Reptiles are important models for studying phenotypic plasticity because they are quite sensitive to environmental conditions experienced during development, and naturally experience a broad range of environmental conditions during this time. There are a number of interesting biological traits of reptiles that make them particularly interesting models for research on phenotypic plasticity. For example, temperature-dependent sex determination, where incubation temperature irreversibly determines sex during development, is a fascinating polyphenism that is widespread among reptiles. Additionally, the sensitivity of developing embryos to environmental factors (like temperature or hydric conditions) has been implicated as a primary force behind the evolution of various maternal reproductive strategies including viviparity or nest-site choice. Accordingly, there exists a rich literature documenting the effects of embryonic environments on the phenotypes and survival of reptiles during early life.

A major shortcoming of this literature, however, is that the vast majority of studies terminate shortly after hatching. That is to say, our understanding of phenotypic plasticity in reptiles is biased towards phenotypes apparent in early life. Yet we rarely know if these phenotypes are persistent or transient, or if conditions experienced during development have delayed effects, or effects on reproductive traits. Together with my coauthors Fred Janzen and Dan Warner, we have recently published a review that discusses the shortcomings of terminating plasticity studies during early life, and highlights the important contributions that have come from the relatively few long-term studies in existence. We call for studies that specifically look at the effects of embryonic environments on adult phenotypes, and offer a number of approaches to address this problem.

Enter the Anole. There are a number of anole species that are very tractable models for experiments addressing the influence of embryonic conditions on adult phenotypes, reproduction, and survival. Anolis sagrei, for example, readily breeds in captivity, is highly fecund, and reaches reproductive maturity in a matter of months. Anoles are tractable for detailed assays on reproduction in the laboratory, and raising anoles from egg to adult in the lab is entirely feasible under reasonable timelines. Though it is no small task, it is very possible to incubate hundreds of anole eggs under different conditions, mark the babies upon hatching, and then release them into the field in a place where migration is not possible (like a small island). Periodically sampling that island can give insights into the effects of incubation conditions on adult phenotypes and survival under natural conditions. Phil Pearson and Dan Warner recently published a paper using such a methodology.

Of course, we also encourage such long-term studies for everyone working with reptiles, even those that are very long-lived (like turtles). But there is a dearth of studies that address the effects of embryonic environments on adult phenotypes in reptiles, and I hope that anoles are a key group that help address this shortcoming. So let’s get after it, Anolologists!!!!

The Latest in Anole Systematics and Taxonomy

By Jonathan Losos

On October 1, 2018

In All Posts

From Nicholson et al. (2018)

Anole Annals has dropped the ball on staying current with recent papers about the systematics and taxonomy of anoles. So, here are two highlights over the last two years.

Last year, Steve Poe and 10 co-authors from five countries published a phylogeny including all species of anoles based on all available data. It was a tour de force and the paper was far-ranging, discussing the many implications of their phylogeny for anole evolution, biogeography and other topics. Among other topics, the paper sank a number of anole species, presented a phylogenetic taxonomy of anoles and argued that this taxonomy solves the problems posed in the debate on generic classification of anoles in the traditional Linnean taxonomic framework.

Here’s the abstract from that paper, published in Systematic Biology:

Anolis lizards (anoles) are textbook study organisms in evolution and ecology. Although several topics in evolutionary biology have been elucidated by the study of anoles, progress in some areas has been hampered by limited phylogenetic information on this group. Here, we present a phylogenetic analysis of all 379 extant species of Anolis, with new phylogenetic data for 139 species including new DNA data for 101 species. We use the resulting estimates as a basis for defining anole clade names under the principles of phylogenetic nomenclature and to examine the biogeographic history of anoles. Our new taxonomic treatment achieves the supposed advantages of recent subdivisions of anoles that employed ranked Linnaean-based nomenclature while avoiding the pitfalls of those approaches regarding artificial constraints imposed by ranks. Our biogeographic analyses demonstrate complexity in the dispersal history of anoles, including multiple crossings of the Isthmus of Panama, two invasions of the Caribbean, single invasions to Jamaica and Cuba, and a single evolutionary dispersal from the Caribbean to the mainland that resulted in substantial anole diversity. Our comprehensive phylogenetic estimate of anoles should prove useful for rigorous testing of many comparative evolutionary hypotheses.

