Allozyme variation in populations of scallops, Pecten jacobaeus (L.) and P. maximus (L.) (Bivalvia: Pectinidae), across the Almeria–Oran front

Abstract

The scallops Pecten jacobaeus and P. maximus have been traditionally considered as different species which shared a common ancestor some 5 Ma ago. This view has been challenged by recent reports of genetic distances based on allozymes and mitochondrial DNA which are incompatible with such a long period of isolation and are more typical of conspecific populations. To explain the slight allozyme differentiation opposed to a clear morphological distinctiveness, two main hypotheses have been proposed. One hypothesis views interpopulation allozyme differentiation as an outcome of simple population genetic processes such isolation by distance. The other considers that the intertaxa allozyme differentiation reflects a genetic discontinuity associated to the Almerı́a–Oran oceanographic front (AOF), which is related with genetic breaks in other marine species. In order to test these hypotheses, we investigated the variation at 15 allozyme loci in six populations of P. jacobaeus and P. maximus, some close to the AOF and some far away from it. We found that the allozyme variation was distributed according to the “front hypothesis”. There was a major genetic discontinuity in the area surrounding the AOF, and therefore, coincident in space with the morphological discontinuity between the two taxa. The genetic discontinuity was reflected in the UPGMA dendrogram, which defined two clearly separated groups of populations according to taxon affiliation, and in the partition of the genetic variance of allozymes, of which 63% was accounted by inter-taxa differences. In addition, we explicitly rejected the isolation by distance explanation. We also have characterized the genetic structure of P. jacobaeus populations, which appears to be slightly less substructured than P. maximus populations from N Europe. Finally, we report differences between the two taxa in the activity of adenilate kinases and other unidentified dehydrogenases after electrophoresis, which add to the classical morphological differentiation between taxa.