Antarctica as an evolutionary incubator? Phylogenetic comparative study of the amphipod family Iphimediidae on the Antarctic shelf

From about 40 Mya, while Antarctica geographically isolated from the rest of the world, the marine shelf fauna faced a dramatic decrease in water temperatures. Many lineages went extinct, while others adapted and flourished. The Antarctic clade of the amphipod family Iphimediidae was among the successful ones. Here, a range of cutting-edge phylogenetic comparative and morphometric methods are used to investigate the evolutionary processes which generated the exceptional diversity of this clade. To this purpose, three types of data were collected: (1) a novel phylogeny of the family was first reconstructed using a multigene molecular dataset, (2) 3D shape data were obtained by applying 3D-geometric morphometric methods on micro-CT scans of the specimens and (3) stable isotope d13C and d15N were measured and used as variables describing the trophic ecology of the species. First, possible evolutionary correlations between 3D-morphometric dataset of mouthparts (mandible, maxilliped) and stable isotope ratios were assessed. Significant correlations suggest an adaptation of the mandible shape to the food source. Secondly, as a prerequisite to the study of species diversification, species boundaries were investigated using a combination of DNA-based delimitation methods and detailed morphological/morphometric analyses. The latter showed that species diversity in Antarctic iphimediids is greatly underestimated, as most of the described species appear to be complexes of multiple morphologically similar species. Finally, potential changes in the rate of lineage diversification were explored in parallel to the evolution of morphological traits (mouthparts 3D shape data) along the phylogeny. On one hand, we found no evidence of an early burst of lineage diversification. On the other hand, late bursts (appr. 7-3 Mya) were detected in two subclades. Such late radiations could result from the invasion of novel ecological niches, as a late partitioning of mouthparts’ shape diversity along the phylogeny is also observed. Plio-Pleistocene glacial cycles, which have been hypothesized to act as a diversity pump, might also have promoted late diversification events in Antarctic iphimediids. By applying such an integrative approach for the first time on Antarctic invertebrates, this study improves our general understanding of the evolutionary dynamics shaping the standing Antarctic shelf biodiversity.

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