Author: Rich Glor Page 4 of 13

In what should be our final belated post about talks at the Evolution meetings in Ottawa last month, I’d like to share some results from Bryan Falk from Susan Perkins’s Lab American Museum of Natural History.  Bryan has been investigating the diversity of anole malaria parasites (Plasmodium).  Like many other species of vertebrates, anoles have their own strains of malaria (not the same as the ones that effect us humans), and these lizard malaria have been the focus of numerous fascinating research projects over the years (see Schall 1996 for a review).

Bryan’s work investigated phylogenetic relationships among West Indian strains of lizard malaria using sequence data from mitochondrial DNA plus six nuclear loci.  He found that Plasmodium samples on most islands form monophyletic groups, although some clades are found in both Florida and Cuba, suggesting travel between these two regions.  Bryan also reported very low overall genetic diversity, the presence of most genetic variation among (rather than within) populations, and no evidence for purifying selection.  Bryan’s previous work used tree-based delimitation to diagnose previously unrecognized or ambiguous taxa of Plasmodium on Hispaniola, and his new work uses a similar approach across a broader geographic scale.  In the new study, species tree analyses tend to recover island-specific clades and identify 11 potentially unrecognized species within Plasmodium floridense (see Perkins 2000 for more on species delimitation in Plasmodium). Bryan’s time calibration work suggests that intra-island divergences are very young and his demographic analyses suggests that recent divergence and serial bottlenecks may be responsible for low diversity with in populations but high divergence among populations.  It seems like more exciting new results with anole malaria on are on the horizon from Bryan and his collaborators.

Laura Rubio-Rocha, a masters student at UT Knoxville, presented an anole double-header at Saturday evening’s poster session.  In one poster, Laura presented work on thermal physiology of two anole species that occur in the United States: the native green anole (A. carolinensis) and the introduced brown anole (A. sagrei).  By sampling each species from a number of localities across a latitudinal transect extending from southern Florida to southern Georgia, Laura was able to test whether populations or species vary in their ability to tolerate cold temperatures.  To evaluate each species’ ability to tolerate cold temperatures, she used a simple behavioral assay that diagnosed the lower critical thermal minimum (CTmin) as the temperature at which a species no longer responded to physical stimuli.  Although anoles readily recover from this state in captivity, anoles in field aren’t likely to last long when incapacitated.

When CTMin was assessed in the field, Laura found that thermal tolerance largely mirrored the latitude of sampled populations.  Within each species, those that occurred further north were better able to cope with lower temperatures than population sampled further south.  A strong difference between the two species, with green anoles (A. carolinensis) being able to tolerate much colder temperatures than brown anoles (A. sagrei).  The story, however, doesn’t end here.

Recognizing that some degree of variation in a given individual’s cold tolerance might result from acclimation and plasticity, Laura conducted her analyses both in the field immediately after capture and after allowing the animals to acclimate to the conditions of a common garden laboratory environment for several months.  When Laura analyzed her laboratory data, she found that the strong difference in cold tolerance between species persisted, but the differences within populations of each species from localities across a latitudinal gradient were no longer evident.  Her results suggest that some degree of cold tolerance is genetically determined, but that variation within species along a latitudinal gradient is likely a plastic response.

A terrible photograph of Blair Hedges’ presentation (sorry, I was stuck in the back of the room).

Blair Hedges used his talk at WCH7 to present his exciting new results with Caribbean skinks.  You can read more about this work in the paper he published in Zootaxa earlier this year with Caitlin Conn (pdf link from Hedges webpage) and in Jonathan’s previous AA post on this paper. Using analyses of a dataset that includes four genes (three mitochondrial and one nuclear) from 136 individuals representing 14 of 16 genera, Hedges and Conn report discovery of dozens of previously unrecognized species and advocate recognition of 16 genera of skinks across the new world.  Unfortunately, many of the new species identified by Hedges and Conn seem to already be extinct, and Hedges showed some very compelling data to support the hypothesis that the decline of skink populations was a response to the arrival of the mongoose.

Photos from Cádiz and Birds report in IRCF Reptiles & Amphibians.

Trunk anoles are the least diverse and most enigmatic of the six replicated anole ecomorph categories (the others are grass-bush, trunk-ground, trunk-crown, twig, and crown giant). Numerous species of trunk anoles belonging to the distichus species group are a dominant component of Hispaniola’s anole fauna, but Cuba has only one very rare trunk anole species and Jamaica and Puerto Rico have no trunk anoles whatsoever.  In the latest IRCF Reptiles & Amphibians: Conservation and Natural History, Cádiz and Bird report the first occurrence of the Cuban trunk anole – A. loysianus – on Isla de Juventud.  This is the first time that this spectacular little anole species has been reported from Cuba’s largest satellite island.

