years. These videos were of free-living male lizards performing territorial advertisement displays. Her idea was to develop an objective method for identifying which movements used in the head-bob, push-up or dewlap display had the potential to convey species identity. She’s just published her solution in Animal Behaviour.

Claire (above) doing a balancing act with some non-anoles

Claire used this archive of display videos to create Display-Action-Pattern graphs, a method developed by Carpenter back in the 60s. These track the up-and-down movement of head-bobs and push-ups as well as the extensions and retractions of the dewlap during the territorial display. To keep it manageable, she limited her efforts to anole species on Puerto Rico, and graphs of about 10 territorial advertisement displays per male. But there was an important biological reason for selecting this number of displays as well. It effectively mimicked the number of displays an anole might typically see on first encountering another lizard. That is, anoles likely make judgements on species identity from only a handful of displays.

From these Display-Action-Pattern graphs, Claire took a host of measures, ranging from the duration and number of movements used, to variation in amplitude and pauses between movements. She also noticed that anoles tend to perform certain combinations of movements together in what she came to call ‘motifs.’ After many many hours of effort, Claire accumulated a huge amount of data for nearly 20 different types of display movement for eight Puerto Rican Anolis species, and in many cases, for different populations of the same species.

Claire asked me for advice on how to analyse it all. I have to admit I was completely useless on this front. I muddled something about using coefficients of variation and some other nonsense, but really I had no idea. I was still in shock over how much data she had accumulated, and the novelty (and implications) of discovering motifs in the displays. She knew what she was doing, though. Her analytical solution was vastly superior to anything I could have suggested.

Claire investigated a variety of approaches, but in the end she settled on the method of random forest tree classification. It’s a sophisticated machine learning algorithm that, in a nutshell, takes data and groups like with like. It doesn’t require any prior direction or preconceived notion on how data should be grouped. It just uses the variation in the data itself. You could view the algorithm as an anole brain using basic rules of variation to make judgements on which displays are likely to be different and which displays are likely to be the same.

The outcome was impressive. The algorithm correctly assigned the vast majority of lizards to their correct species based on just a handful of displays. Where errors occurred, it was partly because lizards were assigned to the right species, but the wrong population. This means anoles from different populations tend to share some display features because they’re still from the same species. Yet the algorithm was able to correctly assign most lizards to the right population. In other words, there was still enough variation in the displays between populations of the same species to identify them as belonging to separate populations. This is very interesting!

Random forest tree classification (above) can assign over two thirds of displaying lizards to their correct species.

The evolution of new species begins with individuals of the same species starting to segregate from each other in some way. Often it’s physical separation (on opposite sides of a mountain range), but changes in social signals can also prompt behavioral separation as well. This could be the case for some anoles on Puerto Rico. Once individuals stop recognising each other as the same species, they no longer reproduce with one another, and the door to speciation is propped open.

The other discovery Claire made was the apparent lack of any common display feature that could be used to identify species (and population identity). Instead, different features were important for different species. The duration and number of headbob movements were features that could be used to identify the territorial displays of Anolis poncensis—a species that is striking in its use of lots of extremely rapid, up and down body movements—whereas the way the dewlap was extended was influential in identifying different populations of Anolis gundlachi—a species that has an unusually long dewlap display. Other species like Anolis pulchellus and Anolis krugi were best identified by effectively considering features of the entire territorial display.

Whether or not anoles actually use the features identified by the algorithm in species recognition remains an open question. But Claire has managed to identify the potential candidate cues that could be used. It is now possible to develop a focussed research program to test whether, and how, anoles used these features to identify species. Again, the obvious way to do this would be to ask the lizards themselves using robot playbacks.

Random forest tree classification sounds awfully complicated, and it is very sophisticated, but it’s actually easy to implement. Any dummy can do it. I taught myself how and wrote a step-by-step tutorial so you can as well. We’ve published this tutorial alongside Claire’s paper in Animal Behaviour. Give it a whirl!

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

By Jonathan Losos

On March 23, 2022

In All Posts

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?

Shape Variation of the Pectoral Girdle of Anolis Ecomorphs

By Alex Tinius

On March 16, 2022

In Education and Anoles, Research Methods

The first three paragraphs of Jane Peterson’s contribution to the Second Anolis Newsletter.

Jane Peterson’s contribution to The Second Anolis Newsletter remains one of the most comprehensive exemplars of functional-morphological research of the anoline appendicular girdles. In just a few short paragraphs Peterson (1974) outlined the key differences in pectoral girdle morphology between the Anolis ecomorphs, drawing information from both field observations and anatomical dissections of anoles from all four Greater Antillean islands. The outlined study could have formed a major contribution to our understanding of ecomorphology, had these brief observations ever been expanded into a scientific publication. Sadly, they remained as notes, confined to a brief communique on an informal basis (that continues to be formally cited). Several intriguing studies hence have examined anole appendicular morphology, but rarely allowed for implications that reach across multiple island radiations (e.g. Anzai et al. 2014, Herrel et al. 2018).

