Genetic variation within Littorina saxatilis (Olivi) and Littorina neglecta Bean: Is L. neglecta a good species ?

Abstract

Genetic variation was compared within- and between-samples of Littorina saxatilis and L. neglecta from five geographic regions of western Europe (Iceland, Norway, Sweden, Isle of Man and Anglesey). The variation at five highly polymorphic enzymes (Aat-1, Pgm-1, Pgi, Mpi and Np) were revealed in samples from eleven vertical transects extending upshore from the barnacle zone to the upper littoral fringe. Both morphological types, L. saxatilis and L. neglecta, were present in all geographic regions except in Sweden.

The results of the genetic analyses show that at four of the five loci between 83 and 95% of the between-sample variation was due to differentiation between geographic areas, while only 4% or less was attributable to differentiation between barnacle zone and high littoral fringe samples. An accompanying morphological study revealed that the barnacle zone snails were mostly of L. neglecta type, except in the Swedish locality (where although they were distinct from the upper shore snails they were not in accordance with the description of L. neglecta), and the littoral fringe snails were of L. saxatilis type. The conclusion is therefore that there is more gene flow between L. neglecta and L. saxatilis type snails within the same locality than there is between snails of similar morphological type, but from geographically separated shores. Although we have not examined material from the type locality of L. neglecta, we suggest it to be a junior synonym of L. saxatilis

One locus, Aat-1, was, in contrast to the other polymorphic loci, more differentiated over the vertical transects (68% of the between sample variation was attributable to differences between barnacle and high littoral fringe samples) than over the different geographic areas (2l%). However, two observations indicated selective rather than stochastic differentiation at Aat-1 : (1) The same pattern was found independent whether or not L. neglecta was present. (2) The much smaller degree of differentiation at the other polymorphic loci indicated a gene flow which would prevent such a large differentiation at Aat-1 solely by random genetic drift.