Rare electrophoretic variants in a hybrid zone

Populations of the grasshopper Podisma pedestris were collected from two ends of a zone of hybridization between two chromosome races, at Seyne and Tende in southern France. 21 enzyme and protein loci were detected by gel electrophoresis. Six of these loci showed widespread polymorphism, and a further eleven had very little or no variation. Two loci (Idh, 6Pgd) had rare alleles in different frequencies in the two areas surveyed. The remaining two loci (Mdh-1, Mdh-2) showed a marked increase in the frequency of rare variants, from 1 per cent outside the hybrid zone, up to 5 per cent at its centre. This region of increased electrophoretic variation coincided with the chromosomal cline between the two races, and with a region of decreased viability. It was spread over about the same width as the chromosomal cline. Possible explanations for this extra variation include intragenic recombination and elevated mutation rates.