Abstract
Islands provide exciting opportunities for exploring ecological and evolutionary mechanisms. The oceanic island of São Tomé in the Gulf of Guinea exhibits high diversity of fauna including the endemic caecilian amphibian, Schistometopum thomense. Variation in pigmentation, morphology and size of this taxon over its c. 45 km island range is extreme, motivating a number of taxonomic, ecological, and evolutionary hypotheses to explain the observed diversity. We conducted a population genetic study of S. thomense using partial sequences of two mitochondrial DNA genes (ND4 and 16S), together with morphological examination, to address competing hypotheses of taxonomic or clinal variation. Using Bayesian phylogenetic analysis and Spatial Analysis of Molecular Variance, we found evidence of four geographic clades, whose range and approximated age (c. 253 Kya – 27 Kya) are consistent with the spread and age of recent volcanic flows. These clades explained 90% of variation in ND4 (φCT = 0.892), and diverged by 4.3% minimum pairwise distance at the deepest node. Most notably, using Mismatch Distributions and Mantel Tests, we identified a zone of population admixture that dissected the island. In the northern clade, we found evidence of recent population expansion (Fu's Fs = −13.08 and Tajima's D = −1.80) and limited dispersal (Mantel correlation coefficient = 0.36, p = 0.01). Color assignment to clades was not absolute. Paired with multinomial regression of chromatic data, our analyses suggested that the genetic groups and a latitudinal gradient together describe variation in color of S. thomense. We propose that volcanism and limited dispersal ability are the likely proximal causes of the observed genetic structure. This is the first population genetic study of any caecilian and demonstrates that these animals have deep genetic divisions over very small areas in accordance with previous speculations of low dispersal abilities.
Highlights
Islands are self-contained natural laboratories and provide the backdrop to many of Darwin’s and Wallace’s observations that culminated in the theory of evolution [1]
In order to assess population structuring of S. thomense, we investigated the distribution of genetic diversity using mitochondrial DNA
NADH dehydrogenase subunit 4 (ND4) DNA and 538 additional base pairs of mitochondrial 16S rRNA from 138 and 26 individual Schistometopum thomense, respectively, collected at 24 sample locations across the island of Sao Tome
Summary
Islands are self-contained natural laboratories and provide the backdrop to many of Darwin’s and Wallace’s observations that culminated in the theory of evolution [1] Continuing in this role, they provide contemporary biologists with the opportunity to investigate ecological and evolutionary mechanisms [2,3,4,5,6,7,8]. Isolation and competition for open ecological niches promote speciation [6], placing these locales in a prominent role when considering conservation of biodiversity [9]. In this context, the use of molecular phylogenetics has elucidated insular taxonomic relationships, providing insight into the mechanisms of island colonization and ecological radiations [10,11]. Multiple independent colonization events have contributed to diversity, evidenced by phylogenetic relationships of geckos [22,23], skinks [24], and multiple snake species [25] on Sao Tomeand nearby islands
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