Recently, the book Invasion Genetics: the Baker & Stebbins legacy was published online, covering various aspects of the evolutionary biology of invasive plant, animal, fungus and microbe species. One chapter, coauthored by myself, will particularly appeal to Anole Annals readers, as it provides an extensive review of the genetic, evolutionary and ecological differences between exotic and native anole species. Anoles are highly appropriate for a book on invasion genetics, because of the large body of research on both the genotype and phenotype of anoles, the many species that have exhibited the ability to establish populations outside of their native range, and the exponentially increasing number of exotic anole populations since the onset and intensification of travel and trade in the Caribbean and across the world.
The chapter contrasts what is known about the natural dispersal and colonization processes of Caribbean native anoles to the human‐mediated translocation of exotic anoles in the Anthropocene. Previously, natural colonization events rarely occurred, whereas the rate of new (exotic) anole colonizations has increased drastically. The main argument of the chapter is that the many exotic introductions have eroded the previously strong biogeographic structure of anole assemblages.
An exotic crested anole male (Anolis cristatellus) on the island of St. Martin. (photographer: Wendy Jesse)
It’s that twice in the year opportunity to get AA anole watches at bargain basement prices in honor of today’s clock changes. Get ’em before they run out of stock (or, more importantly, before midnight). Use Code:
Aslam Ibrahim Castellón Maure posted this photo on his Facebook page. Taken in the Zapata Peninsula, it’s a Cuban trogon eating an unidentified anole. The Cuban trogon, or tocoroco, is the national bird of Cuba. But what species of anole? Hispaniolan trogons have also been observed eating anoles. More surprisingly, their lovely relative the quetzal has also been reported to do so, notable because quetzals are thought to be primarily frugivorous.
Figure 1. Anolis cristatellus wileyae; Photograph credit (De Gao)
In his classic work on biogeography, Darlington (Zoogeography: The geographic distribution of animals, John Wiley, New York, 1957) used a small sample of Caribbean island herpetofaunas to show that larger islands have more species. Recently, Gao and Perry reevaluated the regional biogeographical patterns of West Indian native and nonnative herpetofauna by assessing multiple species–area relationship (SAR) models, C– and Z-values (typically interpreted to represent insularity or dispersal ability), and the contribution of area effects towards explaining among-island heterogeneity.
But this time, their sample included over 1600 islands.
Figure 2. Map of the West Indies, showing the distribution of 1668 studied islands
They found that SARs were best modeled using the Cumulative Weibull and Lomolino relationships, both of which can display both convex and sigmoid curves. However, the Cumulative Weibull regressions were more likely to display sigmoid curves within the broad range of island sizes studied – from tiny rocks to major islands like Hispaniola and Cuba. These findings imply that the flexibility of Cumulative Weibull and Lomolino distributions may have been under-appreciated in the literature. Z-values for all herpetofauna in the current study were lower than those reported by Darlington, perhaps because the earlier study oversampled larger islands.
Figure 4. Comparison of Z-values with previous studies
Broadly consistent with previous studies, Z-values reported by Gao and Perry were ranked: (1) native > nonnative; (2) reptiles > amphibians; (3) snake > lizard > frog > turtle > crocodilian. Area had a weaker effect on among-island heterogeneity for nonnative species than for native species, as might be expected given the different processes of species accumulation in the two groups. Lower extinction rates could contribute to low between-island heterogeneity for native species. In contrast, the arrival of non-native species is more closely related to economic activity than to island size. For most small islands less affected by human activities, extinction and dispersal limitation are the primary processes producing low species richness. High levels of among-island heterogeneity underlie the high value of this region as a biodiversity hotspot.
So what does this tell us about anoles? To the extent that the lizard patterns reflect the large number of Anolis species in this region, the findings imply that within-island speciation, rather immigration related to island area, is the main source of new native species in this region. Not surprisingly, perhaps, human activities accelerate the rate of over-water dispersal of both native and non-native species and weaken the area effect within the region. This leads to increases in among-island heterogeneity under human-mediated conditions. Anoles may be more likely to be affected by the increase in extinction rates that is typically seen on the smallest islands.
