A genetically distinct population of three-lined salamanders, Eurycea guttolineata, in the Florida Parishes of Louisiana
Thomas W. Cannon, Nathaniel T. Akers, Tamara L. Walker and David A. Beamer
Mathematics and Science Department, Nash Community College, Rocky Mount, NC
Discussion
Bayesian analysis of our cytb haplotypes suggests that three-lined salamanders are characterized by extreme genetic homogeneity (Fig. 2). Cryptic diversity has been a common theme in salamander systematics ever since molecular techniques have been applied. Thus, our finding of very shallow levels of divergence over such a large area is somewhat surprising. The low levels of genetic diversity we have uncovered stands in sharp contrast to patterns of genetic diversity in other co-distributed plethodontid salamanders. Two other members of the genus Eurycea (quadridigitata complex and bislineata complex) as well as several Desmognathus species are characterized by very strong and deep regional genetic structure.
Despite the low levels of genetic diversity there are some patterns of geographic structure. There is a shallow break associated with the Apalachicola River though this area also appears to be a zone of mixing between haplotype clades. There is also some genetic structure associated with the Savannah River basin (this basin has played a much more prominent role in the histories of other plethodontid salamanders). There is also a clade (represented by only a few samples) that is associated with the fall line in Alabama and Mississippi. Finally the most divergent populations are located in the extreme southwestern portions of the range in Louisiana and adjacent Mississippi.
We have limited data on the hybridization between E. longicauda and E. guttolineata. Several samples from the areas of putative hybridization in northern Alabama had mtDNA haplotypes characteristic of E. longicauda. Though these populations were identified as E. longicauda in the field and the mtDNA matches this assertion it will be necessary to gather nuclear data to rule out the possibility of hybridization in this region. We did detect a haplotype closely associated with E. guttolineata in a population morphologically typical of E. longicauda in the Ocoee River gorge of Tennessee. This result would appear to represent a clear case of hybrid introgression.
Acknowledgments
We would like to thank the following people for their assistance in the collection of the molecular data: Larry L. Cross, Jason H.Tant, Christopher Moss, Christina Peroutka, Trip Lamb, Kallie O'Steen, Gerardo Tinajero, Thomas Bridgers, Christopher Hadley and Ashley Short. The following people provided some important samples; Steve Bennett, Dirk Stevenson and Jeff Camper. The SC Department of Natural Resources and Nash Community College Department of Mathematics & Sciences provided funds for the collection of the molecular data.
Abstract
The three-lined salamander, Eurycea guttolineata, is a wide-ranging lungless salamander in the southeastern United States. We have sampled seventy-seven populations spanning the entire distribution of the three-lined salamander. We have amplified, purified, and sequenced 872 base pairs of the mitochondrial gene Cty-b and used this data to reconstruct the evolutionary history of three-lined salamanders. Our Bayesian phylogenetic reconstruction reveals a pattern of extreme genetic homogeneity across the range of three-line salamanders. However, in the Florida Parishes of Louisiana, we found that populations are highly differentiated genetically from adjacent areas. Here we examine potential biogeographic barriers that may explain this pattern of differentiation.
Introduction
The three-lined salamander is a member of the brook salamanders (Eurycea). They are widely distributed across the southeastern United States, occurring in a wide variety of habitats from the high Appalachian mountains to low coastal swamps. Despite their wide distribution, three-lined salamanders have not been the subject of a molecular phylogenetic survey.
Until recently the three-lined salamander was treated as a subspecies (E. l. guttolineata) of the long tailed salamander (E. longicauda). However, the most recent taxonomic treatment afforded specific status (E. guttolineata). There are some records of potential hybrids between longicauda and guttolineata near the contact zone in Alabama and Georgia. Here we examine genetic variation among populations of three-lined salamanders throughout their range (including populations within the putative hybrid zone with longicauda).
Methods
We focused our collecting efforts along independent river drainages, which we consider to be an appropriate sampling unit for a geographic survey of three-lined salamanders. We collected specimens from 77 populations currently recognized as E. guttolineata (n = 71 populations) or E. longicauda (n = 6) (Fig 1). Sequence data were obtained for 872 aligned bases of the mitochondrial cytochrome b (cytb) gene for individuals from each population.
Sequences were edited in Geneious 6.0.5 and the resulting consensus sequence was aligned with Clustal X. Phylogeny reconstruction utilized Bayesian inference (Fig 2). Phylogeny reconstruction involved Bayesian inference. We used MrModeltest to identify the model of sequence evolution most appropriate for each of the three codon positions in the cytb segment. We analyzed these data using MrBayes 3.1.2, incorporating general model information identified for the three partitions by AIC. The analysis involved two simultaneous runs – each for eleven million generations – sampling Markov chains every 100 generations. Trees retained after “burn-in” and convergence (9 million generations) were used to create a majority rule consensus showing all compatible partitions.
Fig. 2 Bayesian phylogenetic reconstruction based on 872 nts of cyt-b mtDNA.
Thickened branches represent posterior probabilities greater than 0.95.
Fig. 1 Distribution of clades recovered in phylogenetic reconstruction;
range extent of E. guttolineata shown in light green. Populations numbers and clade
colors match those in Fig. 2.