Drifting fronds and drifting alleles: the genetic architecture of the estuarine seaweed Fucus ceranoides L.

In this thesis I investigate the population genetic structure of the estuarine seaweed Fucus ceranoides. My main objective was to look into the processes shaping its genetic make-up, particularly the effects of limited dispersal, past range-dynamics and introgression. In CHAPTER II a combination of molecular markers was employed to infer the patterns of genetic exchange between F. ceranoides and two parapatric congeners, and to assess its consequences for the species intraspecific phylogeography. The results revealed the historical introgression of F. vesiculosus organellar genomes into the F. ceranoides gene pool, and their extensive spread concurrently with the species’ northwards, post-glacial range expansion. This was the first study documenting the role of genetic surfing in producing extensive introgression in a marine organism. In CHAPTER III, the population genetic structure of F. ceranoides is examined throughout its entire distributional range with microsatellite markers with the objective of assessing the effects of limited dispersal and past range dynamics on the geographical organization of its genetic variation. This study showed that restricted dispersal has contrasting effects at the scales of meta-population (connectivity) versus range dynamics (habitat tracking), and that dispersal restrictions can result in either genetic divergence (refugial areas) or homogeneity (recently colonized areas) depending on the maturity and demographic history of the populations. In CHAPTER IV, the fine-scale phylogeographic structure of F. ceranoides in NW Iberia was described using both microsatellites and mtDNA sequence data. Iberian F. ceranoides were subdivided in three genetic groups displaying a nearly perfect parapatric distribution. The lack of evidence for dispersal barriers suggests that the sharp genetic discontinuities detected correspond to secondary contact zones of formerly vicariant sub-populations, which are retained due to persistent founder effects. Taken together, these results confirm the very limited connectivity of F. ceranoides populations and the primacy of history and drift over gene-flow.