I recently received an email from Chris McMartin, the director of the Southwestern Center for Herpetological Research, about a population of brown anoles near his home in Montgomery County, Texas, just north of Houston. Chris has done a lot of preliminary research to understand how the Montgomery population is spreading, and would like to know how these lizards are related to the larger population in Harris County.
Interested? Keep reading!
With Chris’ permission, I’ve copied part of his email below:
“I’ve been casually (in my free time, mostly in the summer) researching Brown Anoles (Anolis sagrei) and their spread in southern Montgomery County where I live. As I amass observational data, I’ve noticed the lizards are abundant in some yards/neighborhoods, but nonexistent in adjacent yards/neighborhoods. I’m slowly trying to piece together additional factors (presence of outdoor cats, prevalence of certain landscaping features including decorative rocks and tropical plants, age of house/neighborhood, use of pesticides, etc.) which may explain not only the disparity in abundance but provide clues as to how to control their expansion.
One big question I have is whether the lizards are naturally expanding from a single introduction long ago (e.g. rapidly moving northward from Harris County, where they occur in densities many times higher than the highest I’ve observed in Montgomery County), or are an amalgamation of numerous discrete introductions (e.g. when a home installs new tropical plants from a nursery/home improvement store). Brown Anoles first showed up in my yard a little over a year ago, marking an expansion northward of about ¾ of a mile from my previous northernmost observations the year prior.
I have corresponded with Dr. Benson Morrill, who owns Rare Genetics Inc. offering DNA analysis for (at this time) colubrid snakes (primarily for sex determination) and inquired as to the possibility of sending him samples from various neighborhoods in my area in an effort to determine whether they represent a contiguous related population or are the result of discrete introductions. He says the process to conduct this analysis would be cost-prohibitive for a private individual such as myself, but perhaps a university student would like to take on the project.
As it is, I currently spend what surprisingly-little free time I have in the summer exploring neighborhoods in my neck of the woods and documenting my obervations—around 60 hours this past summer between field work and analysis—and I’m approaching my limit of resources in time (and definitely money, if considering DNA analysis as part of the project). This is where I think perhaps a graduate student might be interested in taking on a study of Brown Anoles as a thesis project…lots of possible threads to pull (competition with natives, rate of range expansion, effect of occasional hard freezes on population, etc.).
I’ve published articles in local magazines/newspapers about the lizards and have a public-opinion survey from a year ago (still awaiting analysis) trying to find any links between various conditions (age of neighborhood, presence of outdoor cats, etc.) and occurrence/prevalence of browns, especially with respect to A. carolinensis. Some interesting things seem to be occurring. Anecdotally, browns are eating greens (hatchlings), Broad-headed Skinks are eating browns, and greens are eating the skinks (with photographic evidence)! Sort of a three-way lacertilian arms race.”
If this sounds like just the opportunity you’ve been looking for, contact Chris at yall [at] mcmartinville.com.
This pale little lizard is one of the world’s first genetically edited non-avian reptiles. Image Credit: Courtesy of Ashley Rasys, University of Georgia
Compared to mammals, reptiles have a weird way of reproducing—and in the spring of 2017, that put Ashley Rasys in something of a pickle.
For months, the University of Georgia biologist was struggling to come up with a way to tinker with the genes of the brown anole (Anolis sagrei), a petite, pointy-faced lizard native to Cuba and the Bahamas.
The reptile had initially caught Rasys’ eye because of, well, its eyes. People with albinism often have poor vision due to problems with their foveae, the dense pits of cells at the back of the eyes that confer visual acuity. While foveae are lacking in most mammals, they’re present in lizards—making them intriguing candidates for studying the genes that impact foveal function.
There was just one problem: Reptiles aren’t easy to genetically manipulate. In other common laboratory animals, like mice and zebrafish, a tool called CRISPR has made DNA editing a breeze. The procedure typically involves injecting freshly fertilized eggs with gene-editing machinery, creating a change that would propagate when the cell divided.