Figure 3A. Linear Regression_ lizard
Figure 3B. Linear Regression_ lizard native
Figure 3C. Linear Regression_ lizard nonnative
Figure 5A. SAR and additive diversity partitioning_ lizard
Figure 5B. SAR and additive diversity partitioning_ lizard native
Figure 5C. SAR and additive diversity partitioning_ lizard nonnative
One of last year’s winners, Anolis sheplani by Carlos de Soto
Thank you to everyone who has sent in photos for our calendar contest, we’ve been getting some excellent submissions! There are FIVE DAYS left before the deadline (this Friday, November 4) so if you plan to submit, be sure to do so soon!
As a reminder, here are the contest rules:
Submit your photos (as many as you’d like) as email attachments to anoleannalsphotos@gmail.com (note the change in email address from last year). To make sure that your submissions arrive, please send an accompanying email without any attachments to confirm that we’ve received them. Photos must be at least 150 dpi and print to a size of 11 x 17 inches. If you are unsure how to resize your images, the simplest thing to do is to submit the raw image files produced by your digital camera (or if you must, a high quality scan of a printed image). If you elect to alter your own images, don’t forget that it’s always better to resize than to resample. Images with watermarks or other digital alterations that extend beyond color correction, sharpening and other basic editing will not be accepted. We are not going to deal with formal copyright law and ask only your permission to use your image for the calendar and related content on Anole Annals (more specifically, by submitting your photos, you are agreeing to allow us to use them in the calendar). We, in turn, agree that your images will never be used without attribution and that we will not profit financially from their use (nobody is going to make any money from the sale of these calendars because they’ll be available directly from the vendor). For good quality printing of your images check pricing and options online from the convenience of your home.
Please provide a short description of the photo that includes: (1) the species name, (2) the location where the photo was taken, and (3) any other relevant information. Be sure to include your full name in your email as well. Deadline for submission is November 4, 2016.
Studying Caribbean lizards when you are based in Northern Europe is maybe not the most obvious thing to do. But I couldn’t resist the charm of Anolis and embarked on a postdoc project with the aim of unlocking some of their mysteries. Since I have a background in comparative genomics, I was particularly excited about one odd feature of the green anole genome: unlike other vertebrates, it is remarkably cluttered with transposable elements.
Transposable elements (or TEs for short) are popularly referred to as jumping genes because they can copy and paste themselves within a genome. Traditionally TEs have been considered to be a ‘junk’ part of the genome, selfishly proliferating in an arms race with the host genome that is trying to keep TEs in check. As a defense, the host genome is usually restricting TEs from entering functionally important regions. But in the green anole even the Hox gene clusters, developmental control regions of the genome that are usually kept neat and tidy, got invaded by these TEs.
Even junk can become valuable in a different context. Indeed, there is circumstantial evidence that TEs can contribute to diversification and adaptation. For example, genomic incompatibilities arising from TE insertions have therefore been suggested to promote reproductive isolation. In other words, proliferation of TEs should be positively associated with speciation. Furthermore, some evolutionary innovations, like the mammalian placenta, appear to involve co-option of TEs for gene regulation.
Does the odd feature of the green anole genome indicate that something interesting is going on with TEs also in the evolutionary history of Anolis lizards? My study published in the Proceedings of the Royal Society of London B is a first attempt to take a closer look.
To this end, I compared the DNA sequences of Hox gene clusters of 30 lizard and snake species, including 20 Anolis species. I reconstructed the history of TE invasions of Anolis lizards and linked this to patterns of diversification across the phylogeny. The results revealed that there was a burst of TE activity in the lineage leading to extant Anolis. It did not stop there – TEs have continued to accumulate during speciation events, such that extant Anolis whose evolutionary history is characterized by many speciation events also have accumulated more TEs than lineages with relatively fewer speciation events. This finding supports the hypothesis that proliferation of TEs contributes to reproductive isolation, but what is cause and what is consequence remains to be seen.
