Surfing the wave on a borrowed board: range expansion and spread of introgressed organellar genomes in the seaweed Fucus ceranoides L.

For many taxa, introgression represents an important source of genetic variation, but the specific contexts allowing locally introgressed material to spread and largely replace native allelic lineages throughout a species range remain poorly understood. Recent demographic-genetic simulations of spatial expansions show that the stochastic surfing of alien alleles during range expansions may constitute a general mechanism leading to extensive introgression, but empirical evidence remain scarce and difficult to distinguish from selection. In this study, we report a compelling case of such a phenomenon in the estuarine alga Fucus ceranoides. We re-assessed the phylogenetic relationships among F. ceranoides and its marine congeners F. vesiculosus and F. spiralis using nuclear, mitochondrial and chloroplast sequence data, and conducted a mtDNA phylogeographic survey in F. ceranoides. Our phylogenetic analyses revealed a recent and asymmetric introgression of a single F. vesiculosus cytoplasm into F. ceranoides. The phylogeographic scope of introgression was striking, with native and introgressed mtDNA displaying disjunct distributions south and north of the English Channel. A putative Pleistocene climatic refugium was detected in NW Iberia, and the extensive and exclusive spread of the alien cytoplasm throughout Northern Europe was inferred to have occurred concurrently with the species post-glacial, northwards range expansion. This massive spread of a foreign organelle throughout the entire post-glacial recolonization range represents good empirical evidence of an alien cytoplasm surfing the wave of a range expansion and the first description of such a phenomenon in the marine realm.