This year in Zootaxa, Nicholson, Crother, Guyer and Savage published a paper following on Poe et al.’s. The paper does not have an abstract, but the title says it all: “Translating a clade based classification into one that is valid under the international code of zoological nomenclature: the case of the lizards of the family Dactyloidae (Order Squamata).” In short, the article argues that the traditional Linnean classification system is not going away any time soon, and that the clades recognized by Poe et al. are easily translatable into genera in the traditional system, as revealed in the figure above.

In a post just published, an administrator for Wikispecies asks if they should follow this proposed reclassification of anoles. If you have comments, please make them on that post.

Nomenclature of Dactyloidae: Revisit and Opinions Wanted

By Scott Thomson

On October 1, 2018

In All Posts, New Research

Hi everyone. I recently received and have to determine what to do with the following paper (editor’s note, for background, see this recent post):

Nicholson, K.E., Crother, B.I., Guyer, C., & Savage, J.M. 2018. Translating a clade based classification into one that is valid under the international code of zoological nomenclature: the case of the lizards of the family Dactyloidae (Order Squamata). Zootaxa 4461 (4): 573–586. DOI:10.11646/zootaxa.4461.4.7

As an administrator and bureaucrat at Wikispecies I have to decide how to proceed with this group of reptiles. I have made a tentative start here but please realize this is a simple start easily undone.

I recall the last time this came up, in 2012. I joined the discussion at the time. However, despite my comments at the time, I did not follow splitting the genus up then. In the end, my view is for stability and consensus. By stability, I mean the actual meaning of stability under the ICZN code, which does not apply here. But consensus could.

Why is this paper different? Well, first up, last time it was a PhyloCode paper and as such is relatively easy to ignore, as it does not submit to the rules of nomenclature. However, this time it is an ICZN compliant paper so you cannot ignore it. As stated many times, names are to considered as valid on publication or refuted–there is no ignore. So the above paper may be refuted, but not ignored.

Last time, many argued that the genus is monophyletic. This is not really an argument against splitting. It’s a position statement. The order Testudines is also monophyletic, should every turtle species (275 living species) all go back into the genus Testudo? The current genera or lack of them present are only a reference to the history of research. It does not mean it is the most suitable arrangement.

More importantly is diagnosibility. Can the new proposed genera and their inherent species be adequately diagnosed? This is a more important question.

Note that a genus with some 500 species is generally considered too big. Many writers over the years have deemed between 100-200 species about the maximum size wanted. However, this does still need to address the previous point on diagnosibility.

Another point people brought up last time was stability. Well, stability actually refers to the mononomial and whether a name can be replaced by a forgotten name. It is used as a reason to reverse priority. This is the code purpose of stability. Note that the combination first up does not have to be stable, and second is a taxonomic decision, not a nomenclatural one. Hence outside the code.

So what I am after: Basically I want to see through any commentary if the people who work on anole’s are likely to use this new nomenclature. If they are, I will adopt it at Wikispecies. That will require the moving and reorganisation of some 550 pages. I do not take that on lightly. Hence I am asking you, the people who work on anoles, first. My decision will be based on the answers I get. I do not work on anoles. I am a turtle and tortoise specialist. But I do have a job to do at Wikispecies.

For your information, I have discussed this briefly with Peter Uetz at Reptile Database also. He also was not sure what to do, but remembered the last time it came up here. So I am reaching out to all of you on this issue. I am after consensus, not stability. As I said, stability does not apply here. But I will say that to reject the nomenclatural proposals of Nicholson et al. (2018) does require a refutation. They have presented to science in good faith in a very good journal, Zootaxa. We cannot ignore this and as a taxonomist, I will not.

In advance, I thank everyone for their comments. I think this issue needs to be openly debated.

Caption Contest: a Festive Anole, a Keypad, and a Card Swipe Add up to…?

By Jonathan Losos

On September 28, 2018

In All Posts

Photo by Steen Christansen.

