Genetic differentiation, origin and dispersal of Gammarus gauthieri from the Iberian peninsula and North Africa (Crustacea, Amphipoda)

Genetic differentiation among population samples of G. gauthieri on both sides of the Strait of Gibraltar was investigated by enzyme electrophoresis at 20 enzyme loci with reference to G. gauthieri and G. ibericus from different areas in the Iberian peninsula. Levels of divergence resolved suggest that G. gauthieri reached North Africa from the Iberian peninsula, and not vice versa. With reference to geological evidence, this was likely to occur during the Messinian (6—5 My ago). As the opening of the Strait of Gibraltar represents a well dated geological event, an attempt has been made to calibrate the molecular clock for the G. pulex-group. Indirect estimations of gene flow levels were shown to be extremely low. Probably as a consequence of frequently occurring semi-arid conditions, populations of G. gauthieri are strongly subdivided and likely to be subject to stochastic processes. Therefore, even within areas, population samples may be genetically markedly different. Dendrograms of genetic distances were compared with geological evidence. From these data an hypothesis of phylogenetic relationships is proposed.