Phylogeography of the Galician crabs necora puber and maja brachydactyla in the ne atlantic

Resumen

This thesis focus on the phylogeography of the Atlantic populations of two marine crabs: Necora puber and Maja brachydactyla. These species are common inhabitants of the northeast Atlantic European shores, and representative of coastal marine invertebrates under considerable commercial exploitation. So far, no genetic studies have been carried out at the population level, although this information is basic for the design of any effective management and conservation strategy. Thus, our first aim was to describe their genetic diversity and population structure, paying special attention to Galicia, the northwestern region of the Iberian Peninsula, where they are particularly relevant socio-economic resources. Our second aim was to infer the processes that could explain their current genetic patterns, addressing the role of evolutionary forces (e.g. gene flow and genetic drift) and past events (e.g. tectonic movements and glaciations) in shaping the phylogeographic history of these marine species (Chapter 1). With this in mind, we developed a set of highly variable nuclear markers (microsatellites) and optimized mitochondrial markers (partial COI and 16S fragments) suitable for this population analysis (Chapter 2). Mitochondrial diversity for ten samples of N. puber (spanning from France to Portugal) was quite low and very similar across localities (Chapter 3). We did not find any significant mitochondrial differentiation between these populations, although the French sample seems to be slightly distinct. We detected signs of a past population expansion (spatial and demographic) along the sampled range, probably during the Late Pleistocene after the Last Glacial Maximum. Unfortunately we could not contrast these results with nuclear data, as the isolation of polymorphic loci failed despite many attempts. Interestingly, we observed ambiguous positions in the mitochondrial sequences of some individuals, mainly linked between the two markers (COI and 16S) and mainly found in one locality. Although we corroborated that they did not distort population inferences, it was a striking outcome that deserves further assessment. For M. brachydactyla, we were able to cover a wider geographic area, from Ireland in the north to the Canary Islands in the south (Chapter 4). In this case, the 16S fragment revealed invariable across samples but, in turn, we successfully developed nine polymorphic microsatellite loci. Both COI and microsatellites showed the same pattern: high diversity, similar for all populations, and only minor differentiation towards the latitudinal limits of the sampled region. M. brachydactyla populations also fit an expansion model during the Late Pleistocene, but earlier before the Last Glacial Maximum. Taken together, our results suggest that, in both species, gene flow is an important force in the genetic homogenization of their respective populations. Considering that juveniles and adults of these crabs are mainly sedentary, and that the divergence trends observed mainly agree with extant oceanic currents in the northeast Atlantic, the most plausible mean of migration should be larval dispersal. In addition, although our estimates of population sizes are rough, it seems that appreciably large effective population sizes across the range could be preventing the random loss of alleles through genetic drift (Chapter 6). Results also point to a marked effect of Pleistocene glaciations in the current genetic variation and distribution of N. puber and M. brachydactyla, although suggesting that the impact on the former could have been stronger. This hypothesis becomes even more plausible when we note that both species seem to be already present in European coasts since the Pliocene, according to the fossil record. Moreover, the influence of the natural sweepstakes recruitment of marine organisms could contribute to this common pattern of shallow population histories even for deep evolutionary lineages. Finally, in special for N. puber, we cannot rule out the role of selection in reducing its mitochondrial diversity through selective sweeps (Chapter 6). In the case of M. brachydactyla we could confirm this condition of a pre-Pliocene origin but a recent Pleistocene expansion along the northeast Atlantic, once we analyzed this species in the context of the genus Maja in the Eastern Atlantic and the Mediterranean basins (Chapter 5). We addressed this last issue after corroborating (with COI and 16S sequences) that the norhteast Atlantic populations of the spiny spider crab were effectively distinct from all the Mediterranean ones, supporting their identification as a separate species, M. brachydactyla, as proposed by Neumann in 1998. This was an important finding because they were traditionally considered the same species as the Mediterranean M. squinado, because of their remarkable morphological resemblance and shared commercial importance. In addition, a more extensive phylogenetic study was accomplished by including, together with all the European species, three Asian Maja species and two Maja crabs from South Africa. This phylogeny supported the definition of the Eastern Atlantic Maja spider crabs as a single mitochondrial lineage as well as the definition of all the European Maja species as a monophyletic clade that most likely diverged from an Indo-West Pacific ancestor during the Early Miocene. Thus, we suggest a reformulation of the taxonomy of the genus Maja in the Eastern Atlantic and the Mediterranean, and depict their possible biogeographic history for the first time.