Abstract
The diatom Didymosphenia geminata has gained notoriety due to the massive growths which have occurred in recent decades in temperate regions. Different explanations have been proposed for this phenomenon, including the emergence of new invasive strains, human dispersion and climate change. Despite the fact in Argentina nuisance growths began in about 2010, historical records suggest that the alga was already present before that date. In addition, preliminary genetic data revealed too high a diversity to be explained by a recent invasion. Here, we estimate the divergence times of strains from southern Argentina. We integrate new genetic data and secondary, fossil and geological calibrations into a Penalized Likelihood model used to infer 18,630 plausible chronograms. These indicate that radiation of the lineages in Argentina began during or before the Pleistocene, which is hard to reconcile with the hypothesis that a new variant is responsible for the local mass growths. Instead, this suggests that important features of present distribution could be the result of multiple recent colonizations or the expansion of formerly rare populations. The text explains how these two possibilities are compatible with the hypothesis that recent nuisance blooms may be a consequence of climate change.
Highlights
The colonial diatom Didymosphenia geminata (Lyngbye) Schmidt has been abundant in some rivers of the north temperate zone for many years
For one of the samples from the Futaleufú River (FTa), the amplified 28S sequences did not correspond to D. geminata and the rbcl PCR produced negative results
No data could be generated for five samples from four different rivers, because no DNA could be amplified or because the amplified sequences didn’t correspond to D. geminata (28S amplicons from samples DVa and DVb)
Summary
The colonial diatom Didymosphenia geminata (Lyngbye) Schmidt has been abundant in some rivers of the north temperate zone for many years. The facts that D. geminata rapidly invaded many New Zealand rivers and has been reported to be introduced species there[8,9] have suggested that the nuisance growths elsewhere were due to colonization of ecosystems in which the alga was formerly absent. The incipience of molecular studies results in difficulties to generate sequence data because it requires using primers designed based on the few available D. geminata sequences, or primers targeting a wide range of taxa. The later procedure is not a straightforward solution, because other microorganisms and organic debris are usually entangled into D. geminata mats, the material usually chosen for sampling, which results in co-amplification of sequences from other taxa when using nonspecific primers. The tMRCA should be of a few My, based on the oldest fossils known for the species and related taxa worldwide
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