With my 2016 Ph.D. thesis, I set out to quantify what Jane Peterson had observed forty years prior, and must confess that I still fall short of reproducing the multitude of implications that Peterson’s (1974) brief descriptions alluded to. Instead of combining video-recorded movement cycles with morphological descriptions, my comparisons are solely based on three-dimensional shape analysis of the skeletal elements that comprise the breast-shoulder apparatus (BSA): the clavicle, interclavicle, presternum, and scapulocoracoid (Fig. 1). Employing the power of computed tomography scanning, and geometric morphometric analysis, I quantified the shapes of the central elements of the pectoral girdle, and compared these across anole radiations.

As with earlier work, I focused on the Jamaican ecomorph representatives, and sought out their ecomorph counterparts from Hispaniola and Puerto Rico, particularly targeting those species that are the most and least similar to the Jamaican forms. That last line of thought did not reveal any straightforward answers, as the complex structural shape of the BSA allows these anoles to be relatively distinct in some aspects, while being quite similar in others. For example, the general shape of the presternum and interclavicle are quite similar between the two trunk-ground anoles Anolis lineatopus (Jamaica) and A. gundlachi (Puerto Rico), while that of the scapulocoracoid differs quite remarkably between the two. These complex associations will take a more detailed analysis than what is warranted here, so I’ll focus on the bigger picture instead.

Fig. 1: CT-rendition of the skeletal components of the breast-shoulder apparatus of Anolis baleatus in lateromedial view, depicting all anatomical features described in the text. The gray arrow denotes anterior.

Skeletal elements of the BSA in isolation

Previous analysis of the scapulocoracoid in isolation revealed that its shape differs between Anolis habitat specialists, and resembles a particularly dorsoventrally tall shape in twig anoles (Tinius et al. 2020). The other ecomorph groups (trunk-ground, trunk-crown, and crown-giant) show obvious tendencies towards a particular structural organization, but in none of these does the scapulocoracoid resemble a truly characteristic shape.

The New Yorker Features an Anole Cartoon

By Jonathan Losos

On March 12, 2022

In All Posts

John David Curlis

By Anthony J Geneva

On March 1, 2022

In Scientist Profiles

Where do you work and what do you do?

I am currently a graduate student at the University of Michigan, but I conduct most of my research in the tropics of Central and South America. I am broadly interested in trying to answer the question of how to explain patterns of phenotypic diversity found in nature, especially in the context of color and signaling. In other words, why do organisms look the way they do – why do they have certain colors over others, and what sort of information are they conveying by showing off those colors? When not focusing on my research, I spend virtually all of my free time photographing as many animals as I can find, as well as spending countless hours sorting said photos into their respective taxonomic groups. What can I say, I’m a biodiversity nerd!

What aspects of anole biology do you study, and what have you learned?

I study the evolution of color in anole dewlaps. Even with over 400 species, all anoles possess this extendable throat fan, and it’s often brightly colored. Although we have some understanding of how the dewlap functions as a signal (e.g., species recognition, competitive interactions, courtship, predator avoidance/deterrence), it remains unclear why there are so many different colors of dewlaps. To try to tackle this question, I am looking at how the evolution of these colors may be influenced by the light environment. Since the reproduction and/or survival of an anole can depend on whether its dewlap is serving as an efficient signal, it’s easy to see how the light environment might determine which colors are favored by selection. For instance, a bright orange dewlap would likely show up much better than a dull green one under the dense canopy of the rainforest, just as a pitch-black dewlap would probably be an excellent signal in a bright, open field. I am testing this idea using an experimental island study in the Panama Canal. My study species is the Panamanian slender anole, males of which can have a mostly orange dewlap or a mostly white dewlap. By introducing these lizards onto a multitude of very tiny, highly variable islands in the canal, I can test which color will “win out” over time in different light environments.

How and why did you start studying anoles?

I have loved reptiles since I was a child, so it was by no coincidence that the very first lab I worked in as an undergraduate had a breeding colony of anoles. While there, I studied physiology and metabolic rates. While I can say that metabolism work is not for me (shout out to the scientists who love it!), I very much enjoyed taking care of and working with the anoles, so I decided to stick with them throughout my undergraduate and graduate career.

What do you love most about studying anoles?

As someone interested in color, I think that dewlaps are anoles’ coolest feature. I love studying anoles and their dewlaps because I am constantly amazed by the astounding amount of diversity in this little flap of skin. In addition, as a researcher, it’s hard to complain about the incredibly high abundance and ease of capture for many of these species.

What is your favorite anole species?

My favorite anole species would have to be the Meyer’s anole, Anolis johnmeyeri (named after the scientist, not the singer). This species, found in Honduras, has an absolutely gorgeous dewlap in both males and females. While large, colorful dewlaps are possessed exclusively by males in many anole species, female Meyer’s anoles have a dewlap that’s almost as large and equally as beautiful. Female dewlaps are bright yellow with a brilliant blue spot, and male dewlaps are bright red with the same blue spot.

Where can people learn more about you and follow you online?

Website: www.colorinnature.org

Instagram: @johndavidcurlis

SICB 2022: Ecological and Genetic Basis of a Sexual Signal

By Renata Pirani

On February 5, 2022

In All Posts

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

By Aaron Griffing

On February 1, 2022

In All Posts, New Research

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!