Could TE activity also have contributed to the morphological differences that characterize Anolis ecomorphs? Well, I did not find evidence for this as yet, but this hypothesis is much more difficult to test since we need to learn more about developmental genetics to know where in the genome we should look. Nevertheless, I think this study shows that we can begin to unravel the genomics of adaptive radiation of these wonderful lizards!
Gonatodes humeralis on a tree trunk in French Guiana. Photo taken by Tim Higham.
The ability for some lizards to adhere to smooth surfaces has attracted considerable attention from scientists, engineers, and the public for quite some time. Anoles can exhibit considerable amounts of adhesion, although they lack the fancy specializations that most pad-bearing geckos have, such as the upward curling of the digit tips (to detach the adhesive system) before the foot is lifted from the surface. This might be related to the higher adhesive forces exhibited by geckos in comparison to anoles. Unlike anoles, the gecko adhesive system has appeared and disappeared several times. The simplification of the system appears linked to the transition from a climbing to terrestrial lifestyle. However, it has been unclear how this innovation might arise and how the early stages might appear.
The evolution of digit form in Gonatodes
That’s where the genus Gonatodes comes into play. Gonatodes is a reasonably diverse (27 species are currently recognized) and ancestrally padless clade of mostly diurnal sphaerodactylines that is sister to Lepidoblepharis. After examining the microscopic anatomy of a number of species from the genus Gonatodes, it was clear that one species (G. humeralis) was a bit different. Upon investigation of the subdigital micro-ornamentation, we found that the spinules in the vicinity of the digital inflection are longer than in other species of Gonatodes and are expressed as branched, spatulate-tipped setae on the free distal margin of these scales. In other words, it looked like this species of gecko was showing signs of incipient adhesion without actually having any toepads. Now that the morphological differences were identified, we wanted to know how/if this translates into functional and ecological differences.
Gonatodes humeralis in French Guiana. Photo by Tim Higham.
The origin of frictional adhesion in geckos
In collaboration with Anthony Russell and Tony Gamble, We sought to understand how this incipient adhesive system works in nature, whether G. humeralis can generate adhesive force, and what it permits these lizards to do on smooth surfaces in the lab. I traveled to French Guiana with Clint Collins, a Ph.D. student in my lab, in order to collect G. humeralis and examine its adhesive force. After that, Anthony Russell and I traveled to Trinidad & Tobago to collect a number of other species, in addition to G. humeralis, to see how they used their habitat and whether G. humeralis could out-perform the other species in the lab. To our surprise, G. humeralis was found on smooth bamboo stalks, whereas other species lived on the ground or on rough tree trunks. In the lab, G. humeralis could exhibit considerable adhesive force (for its size), exceeding that of skinks, but falling short of anoles and other pad-bearing geckos. That’s quite impressive for a gecko that lacks all of the bells and whistles of a typical pad-bearing gecko! Importantly, no other species of Gonatodes that we collected could generate any measurable force, agreeing with our previous morphological analyses! Now to the locomotor tests. Pad-bearing geckos are renowned for their ability to ascend vertical smooth surfaces, so we decided to test the ability of different species to climb different inclined smooth acrylic surfaces. A closely related species, G. vittatus, was unable to ascend any incline greater than 40 degrees. However, G. humeralis could climb up a vertical surface, as shown above.
T. Higham looking for geckos in Trinidad
What does all of this mean?
Although major transformations in vertebrate evolution are common, and often very complex, their origins are often elusive. We offer a glimpse into the early development of the complex adhesive system of geckos. However, the setae of G. humeralis are effective without all of the muscle, tendon, and vascular modifications that are often associated with gecko adhesion. Much like the anoles, the relatively simple setae of G. humeralis provide a dramatic advantage in areas of the habitat typified by leaves or other smooth surfaces (e.g., bamboo stalks). As noted in our paper, our discovery of a functionally intermediate form in the transition to frictional adhesion in a lineage of geckos highlights a statement by Ernst Mayr back in 1960: “Perhaps most astonishing is the relative slightness of reconstruction that seems to be necessary for successful adaptation to rather drastic shifts of adaptive zones.” The relatively simple morphological modification in G. humeralis has permitted a dramatic shift in biomechanics and likely habitat use.
