Population genetic patterns in the rocky hhore crab Pachygrapsus marmoratus from the Western Mediterranean and Eastern Atlantic: complementary results from mtDNA and microsatellites at different geographic scales

Until recently, it was generally believed that marine species with planktonic larval dispersal would be genetically homogeneous across their geographic range. Nowadays, however, there is increasing evidence for genetically structured marine populations among larval-dispersed species, and thus a higher degree of intra-specific local variability than expected. The sometimes very complex patterns of intra-specific genetic differentiation may be due to historical environmental factors related to habitat, currents and sea level fluctuations, and/or to current species-specific traits and present-day ecological factors. Studies on population genetic structure are commonly based on the geographical distribution of mitochondrial haplotypes or, alternatively, on polymorphism at microsatellite loci. These genetic markers have different molecular and evolutionary properties, and, as a consequence, they may reveal different distribution patterns of the recorded genetic variation. Mitochondrial DNA is a haploid genetic marker, exclusively inherited from the mother, and with a medium to low level of genetic variability. It thus enables the reconstruction of ancient processes. In contrast, microsatellites represent a co-dominant genetic marker with high variability, suitable for the investigation of recent colonization and migration events. The aim of the current study is to investigate the population genetic structure on both sides of the Atlanto-Mediterranean biogeographic boundary of the intertidal crab Pachygrapsus marmoratus (Brachyura: Grapsidae) by sequencing a 650 basepair region of the mitochondrial DNA gene cytochrome oxidase subunit I of more than 200 individuals from 15 populations throughout the western Mediterranean Sea and eastern Atlantic Ocean. Moreover, for a subset of western Mediterranean populations, we also genotyped about 300 individual crabs at 6 microsatellite loci. While the mitochondrial data show a recent and weak partitioning of genetic variation at a medium to large scale (it is based on rare alleles), the microsatellite loci reveal genetic differentiation among populations at a local scale. The relative advantages of these two genetic marker systems for studies of population dynamics of highly dispersive marine organisms and the importance of using these methods complementarily is discussed.