The photo above was taken by Steen Christansen and brought to our attention by the anole-loving, neuroethology-studying, mayor of South Miami, Philip Stoddard (thanks, your honor). It’s a brown anole, aka, a festive anole.

Who’s got the best caption?

Are Leopards a Type of Anole? They Have Dewlaps!

By Jonathan Losos

On September 19, 2018

In All Posts

Panthera, a fabulous group that works for the conservation of big cats, puts up fascinating posts on their website. This one first appeared in mid-September 2018.

Field Notes

Panthera’s Blog

Lions Have Manes; Leopards Have…Dewlaps?

Johanna Taylor, M.Sc.

Research Associate

September 4, 2018

Mature male leopards are at least 60% larger than females, and broader, with larger chest girths and longer, bulkier heads. But there’s something else you may notice that sets males apart: Their thick necks can be adorned with a loose flap of skin hanging underneath. These skin folds are called dewlaps.

Dewlaps are observed in a variety of species throughout the animal kingdom, most notably in certain birds, lizards, and hoofed mammals. In mature male leopards, a well-developed dewlap is prominent and one of the best ways to determine age.

PANTHERA — This male leopard’s dewlap steals the show as he struts past the camera trap.

However, the function of leopard dewlaps remains unknown and largely unexplored. Since they are a sexually-dimorphic feature—meaning only one sex exhibits the trait—scientists speculate that this enigmatic ornament is linked to sexual selection or male fitness.

Of another conspicuous, sexually-dimorphic feature among cats—the mane of a male lion—Charles Darwin postulated: “[It] forms a good defense against the one danger to which he is liable, namely the attacks of rival lions.”

The idea behind this hypothesis is that adaptations such as body armor or weaponry, like antlers on male deer, increase the ability of males competing against other males. Most often the winner is the lucky male who gets to mate with the female and pass on his genetics.

On the other hand, results of more contemporary research indicate that injuries sustained during fights between lions don’t appear to differ between maned and non-maned sub-adult males or even females. This outcome suggests that the mane/neck area is not a high-target region during fights. Instead, observations find that the back and hindquarters seem to be the targeted areas during confrontations.

PAUL FUNSTON/PANTHERA — Male lions brawling.

Many biologists believe that a male lion’s mane serves as an indicator of sexual fitness. Studies using life-sized toy lions sporting contrasting mane colors and lengths found that male lions were more likely to approach the imposters with lighter, shorter manes, while female lions were drawn more to models with darker manes. These results suggest that males with shorter, blonder manes are less intimidating to rival males, and thus perceived to have lower overall fitness.

We can apply this theory to hypothesize about the functions of a dewlap on a male leopard. However, fights between elusive male leopards are rarely observed and documented by scientists. This makes it difficult to confirm if the leopard’s dewlap has any defensive functions. Likewise, it’s hard to determine if it serves a similar purpose to that of a large, dark mane in sexual selection, as an indicator of fitness and longevity.

CRAIG TAYLOR — Dewlaps: key for defense or wooing females?

Another speculation about the dewlap is that it simply makes mature leopard males appear larger and more intimidating to rivals. Typically, as a male leopard ages, his dewlap enlarges. Mature males hold wider territories and have more opportunity to mate with females.

This also means that there are more opportunities to come into conflict with other males seeking the same territory and females. It would be ideal to avoid physical confrontation, and potential injury, by being able to display fighting capability and fitness, thus deterring potential rivals without physically engaging them. Fighting with other males has potential to lead to injury or death. Even the smallest injuries can fester and inhibit hunting abilities, leading to a decline in body condition that could result in death.

Younger and smaller males may be dissuaded from engaging in a fight with a larger male sporting a more pronounced dewlap. Alternatively, estrous females may select a male with a larger dewlap, perceiving him to have greater fitness than his peers. As of yet, no studies have been conducted to determine if dewlap size correlates with testosterone levels or is linked to reproductive success as an indicator of longevity.

Photo of a Curly-Tailed Lizard Eating an Anole

By Jonathan Losos

On September 17, 2018

In All Posts

AA reader John Thomas questions whether curly-tailed lizards (Leiocephalus carinatus) actually eat anoles, based on his own observations in Florida (see comments on this post). Let’s see proof, he says! Bob Powell rightly points out the scientific literature on this topic and refers to the Schoener et al. paper from 1982 on the diet of curly-tails in the Bahamas. And here’s a photo from that paper! I’ve seen at least two other photos people have taken of such predation–anyone got more?