But a few quirks ruled out that particular strategy in these lizards. Female anoles can store sperm for many months before fertilizing their eggs internally, making it difficult to time the introduction of the CRISPR cocktail. Anole fertilization also cues the formation of a soft, delicate eggshell that’s hard to penetrate without damaging the embryo.
That meant Rasys and her advisor, Doug Menke, had to get creative. So they decided to shift the injection back a developmental step, targeting eggs still maturing in the females’ ovaries. “At this point, they’re just hanging out in the lizard, waiting to be fertilized,” Rasys says.
Thanks to CRISPR gene editing, one of these brown anoles isn’t exactly brown. Image Credit: Courtesy of Ashley Rasys, University of Georgia
The procedure took more than a year to perfect. But in the fall of 2018, Rasys, Menke, and the rest of their team hatched the world’s first gene-edited non-avian reptile: a red-eyed albino anole with near-transparent skin. According to the team’s study, published today in the journal Cell Reports, its birth marks a breakthrough for the field of developmental genetics, and hints that similar experiments may be possible in some of the other 10,000-plus species of non-avian reptiles that scuttle the Earth.
“This technology is really important and exciting,” says Martha Muñoz, an evolutionary biologist and anole researcher at Yale University who was not involved in the study. “This really opens up the door for other groups to think outside of traditional model organisms [like mice and zebrafish]…the sky’s the limit.”
With albinism in mind, Rasys and her colleagues set out to mutate the anoles’ tyrosinase gene, which governs pigmentation and has been linked to foveal function in humans. Manipulating this gene, Rasys explains, also made for an easy marker of success: If the procedure ended up generating albino anoles down the line, they’d be pretty tough to miss.
After rounding up 21 female brown anoles from the wilds of Orlando, Florida, the researchers gently anesthetized the lizards and opened them up. In anoles, the ovaries are transparent, making it easy to eyeball their contents “like a train of developing eggs,” Menke says.
The team selected 146 of these growing eggs and injected them with the classic CRISPR recipe: a pair of molecular scissors and a series of DNA-binding “guides” that would show them where to cut—in this case, the tyrosinase gene.
The researchers then had to wait another three months or so for the females to fertilize and lay the eggs. And even when this generation hatched, they thought there’d likely be more work to do, Rasys says. Since the CRISPR concoction had been delivered to eggs that were later fertilized by unaltered sperm, the offspring were expected to be hybrids—half edited, half unedited. These lizards then would need to be bred further to yield albinos, which must inherit the mutation from both parents for the trait to manifest.
But as Rasys watched her first clutch of gene-edited eggs grow, she noticed something strange. About a week before they were due to hatch, most of the embryos had darkened from pink to gray—an indication that they’d started producing pigment. A handful, however, retained their initial pallor, even as they continued to swell in size.
A few days later, Rasys arrived at the lab to find a newly-hatched, inch-long albino, stretching its ghostly pink legs. “It was so exciting to see it,” she recalls. “I thought, ‘It’s so cute.’”
In total, four out of the team’s 146 CRISPR-injected embryos were obvious albinos, surprising the entire team. There’s no way to know exactly what happened, but Menke’s leading theory is that the CRISPR components remained active in some of the eggs long enough to work their magic on both the maternal and paternal copies of the tyrosinasegene.
In their native habitat, brown anoles can blend in pretty easily with tree bark. Such is not the case for albino mutants produced by CRISPR gene editing. Image Credit: Courtesy of Ashley Rasys, University of Georgia
Genetic screening revealed another five embryos to be the half-edited hybrids the team had initially expected. And when the researchers partnered one of these CRISPR mutts with an unmanipulated mate, the mutation was passed on to some of the pair’s offspring, suggesting the edited gene was heritable.
There’s still plenty of tinkering to do, Menke says. As they report in the study, the team’s gene-editing success rate was around 6 percent—a figure that pales in comparison to the near-perfect efficiency rates that have been reported in zebrafish and mice.
