Genetic diversification in the Tropical Western Atlantic Ocean: Phylogeography of the gastropod Bulla occidentalis
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The region under study is the Tropical Western Atlantic (TWA) which includes the Caribbean Sea with adjacent coastlines, the Gulf of Mexico, Bermuda and the coast of South America down to the tropical/temperate transitional zone near Uruguay. There are several examples of genetic breaks within the Caribbean that have been attributed to oceanographic factors, transient allopatry, as well as ecological factors, but no common biogeographical pattern has been found and mechanisms behind diversification within the region are not fully understood. The aim of this project was to shed light on diversification patterns of shallow-water soft-bottom invertebrates in the TWA by using the gastropod Bulla occidentalis as a model species. The following questions were adressed: (1) Is B. occidentalis a homogenous genetic entity or is it made-up of more than one genetic partition (ESUs) as hypothesized by Malaquias and Reid (2009)? (2) What caused the pattern of genetic discontinuities (ESUs)? (3) Are the genetic breaks shared with other species? and (4) Are there periods of major expansion or contraction in population size and what may have caused these events? Material was obtained from museum collections and through fieldwork, and sequences fragments of the cytochrome c oxidase subunit I (COI) and 16S rRNA mitochondrial genes were amplified and sequenced using standard methods. Population genetic indices such as number of haplotypes, haplotype diversity, nucleotide diversity, and fixation indices were estimated, and statistical parsimony haplotype networks for the individual genes were constructed to assess the population structure. The B. occidentalis species genealogy with divergence times between main lineages was estimated based on calibration with the oldest known fossil attributed to the B. occidentalis lineage (Early Miocene, 20,43 - 15,97 Mya) under the assumption of a strict molecular clock. Isolation-by-distance methods were employed to test correlation between genetic differentiation and geographic distance. The demographic history was reconstructed using a Bayesian Skyline method. The B. occidentalis population showed a structured genealogy with three ESUs (A: all coastline samples from Brazil to Eastern Florida, including Yucatan and the islands of Guadeloupe and Bermuda; B: all samples from the Florida Keys; C: predominantly Cuban samples). The three lineages had an average genetic distance of 4,6% - 5,9% (uncorrected pdistance). Divergence between the three lineages was dated to the Late Miocene (11,06 - 6,11 Mya), and may have been caused by vicariance related to the Panamanian Isthmus up-lift. The mechanisms maintaining divergence of these lineages are difficult to pinpoint because no direct link was established between the geographical subdivision and present oceanographic patterns, ecological factors or Isolation-by-distance. Genetic divergence of the Florida Keys-lineage mirrors patterns found in other groups. The genealogy and demographic history reconstruction showed an increase in genetic diversification and effective population size during the Pleistocene. This coincides with an increase in the magnitude of glaciation cycles that may have caused periods of transient allopatry likely reducing population connectivity leading to genetic diversification, as well as potentially creating new niche-opportunities during low sea-level stands allowing the population to expand.