Genetic diversity and connectivity remain high in Holothuria polii (Delle Chiaje 1823) across a coastal lagoon-open sea environmental gradient

Coastal lagoons represent habitats with widely heterogeneous environmental conditions, particularly as regards salinity and temperature,which fluctuate in both space and time. These characteristics suggest that physical and ecological factors could contribute to the genetic divergence among populations occurring in coastal lagoon and opencoast environments. This study investigates the genetic structure of Holothuria polii at a micro-geographic scale across theMar Menor coastal lagoon and nearbymarine areas, estimating the mitochondrial DNA variation in two gene fragments, cytochrome oxidase I (COI) and 16S rRNA (16S). Dataset of mitochondrial sequences was also used to test the influence of environmental differences between coastal lagoon andmarine waters on population genetic structure. All sampled locations exhibited high levels of haplotype diversity and low values of nucleotide diversity. Both genes showed contrasting signals of genetic differentiation (non-significant differences using COI and slight differences using 16S, which could due to different mutation rates or to differential number of exclusive haplotypes. We detected an excess of recent mutations and exclusive haplotypes, which can be generated as a result of population growth. However, selective processes can be also acting on the gene markers used; highly significant generalized additive models have been obtained considering genetic data from16S gene and independent variables such as temperature and salinity.

West versus East Mediterranean Sea: origin and genetic differentiation of the sea cucumber Holothuria polii

We studied the genetic structure of the sea cucumber Holothuria (Roweothuria) polii (Delle Chiaje 1823) by analysing the mitochondrial DNA variation in two fragments of cytochrome oxidase I (COI) and 16S genes. Individuals were collected in seven locations along the Mediterranean Sea, which cover a wide range of the species distribution. We found high haplotype diversity for COI and moderate diversity for 16S, and low nucleotide diversity for both genes. Our results for the COI gene showed many recent and exclusive haplotypes with few mutational changes, suggesting recent or ongoing population expansion. The Western and Eastern Mediterranean populations exhibited slight but significant genetic differentiation (COI gene) with higher genetic diversity in the East. The most ancient haplotype was not present in the westernmost sampling location (SE Spain). The oldest expansion time was observed in Turkey, corresponding to mid-Pleistocene. Turkey had also the highest genetic diversity (number of total and exclusive haplotypes, polymorphisms, haplotype and nucleotide diversity). This suggests that this region could be the origin of the subsequent colonizations through the Mediterranean Sea, a hypothesis that should be assessed with nuclear markers in future research.

Allozyme and mtDNA variation of white seabream Diplodus sargus populations in a transition area between western and eastern Mediterranean basins (Siculo-Tunisian Strait)

To investigate the possible influence of the Siculo-Tunisian Strait on the genetic structure of white seabream Diplodus sargus, 13 polymorphic allozyme loci and a fragment of the cytochrome b mitochondrial DNA were analysed. Allozyme data indicated a moderate but significant differentiation between some north-eastern (Bizerta, Ghar El Melh Lagoon and Mahdia) and southern (Gabes Gulf and El Biban Lagoon) samples. This heterogeneity was also highlighted after removing PGM* and PGI-1* loci which may be under selection. These results can be explained by the chaotic genetic patchiness hypothesis. In contrast, the mtDNA data indicated genetic homogeneity among localities showing the absence of structure in white seabream populations across the Siculo-Tunisian Strait. Historical demography of this species suggests that it has undergone a recent population expansion as a consequence of a bottleneck event during the Pleistocene glaciations.