But just showing gene-editing is possible in this system is a big deal, says Ambika Kamath, a behavioral ecologist at the University of California, Berkeley who was not involved in the study. Albinism implications aside, anoles have long been studied by evolutionary biologists and ecologists. In their native Caribbean, the lizards have split into many lineages, but understanding this diversification “has primarily been a historical science…involving stitching together patterns that happened a long time ago,” Muñoz says. “By extending CRISPR to Anolis, we can now mechanistically test some [evolutionary] hypotheses.”
As more applications surface, however, “we don’t want to be releasing CRISPRed lizards into the wild willy-nilly,” Kamath says, without a better understanding of how these sorts of introductions would affect the population at large.
An albino anole produced by CRISPR gene editing (left) next to a typical brown anole. Image Credit: Courtesy of Ashley Rasys, University of Georgia
And it might be more than lizard lives at stake. Menke thinks the team’s technique is likely to work in a variety of reptiles, many of which share the anole’s mode of reproduction. There’s even the possibility, he says, that the method could be adapted for birds, which are cut from the same evolutionary cloth. Scientists have hatched CRISPant chicks in the past, but as in lizards, bird embryos are hard to pinpoint at the single-cell stage, making current editing procedures complex and laborious.
Carolyn Neuhaus, a bioethicist at the Hastings Center who was not involved in the study, cautions that as CRISPR continues to be debuted in more and more organisms, the how, when, and in whom of gene editing will need to remain transparent. Though many experiments—including the ones in this study—have the potential to advance science and human health, she says, technology like this shouldn’t be used in a new species “just because it’s there.”
“We rely on scientists to create accurate and reliable knowledge, and that’s a huge responsibility,” she says. “With the CRISPR craze…I just hope it happens as mindfully and carefully as possible.”
When I first encountered Anolis baleatus, this Hispaniolan crown-giant was mostly an inconvenience. At the time I was gathering data for my doctoral thesis by cycling preserved anoles through a µCT-scanner. Most of the adult specimens of A. baleatus were just too large to easily fit into the scan chamber, so it took a lot of patience and creativity to acquire any decent images of the appendicular girdles, which are the body parts I was interested in.
During that process I also acquired radiographic images of the head skeleton, and found unusual patterns of crenulation in this species. The cranium of Anolis baleatus displays a great degree of seemingly asymmetrical (or at least somewhat irregular) ornamentation across its dorsal surface. This is especially pronounced on the prefrontal and frontal bones, and completely obscures all superficial distinction between them in adult lizards. In adults, cranial ornamentation is also borne by the paired nasals, maxillae, and postorbitals, and the parietal (see figure).
Both Steven Poe (1998) and Susan
Evans (2008) mentioned this ossified garnish, but a thorough account of their
variation among anoles remains absent from the primary literature. Richard
Etheridge and Kevin de Queiroz (1988) were probably the first to report on skull
ornaments in anoles (as part of a discussion of several iguanian lizards with
similar cranial adornments), and remarked that the distribution patterns of
dermal rugae may reflect those of the topographically associated epidermal
scales.
Overall, this ornamentation appears
to be relatively uncommon among anoles, especially to the degree expressed in Anolis baleatus (and several other
crown-giant ecomorph anoles). Considering the osteologically robust appearance
of crown-giants, even at early stages of ontogenetic development, this gives
rise to questions regarding the development of these ornamental patterns.
Thanks to the collection efforts of Luke Mahler (University of Toronto), and a
postdoctoral position in his lab, I was able to acquire CT-image data
representing an ontogenetic series of this species, ranging from very young
juveniles to skeletally mature adults.
While parts of the paired frontals
of juveniles are covered in modest eminences, prominent cranial ornamentation
is absent from small specimens (see figure). Likely, growth of these ornaments
begins very late during ontogenetic development. Ornaments on the prefrontals
and parietal are only evident in specimens that, to the best of our judgement, are
approaching sexual maturity. We looked at fifteen specimens per sex,
representing a range of juvenile and subadult sizes, and this general pattern is
consistent throughout the image data. Schwartz (1974) inferred that anoles in
the ricordii group reach sexual
maturity between 100 and 110 mm snout-vent length (SVL), and we observed the
first prominent ornaments at sizes between 90 and 95 mm SVL. Assuming that
differences in size directly represent ontogenetic growth, these findings imply
that Anolisbaleatus starts to grow elaborate ornamentation as it approaches
sexual maturity, and that expansion and growth of these ornaments then
continues into skeletal maturity. Interestingly, both males and females appear
to develop them at roughly the same body size.
