Analisi del differenziamento genetico tra popolazioni di Octopus vulgaris Cuvier, 1797 mediante marcatori nucleari e mitocondriali

The common octopus, Octopus vulgaris Cuvier, 1797, represents an important commercial fishery marine resource all over the world, characterized by increasing requests from the markets. In the Mediterranean Sea, especially for the Italian fishery, this species constitutes an important portion of trawling and artisanal landings. In Italy, despite the great socio-economic interest, specific management measures do not exist at the national level for this resource. The common octopus is a very interesting topic both for applied studies and for the basic research, especially at the taxonomic level, because of the uncertainties regarding the occurrence of a single species, with cosmopolitan distribution, or the existence of multiple species forming a ‘species complex’. The principal aim of this work is the genetic characterization of seven populations of O. vulgaris along the coasts of Sardinia in order to obtain useful indications for the selection of correct management units. The genetic analyses have been realized using three different genetic markers, two mitochondrial (the Cytochrome Oxidase I (COI) and III (COIII) genes), and one nuclear marker (five microsatellites loci). All markers proved to be useful in investigating the genetic structure of the populations. In particular, the mitochondrial sequences were useful in comparisons with homologous ones from the GenBank database to evaluate the species’ taxonomy. The results of AMOVA show a substantial homogeneity in Sardinian populations that display low levels of differentiation, both with the nuclear marker (FST=0.004 ns), and the mitochondrial ones (ΦSTCOI=0.003 ns; ΦSTCOIII=0.0002 ns). The pairwise analyses show low levels of differentiation and not significant values for all the genetic markers. The lack of significant genetic differentiation in the Sardinian samples is further confirmed by DAPC, PCA analyses and the Bayesian clustering of the software STRUCTURE. The demography was investigated using all the genetic markers, which pointed out the lack of demographic changes in recent time. The multimodal mismatch distributions reaffirm the occurrence of populations demographically stable. The stability of the populations was confirmed by the haplotype network analyses with the two mitochondrial markers, highlighting a common situation in stationary populations: several principal haplotypes, shared by all the locations, and an increasing number of new secondary haplotypes arising from mutational events. The COI and COIII sequences permitted the comparison of the Sardinian haplotypes with the O. vulgaris sequences available in GenBank. Both markers highlight a genetic affinity among Sardinian specimens and the sequences from the Mediterranean Sea (France, Spain, Central Mediterranean and Turkey), the Eastern Atlantic Ocean (Morocco, Senegal and Galicia), the Southern Atlantic Ocean (South Africa and Tristan da Cunha) and the Southern Indian Ocean (Amsterdam and Saint Paul Islands). This substantial genetic homogeneity contrasts with some sequences (from specimens collected in Turkey, Japan, China, Brazil and Venezuela) that resulted to be highly divergent from all the others. This finding reaffirms the potential existence of several O. vulgaris populations, just partially interconnected, or the even occurrence of distinct species, and emphasizes the need for more detailed phylogeographic and taxonomic studies of the Octopus genus to confirm or exclude the presence of cryptic species within the taxon, as suggested by several authors.