As impressive as their current diversity may be, anoles have a rather pathetic fossil record.  Aside from a smattering of subfossils that are less than a million years old, all we have to work from are a few amber specimens from the Dominican Republic preserved over a narrow temporal window (15-20 mybp).  This lack of fossils is problematic because molecular genetic data suggest that that Anolis may be more than 50 million years old.  The fossil record for anole relatives isn’t much better, with only a handful of well-characterized fossils that are greater than 10 or 20 million years old existing across Iguania.  Although they are relatively uncommon, previous reports of iguanian fossils have occasionally been sloppy about assignment to extant clades.  Daza et al. (2012) have done a remarkable job of clarifying our understanding of fossil iguanians by providing not only a remarkably detailed and nuanced description of a new fossil iguanian from the Late Pliocene, but also by conducting quantitative phylogenetic analyses that place this fossil in a large tree that includes a diverse range of other fossil and extant iguanians.

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Anolis carolinensis is increasingly used as a genetic model organism, but we know suprisingly little about the distribution of geographic genetic variation in this species across its native range.  At this year’s evolution meetings, Marc Tollis presented his recently published work on phylogeography of Anolis carolinensis.  His work provides basic information on geographic genetic diversity within A. carolinensis, and permits tests of hypotheses about the contribution of riverine barriers, sea-level changes, and southern refugia to this diversity.  Tollis sampled 190 anoles from 9 states and obtained sequence data from mtDNA and 10 novel nuclear loci (4 introns and 6 anonymous loci).  Using phylogenetic analyses and the Bayesian clustering algorithms in Structurama, Tollis identifies four major clades that appear to have diverged from one another around 2 million years ago: North Carolina, Gulf-Atlantic, Suwannee, and Everglades.  Although these populations appear to have experienced range expansions, Tollis rejects the southern refugium hypothesis because expansion events predate the inter-glacial, genetic diversity is no greater in the south, and there is no consistent pattern of northern genotypes nested within southern genotypes.  Instead, Tollis’s data points to a rapid and recent westward expansion.    Tollis’s work also rejects the hypothesis that rivers are important barriers to Anolis carolinensis dispersal, a result that he suggests is not surprising given the group’s well-established overwater dispersal capabilities.  Because this phylogeographic work on Anolis carolinensis rejects both the riverine barrier and refugium hypotheses, it appears that the distribution of genetic diversity is somewhat unique and not widely shared with other taxa distributed across the same region.  Phylogeographic analyses of A. carolinensis are long overdue and Tollis’s presentation and associated publication are a most welcome contribution to the field.

Tollis M, Ausubel G, Ghimire D, & Boissinot S (2012). Multi-Locus Phylogeographic and Population Genetic Analysis of Anolis carolinensis: Historical Demography of a Genomic Model Species. PloS one, 7 (6) PMID: 22685573

The first poster session at Evolution 2012 got off to a great start last weekend with Rosario Castañeda’s poster on ecomorphological evolution in mainland Anolis.  Currently a post-doc in the Losos Lab at Harvard, where she’s working to bring Anolis to the Encylopedia of Life, Rosario’s Ph.D. thesis with Kevin de Queiroz and associated publication investigated phylogenetics and ecomorphological diversification of the Dactyloa clade of Anolis.  For those who aren’t already familiar with the Dactyloa clade, this group of impressive anoles can be found in the Lesser Antilles and South America.  In her poster, Rosario presents results obtained by combining her multi-locus phylogeny with morphometric data.

Using UPGMA analyses of principal component axes extracted from morphological measurements of 50 species of Dactyloa and 28 species from the Greater Antilles, Rosario initially reported recovering nine distinct morphological clusters (four of which include only one species).  Rosario further reported that these phenotypic clusters do not correspond with monophyletic groups on her phylogenetic tree, suggesting that each cluster did not simply evolve a single time.  Finally, Rosario used distances among species in morphometric space to show that fifteen species in the Dactyloa clade are similar to one of the replicated Greater Antillean anole ecomorphs.  She specifically reported that species from the Dactyloa clade can be assigned to trunk-crown, trunk-ground, and twig ecomorphs.