Request for Anole Habitat Data

By Joe Veech

On September 16, 2018

In All Posts

Anolis chlorocyanus in its habitat. Photo by Jonathan Losos.

Hello All, I’m an ecologist that studies species-habitat relationships, among various other things. I’m presently working on a book that will describe 10 methods of statistically analyzing habitat and also try to provide a conceptual unification of how capital-E ecologists and wildlife ecologists have traditionally viewed habitat and its importance. I’m looking for real empirical datasets to analyze. Data for Anolis species would be great given their importance in testing ecological and evolutionary theory, and their specific habitat preferences. The dataset could be for a single species or multiple species. It could have either microhabitat data (e.g., observations of Anolis individuals at different perch heights and stem diameters, tree species perched on) or macrohabitat data (e.g., amount of vegetation or canopy cover within a 10 m diameter of lizard location). Really any kind of environmental or habitat data would work. Ideally, the dataset would also have environmental data from locations where lizards were not observed (so-called absence points). If you have a dataset that you’d be willing to share please contact me at joseph.veech@txstate.edu. THANKS!

Invasive Species Removal Aids Endemic Lizards on Remote Caribbean Island

By Jonathan Losos

On September 9, 2018

In All Posts

Great news from the conservation group Island Conservation, where this post appeared. Check out all their great work! Also, see Anole Annals posts on this project.

31 JUL 2018

ISLAND CONSERVATION

NEWS, PRESS RELEASES & EVENTS

Magical Transformation Spells Brighter Future for Redonda’s Fantastic Beasts

Campaign to remove invasive rats and goats from Redonda has yielded spectacular results for the island’s unique and special wildlife.

Media Release

If conservationists had waved a magic wand, the results could hardly have been more spectacular. Within 12 months of starving goats and thousands of ravenous rats being removed from Redonda¹, this remote Caribbean island has witnessed a miraculous transformation.

Since the ambitious restoration programme² was rolled out, the rock of Redonda has been transfigured from an inhospitable lunar landscape to a greener haven. More importantly, the unique plants and animals native to this isolated, uninhabited outpost of Antigua and Barbuda are making a rapid recovery after being freed from an alien invasion.

The evocatively named Redonda ground dragon, a rare black lizard found nowhere else on the planet, has doubled in number – just one of the many fantastic beasts that have been pulled back from the brink of extinction by removing the predatory black rats and plant-devouring goats.

Redonda before restoration. Credit: Ed Marshall

And it doesn’t stop there. In less than a year, numbers of the equally rare Redonda tree lizards have tripled, hundreds of new trees have sprung up, land birds have increased tenfold, and the island’s globally important seabird colonies – including magnificent frigatebirds and several booby species – are having their best breeding year on record.

Speaking on behalf of the Department of the Environment, Dr Helena Jeffery Brown said:

The Government of Antigua and Barbuda considers the return to life of Redonda as a shining beacon in our collective efforts towards ecosystem restoration and biodiversity conservation that will bring us another step closer to attaining some of the Aichi Biodiversity Targets³.”

The project’s coordinator, Shanna Challenger, of the Environmental Awareness Group (EAG) and Fauna & Flora International (FFI), added:

This has been the opportunity of a lifetime – witnessing an island be reborn. Changes forecasted to happen in five years occurred within months. Our conservation efforts really show the benefits of invasive species removal on Caribbean island ecosystems.”

View heading north post-eradication Nov 2017. Credit: ShannaChallenger_FFI

“Blood, sweat, toil and teamwork”

In the field of conservation, where successful outcomes can take years if not decades to materialise, the spectacular results on Redonda appear remarkably swift.

The reality, of course, is that this ‘overnight’ transformation was a long time in the making. It took seven months of blood, sweat, toil and – above all – teamwork to catch dozens of nimble goats and remove over 6,000 rats from every inch of Redonda’s rugged terrain. This Caribbean island makeover involved meticulous planning, ingenuity, and edge-of-the-seat maneuvers that included abseiling down sheer cliff faces to lay down rat bait and, thanks to the skilled pilots of Caribbean Helicopters Ltd, landing equipment in very tight spots.