Figure 1 from Ng et al. 2016 showing the transect sampling spanning Anolis distichus populations differing in dewlap color (T1-4) as well as control transects (C1-4). Pie charts show dewlap color variation (top row), mitochondrial clade membership (middle row) and nuclear genetic cluster assignments (bottom row).
We sure love dewlaps here on Anole Annals! These flashy signals are incredibly diverse in size, color and pattern, and always make for a gorgeous image (e.g. 1, 2). Yet, we still have much to learn about why there is such a diversity of dewlaps and, furthermore, what are the consequences of such diversity? Previous work by Leal and Fleishman (2002, 2004) suggests that some of this dewlap diversity is due to adaptation for more efficient communication in different habitats. In a recent paper, we sought to identify whether the consequence of such adaptive trait divergence was speciation, or whether locally adapted dewlaps are maintained despite gene flow.
Anolis distichus shows remarkable geographic variation in dewlap color that predictably varies with habitat in a manner consistent with adaptation (Ng et al. 2013). This variation in color across Hispaniola gave us a great opportunity to conduct replicated analyses to identify whether adaptive differences in dewlap color consistently leads to the same genetic outcome.
We sampled populations in the Dominican Republic along five transects that transitioned from populations with orange dewlaps to those with cream or yellow dewlaps. For a comparison, we also sampled four ‘control’ transects where all populations shared a similar dewlap color. If dewlap differences are associated with speciation, we expected to see genetic differentiation between populations at either ends of the transect as this would suggest some level of reproductive isolation. Otherwise, transects showing no evidence of genetic structure would suggest that individuals are freely mating regardless of dewlap color.
Looking at the genetic structure of both nuclear and mitochondrial DNA along each transect, we found that geographic variation in dewlap color is associated with both speciation and gene flow. Three transects showed distinct genetic structure consistent with speciation, with one in particular only showing evidence of hybrids at one site which was a mere 0.89-1.55km away from other sampled sites. On the other hand, the other two transects did not look much different to the control transects, suggesting ongoing gene flow regardless of phenotypic differences.
Considering all transects together, I think there are two main take-aways from our results. First, finding evidence of gene flow across a sharp geographic shift in dewlap color must mean that strong selection is maintaining geographic variation in dewlap color; perhaps due to adaptation to different habitat types. Second, it appears that dewlap divergence does not necessarily lead to speciation. More work, however, is needed along these lines to understand whether the dewlaps we are characterizing as different are actually different from an anole’s perspective or in particular light environments (e.g. 1).
Anolis carolinensis from North Carolina. Photo from Carolina Nature.
One of the most well-known species of anole lizard is Anolis carolinensis, AKA the green anole, which is the only anole native to the continental United States. As a classic model for ecology and behavior, this lizard was the first species of reptile to have a complete genome sequence. Interestingly, only after it became a genomic model, numerous studies (Tollis et al. 2012, Campbell-Staton et al. 2012, Tollis & Boissinot 2014) sought to understand how genetic variation is structured across the geographic range of A. carolinensis, and to infer historical migration patterns and demographic events to explain the current distribution of green anoles. However, these studies still left many questions unanswered, mostly due to the fact that they were limited in terms of numbers of genetic markers. Now, we have published a new paper in Ecology and Evolution that used a targeted enrichment method to capture more than 500 sequence markers and provide a clearer picture of A. carolinensis historical biogeography.
