Anole Annals

Though temperature-dependent sex determination is one of the most interesting things about reptiles, this mode of sex determination unfortunately does not extend to anoles. In iguanid lizards, sex determination has long be known to be a consequence of sex chromosomes, males being the heterogametic (XY) sex.

Reptilian sex chromosomes occupy a strange middle ground within vertebrates: on one hand, amphibian and fish sex chromosomes are marked by rapid turnover in precisely which chromosomes determine sex ; on the other hand, bird and mammal sex chromosomes are characterized by their stability over millions of years.  In an early-view paper in Evolution, Rovatsos et al. (2014)  show that sex chromosomes are stable in at least some reptiles–in anoles, they have been conserved since before the diversification of the genus. 

The authors began by picking five X-linked and three autosomal genes from the recently published Anolis carolinensis genome, and use quantitative PCR to confirm that the X-linked but not the autosomal genes have double the gene dosage values in female vs. male A. carolinensis. Next, they extend their sequencing efforts to seventeen other species from across Anolis as well as three species of phrynosomatid lizards. Remarkably, similar patterns of gene dosage differences between males and females are seen across the sampled taxa, suggesting that the same genes are X-linked in all these species. This result implies the stability of the X-chromosome for at least 70 million years, pre-dating the divergence of Dactyloidae and Phrynosomatidae.

This finding puts a dent in a long standing hypothesis for why birds and mammals have stable sex chromosomes–their stability was attributed to “the lower susceptibility of homoiotermic endothermic vertebrates (mammals and birds) to thermally-induced sex reversals due to their effective thermoregulation.” Rovatsos et al. (2014) call for new explanations for “why some vertebrate lineages possess frequent turnovers of poorly differentiated sex chromosomes, while others show a long-term stability of sex chromosomes connected with their progressive differentiation,” explanations that must take into account the stability of sex chromosomes across anoles and potentially across all iguanian lizards.

The team at Tropical Herping has done it again! This time, a fabulous, lavish, luscious, information-packed guide to the spectacular herpetofauna of Mindo Parish, Ecuador. Originally available online, the book is now available in print. I had the privilege of writing the foreword, appended below. More information is available on the TH website, as well as an order form.

Foreword:

Small in size, but a global giant in biodiversity, Ecuador is awash in all manner of fauna and flora. Birds, butterflies, trees—the country is a hotspot for just about everything. But no group of organisms is more beautiful, more charismatic, more scientifically captivating than Ecuador’s reptiles and amphibians. The Amazon rainforest dominates the attention of the public, but other parts of the country, especially the mountainous regions, are just as biologically rich. One such area is the small parish of Mindo in Pichincha Province, home to 102 species of creepy crawlies. And what an ensemble! Brilliant colors, toxic skin and venom, sweet serenades, menacing looks, gorgeous displays—this region is an encyclopedia of herpetology in just 268 square kilometers.

Field guides play an essential role in making the fauna and flora of an area widely accessible. They are at the front line of nature education and conservation, the place where the fruits of scientific exploration are distilled, synthesized, packaged, and presented to the public at large. Since the time of Roger Tory Peterson, field guides have played another role, being a venue for beautiful, yet accurate, scientific illustration, allowing readers to not only understand the identifying marks of each species, but also to appreciate them esthetically.

Despite its bountiful herpetofauna, until now no field guides existed for Ecuador’s amphibians and Reptiles. The Tropical Herping team has brilliantly stepped into this void, producing a guide to the herps of Mindo that hopefully will serve both as a model of how guides should be produced and an inspiration to the production of similar efforts elsewhere in Ecuador and beyond. The Amphibians and Reptiles of Mindo is particularly notable in three respects. First is the breadth and depth of information provided for each of Mindo’s species. These authors know their fauna in exquisite detail and have synthesized that knowledge in a clear and lucid manner. The inclusion of frog calls, recorded by the authors themselves, is an added bonus bridging the paper and digital eras. Second, the public often does not understand the connection between scientific research and the information presented in field guides, magazine articles and nature documentaries. Unlike most field guides, The Amphibians and Reptiles of Mindo makes this link crystal clear, providing citations so that readers know where to turn to learn more. Indeed, especially impressive is the fact that the authors did a great deal of field work themselves to round out knowledge of these species, presenting that information for the first time here. Finally, third, the book is simply beautiful. The photographs are simply stunning and the maps and other illustrations lovely as well.