New Zealand-based Wildlife Management International Limited led the rat eradication team: “We have over 30 years of experience in clearing invasive species from islands,” said ecologist Elizabeth (Biz) Bell, “but having a ground team, rope access team and helicopter team using a combination of bait stations, hand broadcasting and aerial methods to successfully target all of the rats makes the Redonda project unique.”

The British Mountaineering Council (BMC) played a vital role in ensuring that even the steepest cliffs could be reached. Safety was paramount, as CEO Dave Turnbullrecalls:

The volcanic cliffs of Redonda presented an extremely challenging environment for the climbers to operate in; the BMC was very pleased to support this important conservation project and help ensure the safety of the team throughout the work.”

The safe removal of the malnourished herd of rare-breed feral goats presented an altogether different challenge, as Dr Karl Campbell of Island Conservation can attest:

This project was essential for the wellbeing of the goats and to enable the ecological recovery of the island. The translocation of goats has seen a suite of positive impacts, and further benefits will unveil themselves in time.”

One intriguing consequence of removing the predatory rats is that lizards on Redonda are rapidly changing their behaviour, according to scientists from Harvard University and the Museum of Natural History in Paris. “We measured anti-predator behaviour and found that the ground dragons are becoming fierce and fearless now that the rats are gone, chasing and eating even adult tree lizards”, said Dr Colin Donihue. “This new balance of nature is more in line with what we’d expect of these animals before rats were ever on the island.”

Redonda ground dragon. Credit: Jeremy Holden_FFI

The long list of organisations that cooperated in this mission reflects the complex challenges that had to be overcome. The government of Antigua and Barbuda along with the EAG and FFI joined forces with leading technical specialists from the UK, USA and New Zealand.

Thanks to their collaborative efforts, Redonda has been rat free since March 2017 while the feral goats⁴ have been rehoused and are being cared for by the government’s Veterinary and Livestock Division on Antigua. Biosecurity equipment and protocols have also been installed to prevent future invasions.

What’s next for Redonda?

Redonda harbours endemic species that occur nowhere else in the world, including at least five species of reptiles, and globally important colonies of seabirds. Conservationists argue that Redonda’s unique and special wildlife, coupled with the historical remains of one of the region’s largest guano mines, warrants greater protection for the island. They’re not the only ones. A recent nationwide survey revealed that over 96% of Antiguans and Barbudans agree the island should be protected.

Preparations for designating Redonda and the surrounding sea as a reserve are now under way, led by the Redonda Steering Committee, chaired by the Department of Environment.

According to Dr Robin Moore from US-based Global Wildlife Conservation, which has supported this project and is now helping with planning for the protected area, “It’s incredible to see this radical and rapid transformation of Redonda from a bare rock to a carpet of vegetation. As plants and animals continue to rebound, this could truly be a showcase sanctuary for wildlife.”

– END –

High-resolution images are available upon request. For more information and interviews, please contact:
Sarah Rakowski | Head of Communications, Fauna & Flora International | Tel: +44 (0)1223 747 659 | sarah.rakowski@fauna-flora.org

Flowering Prickly Pear Cactus on Redonda. Credit: Salina Janzan_FFI

Redonda tree lizard. Credit: Ed Marshall

Redonda before restoration. Credit: Ed Marshall

Redonda ground dragon. Credit: Jeremy Holden_FFI

Red Footed Booby nesting in Ficus Tree close-up. Credit: Salina Janzan_FFI

Red-billed tropicbird. Credit: Ed Marshall

Native flora on Redonda. Credit: Ed Marshall

Masked Booby chicks survival will no longer be threatened by IAS. Credit: Salina Janzan_FFI

Magnificent Frigatebirds nesting on Ficus Tree. Credit: Salina Janzan_FFI

Magnificent frigatebird colony on Redonda. Credit: Ed Marshall

Magnificent frigatebirds at nest. Credit: Ed Marshall

Notes to editors:

Redonda harbours a number of endemic species that occur nowhere else in the world, including at least five species of reptiles, such as the Redonda ground dragon (Pholidoscelis atrata). In 2015 all of the surviving reptile species were evaluated by IUCN as Critically Endangered, meaning they face an extremely high risk of extinction in the wild. Redonda has regionally and globally significant colonies of seabirds, including brown boobies, masked boobies, red‐footed boobies, magnificent frigatebirds and red-billed tropicbirds. It was internationally recognised as an Important Bird Area in 2009.The black or ship rats (Rattus rattus) that invaded Redonda were among the largest recorded members of this species and were observed hunting and killing the island’s lizards and seabirds. Diet analyses have shown they also consumed enormous quantities of plants and invertebrates.Christopher Columbus named the island in 1493 and claimed it for Spain. Redonda was later transferred to the British Crown and around 7,000 tonnes of seabird guano was harvested annually from 1865 to 1914. It is believed that rats were introduced to the island during this period. The mining community was disbanded during World War I, after which the island was uninhabited. In 1967, Redonda became a dependency of Antigua and Barbuda.
The Redonda Restoration Programme is supported by, among others, the Darwin Initiative through UK Government funding, National Fish & Wildlife Foundation, Global Wildlife Conservation, Betty Liebert Trust, US Fish & Wildlife Service NMBCA and Syngenta Crop Protection AG.The Redonda Restoration Programme is part of a larger effort to protect global biodiversity. Forty-one percent of the world’s most highly threatened vertebrates are found on islands, with invasive species introduced to islands being a leading cause of extinction. Removing invasive species from islands is an effective and proven way to save many of our world’s most vulnerable species. To date, there have been more than 400 successful projects to remove invasive rodents from islands. The pace, scale and complexity of these efforts are increasing in recognition of the threat invasive species pose to biodiversity.
The Aichi Biodiversity Targets form an integral part of the Strategic Plan for Biodiversity 2011- 2020 adopted by signatories to the Convention on Biological Diversity at a 2010 meeting in Nagoya, Japan. They comprise a series of strategic goals that aim to halt biodiversity loss, maintain ecosystem services and protect the variety of life on the planet that is essential to human well-being.
The rat eradication and goat removal work was completed by June 2017, and Redonda was officially declared rat free the following year in July 2018, after an intensive three-week survey failed to uncover any signs of rodents anywhere on the island. This aligns with customary best practice, which is to wait at least one year before declaring a rat eradication operation successful. The entire population of black rats (Rattus rattus) was eradicated using Klerat, a bitter, waxy rat bait containing the active ingredient brodifacoum that has been used successfully to remove rats from dozens of Caribbean islands since the early 1990s. This was readily eaten by the rats on Redonda but ignored by the native reptiles, mammals and birds. The bait was distributed at intervals of not less than 40 metres, even down the high cliffs, to be certain of reaching every rat.The rat eradication team lived on the island for more than two months to monitor bait uptake and remove rat carcasses. The goat operation took more than six months and aimed to bring the healthiest animals back to Antigua alive at the request of the Department of Agriculture. Recent genetic tests indicate the goats, which have unusually long horns, are of Spanish origin. Most of the goats were successfully captured by hand after being shepherded along temporary fence lines, but some were caught using live snares. All of the goats were found to be very thin and stunted due to the lack of food on Redonda, but have since gained weight and begun breeding on Antigua.

A Second Caribbean Anole Species Introduced to Brazil

By Ivan Prates

On September 7, 2018

In Introduced Anoles, New Research, Notes from the Field

The brown or festive anole, Anolis sagrei, is an invasive species in several countries in the Americas and Asia. This species is native to Cuba, the Bahamas, and the Cayman islands. Following introductions, A. sagrei can reach high population densities and undergo rapid range expansion. In a recently published contribution, we provide the first record of this aggressive invasive lizard in Brazil.

In 2017, we recorded specimens of A. sagrei within the limits of an International Airport in the metropolitan area of the city of Rio de Janeiro, southeastern Brazil. The observation of juveniles and mating couples suggests that the species is established locally.

