Abstract
The accuracy of phylogenetic inference can be significantly improved by the addition of more taxa and by increasing the spatial coverage of sampling. In previous studies, the brown mussel Perna perna showed a sister–lineage relationship between eastern and western individuals contiguously distributed along the South African coastline. We used mitochondrial (COI) and nuclear (ITS) sequence data to further analyze phylogeographic patterns within P. perna. Significant expansion of the geographical coverage revealed an unexpected pattern. The western South African lineage shared the most recent common ancestor (MRCA) with specimens from Angola, Venezuela, and Namibia, whereas eastern South African specimens and Mozambique grouped together, indicating a non-sister relationship for the two South African lineages. Two plausible biogeographic scenarios to explain their origin were both supported by the hypotheses-testing analysis. One includes an Indo-Pacific origin for P. perna, dispersal into the Mediterranean and Atlantic through the Tethys seaway, followed by recent secondary contact after southward expansion of the western and eastern South African lineages. The other scenario (Out of South Africa) suggests an ancient vicariant divergence of the two lineages followed by their northward expansion. Nevertheless, the “Out of South Africa” hypothesis would require a more ancient divergence between the two lineages. Instead, our estimates indicated that they diverged very recently (310 kyr), providing a better support for an Indo-Pacific origin of the two South African lineages. The arrival of the MRCA of P. perna in Brazil was estimated at 10 [0–40] kyr. Thus, the hypothesis of a recent introduction in Brazil through hull fouling in wooden vessels involved in the transatlantic itineraries of the slave trade did not receive strong support, but given the range for this estimate, it could not be discarded. Wider geographic sampling of marine organisms shows that lineages with contiguous distributions need not share a common ancestry.
Highlights
Increased taxon sampling recognizably improves phylogenetic inference (Hillis 1996) while in ecology, the understanding of distribution patterns depends critically on the scales at which observations are made (Wiens 1989; Rahbek 2005)
Bayesian inferences (BI) and Maximum likelihood (ML) analyses based on the concatenated dataset recovered identical topologies with the single exception that the relative phylogenetic position of P. viridis as the sister species to the remaining Perna received no statistical support in the latter
BI analysis based on the concatenated dataset recovered three well-supported clades within P. perna (Fig. 2), whereas only a single clade was recovered in the nuclearbased BI analyses (Material S2)
Summary
Increased taxon sampling recognizably improves phylogenetic inference (Hillis 1996) while in ecology, the understanding of distribution patterns depends critically on the scales at which observations are made (Wiens 1989; Rahbek 2005). The analysis of a small subset of species that are presumed to be representative can distort phylogenetic relationships (Tuinen et al 2000; Murphy et al 2001). This can be mitigated by large sequence data sets that improve the accuracy of phylogenetic estimation (Poe and Swofford 1999; Heath et al 2008), but using only a few representatives from a particular group can produce misleading results and incorrect topologies even with increased sequence length (Zwickl and Hillis 2002; Heath et al 2008).
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