The publication of The Amphibians and Reptiles of Mindo could not come at a better time. The Mindo region is a microcosm for all that ails the natural world. Deforestation, habitat fragmentation, pollution, overharvesting—all are threats. Mindo has one thing going for in its favor—it has become a nature vacation travel destination, providing jobs and economic rationale for preserving natural habitats. But, ecotourism can be a two-edged sword, as people and development are drawn to the area with potentially negative consequences. Mindo has the opportunity to show how responsible stewardship can be mutually beneficial to man and nature, and this lovely book shows what is at stake. Three cheers for the three authors of this magnificent volume. Long live the herpetofauna of Mindo!

We all know that some of our favorite anole species are abundant in urban settings, yet many others are not. Why is this? Do species have to evolve and adapt to city living? Maybe not. In what may be a surprising preliminary analysis, Kristin Winchell over on her blog Adaptability suggests that Caribbean anoles ancestrally had what it takes to live in human settings, and not being able to do so is an evolutionarily derived trait. Sounds crazy at first, right? Until you remember that anoles colonized these islands over water, and so to be successful, had to be flexible and able to cope with whatever life through at them–including, apparently, concrete sidewalks, trashcans, cars, and cats. Check out the details on Kristin’s post.

One of the major experimental advances in recent decades has been the battery of methods capable of functionally validating hypotheses regarding the molecular networks that regulate biological processes. For biologists, these emerging methods allow us to move beyond descriptive and correlational studies to new dimensions where we can experimentally validate our observations. Until recently these technologies were, by and large, reserved for the most well developed laboratory model systems (e.g., mouse, chicken, zebrafish, Drosophila), systems that rarely have direct utility to ecologists and evolutionary biologists. But the topography of biology is changing. These methods are rapidly becoming more easily applied to non-model systems, such as our favorite genus Anolis. In an upcoming paper from the Menke Lab, the tools of functional genomics are applied to anole limb development, taking another step towards making Anolis a truly integrative model system.

In situ hybridization showing expression of of early limb genes in A. sagrei.

Park et al. describe a micromass culture system to explore the molecular regulation of anole limb morphogenesis. In their protocol, Park et al. collect cells from early limb buds of A. sagrei, dissociate the cells from one another, and then add them to a dish as a small (i.e., micro) bolus (i.e., mass) of cells with the appropriate growth media. Even when removed from the embryo, these cells maintain the characteristics of limb cells, developing cartilage after about two weeks and maintaining their molecular signature for at least eight days. This small mass of cells can be grown for up to 30 days and, therefore, provide a useful template for experimental manipulations. More details of this protocol are described in great detail in the paper. Compared to other technologies which require far greater investment, their protocol should be accessible to anyone with access to a tissue-culture laboratory.

Anolis is an emerging model of limb development, but previous studies have focused on describing morphometric patterns of limb growth, not the molecular regulation of limb development. In fact, there have been no studies systematically dissecting the molecular regulation of limb development in any squamate species despite broad interest in this topic in the laboratory mouse and chick systems for 40 years. To study the molecular mechanisms regulating limb morphogenesis, Park et al. forced the expression of the gene Pitx1 – a hindlimb-specific molecule in mouse and chick – in micromass cultures derived from both forelimb and hindlimbs. This experiment verified that one step of the limb regulatory network, the relationship between Pitx1 and Hoxc11, is likely conserved among amniote lineages. While at this time this  may have been a proof of principal experiment, this protocol may have future implications for both developmental and evolutionary research in Anolis. For example, multiple transgenes can be readily cloned and incorporated into the micromass cultures. In addition, micromass cultures derived from species with distinct limb morphologies may also open to door to finding pathways that are regulated in novel ways across Anolis lizards.

Binding domains of Pitx1 in the intergenic region of Hoxc11. Note conservation of binding region throughout mammals (shaded arrows), but lack of conservation among amniotes (white arrows).