Gone with Gondwana: amphipod diversification in freshwaters preceded the breakup of the supercontinent

Aim

The evolutionary origins of modern taxa are best understood as arising from the interplay of vicariance and dispersal. Vicariant events have long been considered responsible for Gondwanan distributions; such species are relics of the eponymic supercontinent on which they were thought to have originated. One such set of taxa are the freshwater members of the amphipod superfamily Hyaloidea, which due to their marine relatives and current distributions serve as an excellent model for testing vicariance and dispersal hypotheses. We investigated the evolutionary and biogeographic histories of the Hyaloidea using a molecular phylogenetic approach.

Location

North/South America, Australia

Taxon

Superfamily Hyaloidea

Methods

Publicly available nucleotide sequences from GenBank and BOLD were collected for 148 members of the order Amphipoda. These data, including sequences from two nuclear genes and one mitochondrial gene were aligned in order to examine evolutionary relationships within the order. Phylogenetic trees were reconstructed using both Maximum likelihood analyses and Bayesian inference, Bayesian trees were time calibrated based on previously estimated divergence dates.

Results

Maximum likelihood analyses and Bayesian inference, using two nuclear genes and one mitochondrial gene, reveal the freshwater amphipods within the superfamily (Hyalellidae/Chiltoniidae) as a monophyletic group within the superfamily. Members of this lineage are shown to have diverged from their marine ancestors during the Early Cretaceous period, suggesting a singular invasion of freshwater.

Main conclusions

A single Cretaceous divergence event for these animals rather than multiple Paleogene events is supportive of a vicariant origin following the breakup of Gondwana, instead of a recent marine invasion as has been previously hypothesized. The ancestor to the freshwater Hyaloidea likely invaded what is now Antarctica, allowing for dispersal across southern Gondwana. Fragmentation of the supercontinent would have severed gene flow between taxa, setting hyalellids and chiltoniids on separate evolutionary trajectories. A Cretaceous invasion by the Hyaloidea suggests that freshwater members represent a much older lineage than previously considered, occupying continental freshwaters prior to the gammarids in the Mesozoic and cotemporally with the crangonyctids in the Cenozoic. Our results highlight the utility of amphipods for testing biogeographic hypotheses that infer the origin of freshwater taxa.