Trans-Arctic speciation of Florideophyceae (Rhodophyta) since the opening of the Bering Strait, with consideration of the “species pump” hypothesis

Aim: The opening of the Bering Strait initiated significant biotic interchange that is postulated to have played a major role in phylogeographical patterns of northern marine flora and fauna. In addition, the “species pump” hypothesis asserts that glaciation events promoted speciation due to repeated isolation of populations over the past 2.6 million years. Here, trans-Arctic speciation events in red marine macroalgae (Florideophyceae) were assessed using time-calibrated phylogenies, and the applicability of the “species pump” hypothesis was considered. Location: Species records and sequence data for trans-arctic genera of marine macroalgae were amalgamated and supplemented with sampling from the Northwest Atlantic, Northern Alaska (Beaufort Sea), Norway, and Nome, Alaska (Bering Sea; 2014–2017). Methods: Bayesian and maximum likelihood phylogenies were variously built using the 5′ end of the cytochrome c oxidase subunit I gene (COI-5P), and/or the full-length nuclear internal transcribed spacer region (ITS), and/or the ribulose-1 5-biphosphate carboxylase large subunit gene (rbcL), and nodes were timed using calibrated COI-5P and rbcL molecular clocks. The final dataset represented approximately 184 species, broadly representing 14 trans-arctic lineages. Results: Pacific to Atlantic migration and subsequent speciation was inferred in 11 cases, whereas the opposite scenario, atlantic to pacific, was inferred once; only three speciation events appeared to occur during the Pleistocene. Main conclusions: Our results are in agreement with previous studies in that trans-arctic speciation events postdated the opening of the Bering Strait with a clear pacific to atlantic bias. Evidence for the “species pump” (as applied to trans-arctic interchange) was lacking given the frequency of trans-arctic speciation events was not amplified during the Pleistocene