The origin, geographic extent, and potential for spread of A. sagrei in Rio de Janeiro and Brazil are currently unclear. It is also unclear whether this species will be able to colonize natural habitats, such as the surrounding Atlantic Rainforest.

The establishment of brown anole populations elsewhere has led to shifts in substrate use by native anoles and promoted major shifts in the structure of local insect assemblages. As such, this species has the potential of affecting local ecological communities in Brazil. However, the effects of A. sagrei on the local fauna – including native lizards that we sampled in the area – are difficult to predict.

This is the second case of an established exotic anole species in Brazil. Populations of the Cuban green anole, Anolis porcatus, were recently detected in several sites in the Baixada Santista coastal region, state of São Paulo.

To know more:

Oliveira J.C.F., Castro T.M., Drago M.C., Vrcibradic D., Prates I. (2018). A second Caribbean anole lizard species introduced to Brazil. Herpetology Notes, 11: 761-764.

PDF available here (at the bottom of the webpage).

Evolution 2018: Sex Chromosome Evolution

By kmwinchell

On September 4, 2018

In New Research

“Squamates Playing Poker” – a clever depiction of sex chromosome evolution (image by Anna Minkina shared with permission from Tony Gamble; read more about the image on the Gamble lab website)

Anole Annals contributor Yann Bourgeois reports on the talk by Tony Gamble on sex chromosome evolution. While this talk wasn’t specifically about anoles, our favorite lizards are no stranger to this topic, as discussed previously here on Anole Annals. Gamble builds on his previous work in an impressive analysis of sex chromosome evolution across squamates and snakes. We can look past the anoles taking the back-seat on this one because the talk was fascinating and sure to be of interest to readers here. Yann Bourgeois reports:

How sex chromosomes originate is one of the most intriguing questions of evolutionary biology. It is also a complex one, as the mechanisms that lead to their formation are not clearly understood and our knowledge is mostly based on model species that are not necessarily representative (most of them have strongly heteromorphic sex chromosomes). It is generally thought that sex chromosomes emerge as new alleles at a sex-determining gene stop recombining with each other. Since sex-ratio has to reach an equilibrium, balancing selection maintains the two alleles at high frequencies, and strong linkage locks together loci involved in reproductive isolation and mating. As the two non-recombining blocks diverge they accumulate mutations that can be used to discriminate them using molecular tools. In the most extreme case, one of the two chromosomes degenerates, such as the Y chromosome in humans.

Most sex-determining systems fall into two categories: ZW systems where the male is homozygous ZZ and females are heterozygous, and the classical XY system with XY males and XX females. We know from previous research that anoles fall into the XX/XY category (although some species are XXXX/XXY). However, in general in squamates and snakes in particular, the evolution of sex-chromosomes is poorly known. Early studies mostly focused on heteromorphic chromosomes, which are so divergent that they can easily be identified through cytogenetics. But heteromorphic sex chromosomes like those in anoles and other squamates are not necessarily visible, and other tools are needed to identify which system occurs in a given species.

In his talk, Gamble showed how to use RAD-seq or RNA-seq to identify loci associated with sex, and to discriminate between XY and ZW systems (for more information on this approach, check out his 2015 paper in Molecular Biology and Evolution). For example, in an XX system, all males will be heterozygous at sex-linked loci while all females will be homozygous at the same sites. Depth of coverage will also be twice as low at X-linked loci for males than for females as they harbor a single X chromosome. Using this set of molecular methods, Tony demonstrates that there is a high turn-over of sex-determining systems in squamates and snakes, with more than 40 transitions between ZW and XY, half of those transitions being found in geckos.

It is also possible to map those sex-associated loci to reference genomes to identify syntenic blocks that may be repeatedly recruited during sex-chromosome evolution. For example, in the crested gecko (Correlophus ciliatus), most sex-associated loci map on chicken’s chromosome 10. Some genes may have a higher likelihood to be recruited when a new sex chromosome appears, but whether this can be extended to specific syntenic blocks remains an open question. This is made even more difficult by the fact that sex-determining genes in squamates are not yet known with certainty. This ongoing work on macro-evolutionary dynamics of sex chromosomes will provide interesting findings that may be linked to the evolution of mating strategies and speciation in anoles.