The function and evolutionary cause of these structures remain unknown, and these are questions we are currently investigating. Body size is an important correlate for the occurrence of cranial ornaments, but these structures may also conceivably play roles in defense, feeding, or intraspecific agonistic interactions. Stay tuned!
Videos
A. baleatus, female, 55 mm SVL
A. baleatus, female, 65 mm SVL
A. baleatus, female, 96 mm SVL
A. baleatus, female, 126 mm SVL
References
Etheridge, R. & de Queiroz, K. (1988): A phylogeny of Iguanidae.─ [In:] Estes, R.D. & Pregill, G.K. (eds.): Phylogenetic Relationships of the
Lizard Families: Essays Commemorating Charles L Camp, 283-367; Stanford:
Stanford University Press.
Evans, S. (2008): The skull of lizards and tuatara.─ [In:] Gans, C., Gaunt, A.S. &
Adler, K. (eds.), Biology of the Reptilia, vol. 20:1-347;
Society for the Study of Amphibians and Reptiles, Ithaca, New York.
Poe, S. (1998): Skull characters and the cladistic relationships of
the Hispaniolan dwarf twig Anolis.─ Herpetological Monographs, 12:192-236; The Herpetologists’ League.
Schwartz, A. (1974): An analysis of variation in the Hispaniolan
giant anole, Anolisricordi Dumeril and Bibron.─ Bull. Mus. Comp. Zool., 146:89-146.
Emma Higgins presenting her work at Island Biology 2019
The third meeting of the nascent Society for Island Biology took place recently in stunning La Reunion in the western Indian Ocean. Conference goers were treated to a wonderful venue at the Université de La Réunion in St. Denis, whose campus looks out down the gentle slope to the open sea. Four hundred attendees from around the world reinforced what we already knew— that island biology as a study attracts a large number of researchers from very diverse fields of study. The conference organizers also are leading the way on making our meetings in remote locations more responsible; using live streaming of the sessions meant that some interested scientists could skip the travel and stay home to watch the sessions. But the real asset was that the organizers calculated the air travel carbon footprint for all attendees, finding that ~30 hectares of forest would need to be planted to offset the carbon emissions. Happily, that is exactly what they did! The conference organizers and hosts, in partnership with communities and other organizations, committed to reforesting exactly that amount in La Réunion and Mauritius.
OK, now on to the anoles! Well, given the remoteness of the meeting location (nearly the antipode of the Caribbean*) perhaps it is not too surprising that few anologists attended. But I am happy to report that Emma Higgins did, and gave an excellent presentation on her work with anoles on the island of Utila, one of the Honduran Bay Islands.
Emma is a 3rd year PhD student in Adam Algar’s lab at the University of Nottingham, where her thesis is focused on using emerging technology to study lizard thermal biology under changing conditions (think development, climate change, and species introductions). When I say using emerging technology, I mean using #allthetech; Emma uses 3D printing, drones fitted with thermal cameras, Sentinel satellites, and LIDAR to generate her data! Her motivation follows from asking what factors control the abundance, distribution, and microhabitat of anoles on Utila, and whether these variables might be better estimated at extremely fine scales using emerging technology.
A bit of background, there are four species of anoles on Utila, including the endemics A. bicaorum and A. utilensis. An additional native species is A. sericeus, which also occurs elsewhere in the Bay Islands as well as on the mainland. The fourth species is everyone’s favorite— A. sagrei! Utila has experienced a surge of development in the last 10 years, with new roads and development going up faster than conservationists can keep track of. This is a major threat to the island wildlife, which includes an endemic iguana known as The Swamper (Ctenosaura bakeri) which favors the dwindling mangrove forests.
