Oligocene Odyssey: Molecular Phylogenetic Analyses Describe the Origin and Diversification of Freshwater Gammarus in Eastern North America

Aim

Members of the amphipod genus Gammarus have served as an excellent model for testing biogeographic hypotheses given the number of species within the genus and its unique history, with multiple taxa transitioning from marine to freshwater environments independently. However, while attention has been placed on Palearctic Gammarus, little is known about members in the Nearctic. Given trends observed within the genus, Gammarus from eastern North America (ENA) may represent another independent lineage, likely reaching the continent using a heretofore unknown pathway. We investigate the phylogenetic and evolutionary history of Gammarus spp. in this region.

Location

Eastern North America.

Taxon

Gammarus.

Methods

Sequence data for two nuclear and two mitochondrial loci were obtained for 75 individuals, four of which are newly sequenced. Based on a partitioned dataset, phylogenetic trees were reconstructed using both maximum likelihood and Bayesian inference. Bayesian trees were time-calibrated based on a set of five mixed calibration points. RASP software was used to investigate potential ancestral areas.

Results

ENA Gammarus spp. were recovered as monophyletic, showing affinity to both Palearctic Gammarus and G. duebeni. Newly generated sequences solidify the position of Gammarus bousfieldi, which is found to be related to G. minus. Time-calibrated phylogenies place the crown age of the ENA lineage during the Eocene/Oligocene. These timings suggest a marine origin for the lineage followed by dispersal into freshwaters.

Main Conclusions

Members of the “ENA Gammarus” are a unique lineage within the genus, originating from a marine ancestor during the Eocene/Oligocene and colonising the Nearctic shortly after. The exact route the lineage may have taken is unclear; however, three routes are proposed: (i) the Gulf of St. Lawrence, (ii) the Chesapeake Bay, and (iii) the Mississippi River. Results generated here emphasise the lineages' unique evolutionary history, highlighting novel colonisation pathways not often considered for invertebrate taxa.