The Atlantic-Mediterranean transition: discordant genetic patterns in two seabream species, Diplodus puntazzo (Cetti) and Diplodus sargus (L.).

Sparids are a group of demersal perciform fish of high commercial value, which have experienced an extensive radiation, particularly in the Mediterranean, where they occupy a variety of different niches. The present study focuses on two species: Diplodus sargus and D. puntazzo, presenting a wide distribution from the Mediterranean to the eastern Atlantic coasts. They display similar ecological behaviour and are evolutionary closely related. Both are highly appreciated in fisheries and D. puntazzo is currently under domestication process. However, little is know on their population structure and it is an open question whether any genetic differentiation exists at the geographic level. To address this issue we examined sequence variation of a portion of the mitochondrial DNA (mtDNA) control region in population samples of each of the two species collected over a wide geographic range. In addition to the mtDNA, analysis of nuclear loci (allozymes) was included in the study to compare patterns revealed by nuclear and mitochondrial markers. The studied samples covered an area from the eastern Mediterranean to the Portuguese coasts immediately outside the Gibraltar Strait. The two species revealed a level of sequence polymorphism remarkably different for the control region with the D. puntazzo and D. sargus showing 111 and 28 haplotypes, respectively. Such a difference was not detected with allozyme markers. The two species also showed large differences in their population structure. While D. puntazzo presented a marked genetic divergence between the Atlantic and Mediterranean samples, D. sargus showed little intraspecific differentiation. These results were supported using both mtDNA and allozyme markers, and were interpreted as the consequence of differences in the history of the two species such as fluctuations in the effective population size due to bottlenecks and expansions, possibly combined with present-day differences in levels of gene flow.