Evidence of a genetic cline for Sardina pilchardus along the Northwest African coast

Abstract

Chlaida, M., Laurent, V., Kifani, S., Benazzou, T., Jaziri, H., and Planes, S. 2009. Evidence of a genetic cline for Sardina pilchardus along the Northwest African coast. – ICES Journal of Marine Science, 66: 264–271. In all, 700 Sardina pilchardus from 14 locations throughout the Atlantic and Mediterranean Moroccan coastlines were screened for eight enzymatic loci that show polymorphism. Analysis of allelic frequencies showed a genetic break in the area of the Bay of Agadir, segregating two groups of samples, confirmed by analysis of molecular variance; a first consisting of Sidi Ifni (29°12′N) and southern samples down to the southern limit of the species in Mauritania (19°03′N 16°28′W), and a second starting in the Bay of Agadir (30°48′N) and consisting of northern samples that appeared homogeneous with the Mediterranean samples. Specifically, a survey of the SOD* loci demonstrated a genetic cline in allelic frequencies along the coastline. Using a clinal variation model under a state of equilibrium, we estimate a dispersal rate of 189 ± 52 km per generation between Mauritania and Ras Kebdana. This major break towards the south of the Bay of Agadir offers a new perspective on the management of sardine, which ideally ought now to be managed as two stocks along the Moroccan coast. The break is most likely induced by hydrodynamic influences that virtually isolate southern from northern populations.

Evidence for microsatellite hitchhiking selection in European sardine (<i>Sardina pilchardus</i>) and implications in inferring stock structure

“…Tests for correlation between allele frequencies of microsatellite locus Sp22 (4 main alleles, 19 in total) and allozymic locus SOD (2 alleles) in samples collected from the same geographical areas showed that the microsatellite allele Sp22 216 and allozymic allele SOD* 100 (named as in Chlaida et al 2009) exhibited the strongest correlation (R=0.86, P=0.0020) (Fig. 4).…”

“…We do know that such hydrodynamic barrier exists between the Alboran Sea and the Mediterranean (Almeria-Oran front) (Patarnello et al 2007), and the deep ocean separating the Azores and Madeira from the nearby continental shelf probably also acts as a barrier. The abrupt change in allozymic frequency of SOD* off the African Atlantic coast at ~30°N has also been attributed to a hydrodynamic barrier (Chlaida et al 2009), which may also hold for locus Sp22.…”

“…The unveiling of these barriers, most of which were not evident in the neutral variation, can be used to define the following genetic stocks in sardine: 1) an African one (south of 30°N, as has been demonstrated by Chlaida et al 2009); 2) a NE Atlantic one (north of 30 o N to the North Sea and the Alboran Sea); 3) a Mediterranean one (east of the Almeria-Oran front); and 4) one with the Azores and Madeira, which may actually consists of 2 different stocks.…”

“…A boundary was assumed at the 28°N parallel, close to the genetic break recently described by Chlaida et al (2009). However, formal statistical testing of these differences has not been carried out.…”

“…There have been a number of studies addressing the genetic structure of sardine in different parts of its global distribution, using different molecular markers such as allozymes (Spanakis et al 1989, Chlaida et al 2006, Laurent et al 2007, Chlaida et al 2009), mitochondrial DNA (Tinti et al 2002, Atarhouch et al 2006 and microsatellites (Gonzalez and Zardoya 2007a). The results of these studies, though not completely congruent, suggest a very weak genetic structure, with the exception of the allozymic locus SOD*, which could be subject to selection (Chlaida et al 2006;Laurent et al 2007).…”