Emma’s work involves collecting data both at anole-level as well as above the canopy. She uses a DJI Phantom 4 drone platform fitted with a near-infrared camera to estimate a normalized difference vegetation index (NDVI, a measure of “greenness”) of the forest canopy across habitats, and found that just NDVI explains 28% of the spatial heterogeneity of lizard operative temperatures (in a mixed-model framework). This suggests that her drone can identify suitable thermal environments for lizards from above the canopy. I should mention that her resolution here is 4cm/pixel! She plans to zoom out to space and test whether similar imagery from the Sentinel 2 satellite will also be useful. Below the canopy, Emma is using LIDAR to simultaneously conduct forest shade modeling (for super fine-scale temporal variation in thermal microhabitat). LIDAR also detects perch availability, as it detects tree trunks very well. Emma also uses 3D printing to produce hundreds of anole models, each fitted with an iButton® temperature recorder and placed on perches in the forest. Each lizard print takes 52 minutes, so Emma ended up taking the printer to her flat to print 24/7 in preparation for her field season!
I should mention that Emma was joined at the conference by two other excellent scientists— lab mate Vanessa Cutts and fellow Utila lizard biologist Daisy Maryon, both of whom won awards for their posters at the conference!
Stay tuned for the announcement for the 2022 Island Biology meeting, to be held on either Mallorca in the Balearic Islands or Wellington, New Zealand. Also stay tuned for Emma’s results; we look forward to hearing more about her work!
Climate change is negatively affecting Squamates all over the world, and the perspectives for the next 50 years are worrisome. Although more than 200 studies were published in the last 20 years on this subject, only 23 present some information on Anoles, and none of them focused on population dynamics – until now. In the most recent issue of Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, Diele-Viegas et al. evaluated the possible effects of climate change on two populations of three species of lizards from the Brazilian Amazonia, including Norops fuscoauratus.
The idea of this research came from the first author, Dr. Luisa, during her PhD classes back in 2017. She was taking classes on population ecology and had the idea of to combine environmental thermal adequacy with the b-d model, which considers survival and reproductive rates to calculate population dynamics andevaluate the impact of climate change on population dynamics of her study objects, Amazonian lizards. She thus started to search in the literature for articles that focused on something like what she was thinking, and discovered that this had not been done before, at least not for squamate reptiles. She also noticed that data on Amazonian lizards’ life history is very scarce on literature, which could be a deterrent to her getting the job done. After speaking with her advisors (Dr. Fred Rocha and Dr. Fernanda Werneck), Luisa decided to estimate a tolerance index considering the relationship between the upper temperature limit of the animal activity restriction and the environmental temperature of the microhabitat in which this animal occurs and used this index as an approximation of the populations’ survival rates. This allowed her and her advisors (both coauthors of this article) to circumvent the data scarcity and put Luisa’s idea into practice, leading to the publication of this article.
Considering the tolerance index mentioned, the authors predicted that Norops fuscoauratus is likely to became locally extinct at one of the evaluated sites, Reserva Ducke, which is an almost urban reserve in the Amazonian heart. The hatchlings’ tolerance to environmental change was considered the most sensitive vital trait evaluated, highlighting the species’ vulnerability. Also, considering that the response to selection is likely to be too slow in anoles, an evolutionary change in N. fuscoauratus is unlikely to occur considering current rates of environmental change, which reinforces the species’ vulnerability at local scale. This study represents the first effort to evaluate population sensitivity to climate change among reptiles. The authors highlight the need for new studies focusing on this subject to provide theoretical and empirical basis for biologically informed conservation strategies and actions to avoid the extinction of several species around the world. Also, Luisa highlights that, as scientists, we should always value our ideas – our curiosity is the fuel that moves science into progress.
We’ve had previous posts on parasites of anoles (for example, here), but now a new paper in Herpetology Notes adds to the literature on this topic, reporting blowfly parasitism of Anolis parvauritus from northwest Ecuador.
Imaging the surface topography of lizard skin using gel-based stereo-profilometry. (Baeckens et al. 2019)
The skin surface structure of lizards varies greatly among species, likely because it plays a key role in a range of tasks, such as camouflage, locomotion, self-cleaning, mitigation of water loss and protection from physical damage. Yet, we still know remarkably little about how variation in skin surface structure translates to functional variation. Part of this gap in our understanding can be traced back to the lack of means to perform high-throughput and detailed analysis of the 3D anatomy of lizard skin in a non-destructive manner.
