ABSTRACT
Assessing genetic connectivity among populations in high gene flow species is sometimes insufficient to evaluate demographic connectivity. Genetic differentiation quickly becomes zero as soon as a few dozen migrants are exchanged per generation. This provides little information to determine whether migration can ensure demographic coupling. The resulting difficulties in delineating conservation units for the management of commercially exploited marine fish species are well illustrated in the case of the European sea bass (Dicentrarchus labrax). Previous attempts to assess connectivity patterns in the northeast Atlantic have been hampered by a lack of spatial genetic structure. In contrast, mark-recapture data suggested low migration rates between regional spawning areas. Here, we show how a spatial gradient of introgressed Mediterranean ancestry across the northeast Atlantic reflects cryptic patterns of genetic and demographic connectivity. Using a 1K SNP chip data set in 827 individuals sampled from Portugal to the North Sea, we found null overall genetic differentiation across the northeast Atlantic. We however detected a subtle latitudinal admixture gradient originating at the edge of the contact zone with the Mediterranean sea bass lineage. Two significant breaks in the ancestry gradient at the tip of Galicia and northern Brittany indicated barriers to effective dispersal between demographically distinct units. Moreover, a northward expansion signal in Irish and North Seas was revealed by the surfing of rare Mediterranean alleles at the edge of the species range. Our results show that introgressed ancestry gradients offer a powerful alternative to assess genetic and demographic connectivity when the neutral migration-drift balance is not informative.
Subject(s)
Gene Flow , Genetics, Population , Animals , Genetic Variation , Mediterranean Sea , North Sea , PortugalABSTRACT
The systematic position of Amphidoma caudata Halldal within the genus Amphidoma has remained uncertain as a result of its plate formula and the absence of molecular phylogenetic data. Also, this thecate dinoflagellate taxon has been used to designate two distinct morphotypes. The present study aims to clarify the generic affiliation of Amphidoma caudata and the taxonomic value of two different morphotypes M1 and M2. The new examination of the plate formula using SEM showed that it was the same for both morphotypes and that it corresponded to the tabulation of the recent erected genus Azadinium Elbrächter et Tillmann. Morphometric analysis, using cell size, length of apical projection in conjunction with the cell length, and the ratio of horn and spine showed that M1 and M2 formed two distinct groups. These results were supported by a molecular approach, revealing notable differences in the sequences of LSU rDNA and ITS region between these two morphotypes. Phylogenetic analyses inferred either from LSU and combined SSU, ITS region and COI data positioned M1 and M2 in a sister cluster of Azadinium species while Amphidoma languida Tillmann, Salas et Elbrächter, the only species of Amphidoma for which sequence data were available, was situated in a basal position of the Azadinium clade. Thus, we propose the transfer of Amphidoma caudata to the genus Azadinium and, consequently, the rehabilitation of the original tabulation of the genus Amphidoma Stein. To discriminate the two morphotypes, we propose a rank of variety with the following designations: Azadinium caudatum var. caudatum and Azadinium caudatum var. margalefii.