What we knew about Anolis carolinensis phylogeography
Collecting green anoles for phylogeographic study has been a real hoot, taking us all over the country. Anolis carolinensis ranges across subtropical North America, and consists of five geographically structured genetic clusters supported by both mitochondrial (mtDNA; see Tollis et al. 2012 and Campbell-Staton et al. 2012) and nuclear (nDNA) markers (see Tollis et al. 2012, Tollis & Boissinot 2014). Three of the clusters are found in Florida : one whose distribution primarily hugs the Northwestern coast of the peninsula, another along the Eastern coast of the peninsula, and a third relegated to South Florida. The continental mainland, while making up most of the area of green anole range, harbors only two clusters: one occupying North Carolina and South Carolina, and another from Georgia, west of the Appalachian Mountains and across the Gulf Coastal Plain into Texas.
One confusing result from earlier studies of A. carolinensis molecular phylogeography was the placement of the most basal lineage in NW Florida (Tollis et al. 2012, Campbell-Staton et al. 2012). This didn’t make sense biogeographically, since it is believed that the species dispersed to the continental mainland from western Cuba (Buth et al. 1980, Glor et al. 2005). However, a subsequent nDNA study (Tollis & Boissinot 2014) produced a multi-locus species tree to show that southern Florida harbors the most ancient lineage of A. carolinensis. This discovery of mito-nuclear discordance provided a more satisfying biogeographical explanation that only needs to invoke overwater dispersal to South Florida from Cuba.
Different genetic datasets tell different stories about Anolis carolinensis evolutionary history. (A) Phylogenetic relationships of the major green anole lineages inferred from the ND2 mtDNA locus. (B) Phylogenetic relationships of the major green anole lineages using multi-locus species tree approach (1 mtDNA and 3 nDNA markers). Adapted from Manthey et al. 2016.
From there, things remained unresolved even with nDNA. For instance, while the split between South Florida and the rest of the species received full statistical support in Tollis & Boissinot (2014), the relationships between the other clades were less supported, making it difficult to determine if the A. carolinensis mainland clades arose from separate Floridian sources.
The data used in Manthey et al. 2016
To our knowledge, this is the first Anolis phylogeography study to use targeted enrichment, so I thought I would elaborate on the nature of this kind of dataset. Anchored hybrid enrichment (AHE) relies on probes designed from conserved genomic regions ascertained from a panel of vertebrate genomes – including A. carolinensis – which are flanked by non-conserved regions (the level of conservation in determined by PhastCons scores from the UCSC Genome Browser). DNA samples are pooled, and a set containing thousands of probes is used to enrich libraries that get sequenced on an Illumina platform and assembled into contigs, producing hundreds of homologous loci.
Here’s the breakdown of what we ended up with in the new study: our sample contained 42 individual anoles from 26 localities across eight states, and we were able to obtain 487-512 loci per individual, with an average contig length of 629bp, and an average of 17 SNPs per locus including an average of six parsimony-informative SNPS per locus. Roughly speaking, that’s one parsimony-informative SNP every 100bp for 500 loci, so about 3,000 parsimony-informative SNPS = not bad! For what it’s worth, the 10 nDNA A. carolinensis markers obtained by more traditional PCR/Sanger sequencing contained about one SNP every 100bp as well (see Tollis et al. 2012 and Tollis & Boissinot 2014). Therefore, AHE produced hundreds more informative loci at a fraction of the cost.
New insights into Anolis carolinensis phylogeography using targeted loci
Using different statistical clustering methods (DAPC and Structure), Manthey et al. supports the same five genetic clusters as previously described. However, there is now a fully resolved species tree – arrived at using multiple methods. First, the South Florida clade is the most ancient lineage of green anoles, likely splitting off from the rest of the species during the Miocene or Pliocene. However, there is now 100% support for a sister-group relationship between the mainland clades, massively simplifying the story of A. carolinensis. Green anoles likely remained in Florida until the Pleistocene, dispersing northward and onto the mainland where two lineages evolved independently- one along the Atlantic coast in the Carolinas, and another dispersing across the Gulf Coastal Plain.
(A) Map showing geographic localities of 42 green anoles selected for targeted enrichment. (B) Results of species tree analyses. Colored symbols correspond to the five geographic and genetic clusters. Adapted from Manthey et al. (2016).