To tackle this hiatus, I was fortunate enough to be able to round up a great team of scientists and to start exploring the possibilities of a new imaging technique, termed gel-based stereo-profilometry. In this approach, a deformable transparent gel pad with one opaque surface is pressed onto the object of interest, creating a surface impression. While the gel pad is still in contact with the object, a series of photographs from six different illumination angles are acquired, and a topographical 3D map of the surface is created by merging the acquired images using specialized surface analysis software.
Using this technique, we successfully imaged the 3D skin surface structure of Anolis cristatellus specimens in great detail (pixel resolution of 0.86 µm) and in a short-time frame (average acquisition time of imaging and digital reconstruction combined was 90 seconds). In our new paper, we demonstrate that this technique is exceptionally useful for the rapid 3D structural characterisation of lizard skin surfaces without any specimen preparation, permitting 3D visualization in situ and even in vivo. This technique opens exciting new avenues for investigating structure–function relationships in lizard skin.
In addition to the ability to quantify the micro- and macro-structural details of lizard skin, the 3D data sets acquired using gel-based stereo-profilometry can be directly converted into surface meshes, which can in turn be 3D printed. These tangible models can then be directly employed for studies to investigate the role of scale geometry on animal–substrate interactions, or enlarged for educational purposes to illustrate key differences between different squamate taxa.
3D print of lizard skin.
Baeckens S, Wainwright DK, Weaver JC, Irschick DJ & Losos JB (2019) Ontogenetic scaling patterns of lizard skin surface structure as revealed by gel‐based stereo‐profilometry. Journal of Anatomy235, 346–356.
Transposable elements are DNA sequences that move around in the genome. Do they also play any roles in evolution and development? I answered this question by looking at our favourite group of animals – lizards – and found some surprising answers. My most recent paper in Evolution Letters is the last of a trilogy of papers – one, two, and three – that reveal that Anolis lizards, by breaking the rules, allow us to link TEs to speciation and evolvability.
Mobile DNA sequences – transposable elements or TEs for short – are found in the genome of virtually all organisms. As their name implies, TEs can cut or copy themselves from one location in the genome to another. This can wreak havoc as insertion of TEs may interfere with gene regulation or in fact knock out entire genes. Cells therefore have mechanisms that prevent TEs from jumping, including DNA methylation and other epigenetic tools. Thus, TEs are not roaming freely through the genome, but are restricted from entering functionally important parts. Preventing TE invasion is particularly important when genes are regulated through spatial proximity to each other. The textbook example of this situation are the Hox genes, which are the key players in embryonic development with an ingenious mode of action: Hox genes are arranged in tight clusters and their position in the cluster defines their time and space of expression, and thus their effect on the patterning of the early embryo. It is therefore fitting that Hox gene clusters of mammals and other well-studied vertebrates have been found to be almost completely free of TEs. My new study reveals that Anolis lizards have broken this paradigm. Moreover, the invasion of TEs into Hox clusters of Anolis lizards can be linked to aberrant gene expression and increased rates of speciation.
Ever since the discovery of TEs, people have speculated about their evolutionary implications. One possible consequence of high TE activity is structural genomic variation. This may accelerate genomic incompatibility between populations, effectively making TEs engines of speciation.
Occasionally, TE insertions may also generate phenotypic novelty. As noted above, some genes are regulated through their proximity to other genes, which means that invasion of TEs can change expression of a number of genes simultaneously. Furthermore, since jumping TEs often drag along neighbouring genomic regions, they can translocate regulatory sequences that cause genes to be expressed in new cell types or at different stages in development.
While these are good reasons to expect TEs to promote evolution, examples are few and their role often appears idiosyncratic. An excellent group for a more systematic survey of TE-driven diversification is squamate reptiles, a group that includes lizards and snakes. Squamate genomes do not only appear particularly rich and variable in TEs, but their body plan is also highly malleable. Illustrative examples include the adaptive radiation of Anolis lizards and the repeated evolution of limbless and elongated bodies.
