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Center for Advanced Studies of Blanes (CEAB-CSIC), Acces a la Cala S. Francesc 14, 17300 Blanes (Girona), Spain ABSTRACT: In sedentary benthic invertebrates with pelagic larval stages, the genetic composition of individuals that recruit at a given site is determined by the interaction between factors such as non- random mating, differential reproductive success, dispersal ability, selective pressures, and environ- mental factors. We studied the temporal variation of genetic structure of several cohorts of the sea urchin Paracentrotus lividus (Lamarck) collected over 2 consecutive years (2006, 2007) from a local- ity in the NW Mediterranean. Sea urchins bigger than ca. 1 cm in diameter were collected and aged using growth bands visible in the tests. Small recruits of the year were collected by scraping off algal substrates. A fragment of the mitochondrial gene of the cytochrome c oxidase subunit I (COI) and a fragment of the nuclear gene of the gamete recognition protein bindin were sequenced for 374 and 316 urchins, respectively. Differentiation between cohorts was smaller than between spatially sepa- rated populations for both markers. Likewise, our results do not show a reduction of diversity within cohorts over time or a reduced diversity of the juveniles relative to adult populations. No significant changes in allelic frequencies were detected between cohorts for COI, indicating that sweepstake episodes—in which few individuals dominate reproductive events, resulting in high levels of genetic variance—are not common in this species. In contrast, a stronger signal of differentiation was found for bindin, with many pairwise comparisons among cohorts being significant. This differentiation was mainly due to positively selected codons in this gamete recognition protein, which suggests some degree of non-random mating in Paracentrotus lividus. This can be due to spatial and temporal het- erogeneity in the pool of gametes resulting in inter-cohort differences in the composition of bindin alleles that maximized fertilization. Our results indicate that processes occurring previous to fertiliza- tion are important in shaping the genetic structure of populations. This information is relevant if man- agement plans are to be designed for this commercially interesting species.

Temporal genetic variability in the Mediterranean common sea urchin Paracentrotus lividus