The trunk anoles of the distichus species group
The distichus species group consists of at least six species and more than 20 subspecies of trunk ecomorph anoles found across the Bahamas and Hispaniola. The members of this group are among the most conspicuous and abundant lizards across most of their range, where they can be found everywhere from broadleaf montane forest to xeric semi-desert scrub forest. This group has fascinated anole biologists for decades because of its remarkable variation in dewlap color and pattern (dewlaps are extensible throatfans used by males during territorial defense and courtship).
Because the dewlap is considered important to species recognition and sexual selection, dewlap divergence is often thought to be associated with speciation. Our integrative research using phenotypic, genetic, ecological and behavioral data gathered in nature and in the laboratory suggests that the group consists of numerous populations at varying stages of the speciation process, and provides new insight on the role of the dewlap in anole speciation. We are now using experimental and genomic data to further investigate how and why species occurs in the distichus species group.
Genomic and experimental studies of hybridization
Hybrid zones in nature often provide opportunities to investigate how and why new species form, and subsequently co-exist. Guided by the work of previous anole biologists, we have identified numerous hybrid zones where phenotypically and genetically distinct populations of trunk anoles come into contact. Using molecular genetic data, we have found that some of these hybrid zones are accompanied by abrupt transitions between phenotypically and genetically distinct populations that are appear to represent largely evolutionarily isolated species or incipient species at various stages of divergence (Ng et al. 2011). Genetic differentiation among populations of a Hispaniolan trunk anole that exhibit geographical variation in dewlap colour.
We are now following this up with additional research, much of which is focused on a hybrid zone that occurs along an ecological gradient in the Rio Bani river valley (see image below) with pale yellow-dewlapped populations of A. d. ravitergum in the coastal xeric scrub forest and orange dewlapped populations of A. d. ignigularis in upland broadleaf forest. Although we have now observed frequent hybridization along stream beds where the xeric and mesic environments preferred by the two species come into contact, we see very little evidence for introgression. Using multi-generational experimental laboratory crosses between these populations, we have found evidence for strong intrinsic reproductive isolation. We are now interested in identifying the regions of the genome underlying divergence between these species, and the relative contribution of natural selection to patterns of genomic divergence or introgression.
A captive colony of anoles belonging to the distichus species group is critical to our research because it permits detailed experiments on reproductive isolation and the genetic basis for species differentiation. The colony typically includes around 1000 animals ranging in age from hatchling to adult, and is capable of producing nearly 2000 eggs annually. Work with this colony has already permitted us to diagnose the heritability of dewlap color, pattern, and size. In 2013, we completed our first milt-generation crossing experiment designed to investigate intrinsic reproductive isolation. Backcross progeny from this experiment now make up the bulk of the colony. In 2014, the colony grew to include populations for several additional experimental crosses following field work in the southern Dominican Republic.
Our work with the distichus species group has spawned several collaborative projects. For one such project, members of Hopi Hoekstra's lab at Harvard University are investigating the genetic basis for dewlap differentiation using genomic studies of animals sampled from nature. Two additional collaborations have resulted from our ability to provide material from the captive colony for studies of development and evolution. One of these projects is being conducted in Kenro Kasumi's lab at School of Life Sciences (Arizona State) while another is being conducted by Casey Gilman in Duncan Irschick's Lab at the Department of Biology (University of Massachusetts, Amherst). Another collaboration involved work with an institutional veterinarian to diagnose new adenovirus sequences from A. distichus. We welcome proposals for additional collaborative projects, particularly from groups interested in taking advantage of the opportunities offered by our large captive colony.
Dewlap color evolution
Signals involved in species recognition are thought to play a particularly important role in speciation. Although it is widely assumed that divergent anole dewlap color and pattern is associated with anole speciation, we know relatively little about how and why dewlaps diverge. Previous work on anoles suggests that dewlap divergence may result from adaptation to optimize the efficiency of the dewlap as a signal for intraspecific communication across different signaling conditions.
Our comparative analyses of the widespread Hispaniolan species A. distichus support this hypothesis by recovering a strong correlation between dewlap color and environmental variation; pale yellow dewlaps tend to occur xeric environments while orange dewlaps tend to occur more mesic environments (https://doi.org/10.1111/j.1558-5646.2012.01795.x). This adaptive hypothesis is further supported by our finding that dewlap color is a strongly heritable trait whose phenotype is not strongly influenced by availability of pigment precursors in the diet of adult individuals (http://dx.doi.org/10.1093/jhered/est060). Adaptation to optimize signaling efficiency is not the only process driving dewlap divergence in members of the distichus species group. Our work on several Haitian members of the group supports the hypothesis that dewlap can exhibit reproductive character displacement possibly involved with reinforcement (http://dx.doi.org/10.1111/mec.12292).
Bibliography of our work on the distichus species group
Ng, J., A. L. Kelly, D. J. MacGuigan, and R. E. Glor. 2013. The role of heritable and dietary factors in the sexual signal of a Hispaniolan Anolis lizard, Anolis distichus. Journal of Heredity 104:862-873. [http://dx.doi.org/10.1093/jhered/est060]
Ng, J., E. L. Landeen, R. M. Logsdon, R. E. Glor. 2013. Correlation between Anolis lizard dewlap phenotype and environmental variation indicates adaptive divergence of a signal important to sexual selection and species recognition. Evolution 67:573–582. [http://dx.doi.org/10.1111/j.1558-5646.2012.01795.x]
Ascher, J. M., A. J. Geneva, J. Ng, J. D. Wyatt, and R. E. Glor. 2013. Phylogenetic analyses of novel squamate adenovirus sequences in wild-caught Anolis lizards. PLOS ONE [http://dx.doi.org/10.1371/journal.pone.0060977]
Lambert, S. M., A. J. Geneva, D. L. Mahler, and R. E. Glor. 2013. Using genomic data to revisit a classic example of reproductive character displacement in Haitian Anolis lizards. Molecular Ecology 22:3981-3995. [http://dx.doi.org/10.1111/mec.12292] [see also http://dx.doi.org/10.1111/mec.12377]
Glor, R. E. and R. Laport. 2012. Are subspecies of Anolis lizards that differ in dewlap color and pattern also genetically distinct? A mitochondrial analysis. Molecular Phylogenetics and Evolution 64:255-60. [http://dx.doi.org/10.1016/j.ympev.2010.11.004]
Ng, J. and R. E. Glor. 2011. Genetic differentiation among populations of a Hispaniolan trunk anole that exhibit geographical variation in dewlap colour. Molecular Ecology 20:4302-4317. [http://dx.doi.org/10.1111/j.1365-294X.2011.05267.x]