I decided to study how TEs have shaped the genomes, and in particular, the Hox clusters, of squamates. My first surprise was to discover that lizards possess more Hox genes than all other tetrapods since they retained some genes that other lineages have ditched. The second surprise came when I looked at the TE content of Hox clusters. Despite the high TE content in their genomes, squamates follow other vertebrates in generally protecting their Hox clusters from TEs. But there was one exception: I found massive invasion of TEs in the Hox clusters of two out of three Anolis species, with TE contents almost as high as the average place in the genome.
The relationship between TE content in Hox clusters relative to genome-wide TE content in squamate reptiles. All lizards and snakes restrict TEs from their Hox clusters down to roughly half the genome-wide average. However, two Anolis species show Hox clusters that are invaded by TEs, while a third Anolis species (black circle) follows the general trend.
Why in Anolis? Anolis lizards are famous in evolutionary biology due to their adaptive morphological radiation involving high rates of speciation – amassing close to 400 species. In a previous study (explained in a previous Anole Annals post) I showed that Anolis lineages with more speciation events in the past have more TEs in their Hox clusters. My new genome-wide study reveals that this signature of speciation is indeed pronounced in Hox clusters: only the two Anolis species from amply speciating lineages exhibit unusually TE-rich Hox clusters, while a third species (Anolis auratus, black circle in figure above) follows the norm and keeps its Hox clusters relatively free from TEs. Looking in detail at genome-wide TE landscapes of these three Anolis species, I discovered that the two species with TE-rich Hox clusters had a larger population of young, more active TEs in their genomes. In addition, the inferred timing of peak activity of these TEs broadly coincided with past speciation events.
These
results suggest that – during speciation events – TEs are unusually active and
proliferate throughout the genome. As a result, even crucial regions such as Hox clusters become invaded.
Subsequently, TEs are removed from Hox
clusters by selection until a ‘healthy equilibrium’ of TE content relative to
the genome-wide TE content is reached. This equilibrium appears highly
conserved as the Hox clusters of
almost all lizards and snakes contain close to 50% of the global TE content.
This proposed model generates a number of predictions that can be tested with
genomic data from lineages with variable rates of speciation.
How then do some Anolis species cope with having their Hox clusters invaded by TEs? Clearly, the inflation of Hox clusters – increasing the distance between genes – does not disrupt the patterning of the early embryo. Genes located at one end of the cluster remain expressed early in the head of the embryo, while genes located at the other end are expressed late in the tail. However, the successive activation of Hox genes predicts that disruption, if occurring at all, should be most pronounced towards the end of the Hox clusters. I found that this indeed is the case: one out of four Hox13 genes showed aberrant expression in the two Anolis species with TE invaded Hox clusters, but this gene was expressed as ‘normal’ in other Anolis and more distantly related lizards.
Expression patterns of the posterior Hox gene HoxD13 are showing variation between species with low and high TE content in their Hox cluster: while expression in limb buds is conserved, expression in tail tissue (black arrows) is missing in species with high TE content in their Hox clusters (A. sagrei and A. carolinensis).
My study reveals that, despite being THE textbook example of our conserved developmental toolkit, Hox genes can be tinkered with. What is more, the TE invasions of Hox clusters appear to be intimately linked to diversification. Now that Anolis lizards have shown us that it can happen, perhaps they can also show us why it happens and how.
Feiner N.
2019 Evolutionary lability in Hox cluster structure and gene expression in
Anolis lizards. Evol Letters. https://doi.org/10.1002/evl3.131
Feiner N. & Wood N.J. 2019 Lizards possess the most complete tetrapod Hox gene repertoire despite pervasive structural changes in Hox clusters. Evolution & Development. 2019;21:218–228 https://doi.org/10.1111/ede.12300
Feiner N. 2016 Accumulation of transposable elements in Hox gene clusters during adaptive radiation of Anolis lizards. Proc. R. Soc. B 283: 20161555. http://dx.doi.org/10.1098/rspb.2016.1555