We also found that despite the best resolution to date for the A. carolinensis species tree, incomplete lineage sorting is rampant across these loci, highlighting the need for these kinds of datasets for phylogeographic studies at this evolutionary distance. For instance, the only clade with any gene trees supporting exclusive ancestry was South Florida: meaning on a given gene tree, pre-defined “clades” are often paraphyletic. The reason the species trees agreed in their topologies is due to fact that they probabilistically invoke the coalescent process, which incorporates incomplete lineage sorting. Previous studies, using ≤10 loci, simply lacked enough statistical power to do this confidently.
More work to be done
As with most scientific endeavors, the new study resolves some outstanding questions but also begs new questions. For instance, although we were able to infer gene flow between the Gulf-Atlantic and NW Florida clades, the degree of allele sharing between populations is still not clear. There seems to be some admixture between the Gulf-Atlantic and Carolinas clades south of the Appalachian Mountains in Georgia, suggesting elevational gradients provide a more effective barrier to gene flow in this species than riverine barriers. Also, the divergence times of the green anole clades are still based only on molecular clock models and could benefit greatly from informative fossils calibrations.
A familiar face – a brown anole male from my study site in southwestern Taiwan.
For the past few years the authorities of Chiayi County, southwestern Taiwan, have paid bounties to citizens for brown anoles they collect. Every year the bounty per lizard has decreased and yet they spend their budget and the brown anole persists. This year is the same – a lower bounty – but with a slight difference; the green iguana is now also on the list. In theory, it would be ideal if the invasive lizards can be exterminated, but in reality, I am convinced, they will fail. The brown anole exists in southwestern and eastern Taiwan, and simply targeting them in one location will simply retard their dispersal to new localities (and even with the bounty in place, their distribution is extending). We recently published the results of a study in which we compared brown anole specimens from southern and eastern Taiwan, and we found that there are some variations, most likely due to adaptations to the local habitats (no surprise there!). What this means is that in Taiwan, if brown anoles can reach (either by natural dispersal or with the help of people) open disturbed habitats, with structures that can be used as perches, they will most likely adapt and establish new populations.
Me with a green iguana (Iguana iguana) that was removed by firefighters from someone’s garden in Chiayi City.
A sun skink (Eutropis multifasciata) from Tainan City, southwestern Taiwan.
And then I wonder why is the brown anole singled out for extermination. Eutropis multifasciata, a relatively large invasive skink, also exists in Chiayi County. Due to its size, it has greater abilities than the brown anole to compete with and prey upon native lizards and arthropods, and yet, they are not on the list. People regard Hemidactylus frenatus, a very common gecko species in urban areas in central and southern Taiwan, as a native species, not realizing that it too is an invasive species.
Hemidactylus frenatus is a very common species in southern Taiwan, where they are often seen near external lights on the walls of buildings.
My honest opinion is they have to accept that just like Hemidactylus frenatus, Anolis sagrei will spread in Taiwan and become a common sight in areas disturbed by humans. They will become (and in many ways already are) part of local ecosystems as competitors, predators and prey. Conservation efforts should thus rather be directed at the re-establishment and conservation of large areas of secondary forests in disturbed lowland areas of Taiwan. This would not only contribute to the conservation of native forest species, but such areas will also function as reservoirs for species like Japalura swinhonis that can compete with Anolis sagrei, as well as being barriers for its spread. People should also be encouraged to be more tolerant towards snakes, in particular non-venomous species such as Lycodon (Dinodon) rufozonatum rufozonatum, Lycodon ruhstrati ruhstrati, and Sibynophis chinensis chinensis, which can prey upon brown anoles. And, finally, an important part in the conservation efforts of native urban wildlife is to develop a better appreciation among the general public of native birds and lizards in urban gardens and parks, and to reduce the impact on these animals by their pets, especially domestic cats (Felis catus), which may prey on them.
Just for interest sake, here is a current list of exotic invasive lizards in Taiwan:
