The fitness of hybrids between two chromosomal races of the grasshopper Podisma pedestris

Genetic incompatibilities between two races of the grasshopper Podisma pedestris, which differ by a Robertsonian fusion involving the sex chromosome, have been investigated. Crosses were made between four populations spanning the chromosomal cline; F1 hybrids between the races, though not between separated populations of the same race, showed markedly reduced hatching success. Observations of the fate of chromosomes transplanted across the cline supported this finding. Crosses between grasshoppers from a mixed population in the cline showed a lower average hatching success, but no correlation of this inviability with the chromosomal polymorphism. The karyotypes of pre-diapause embryos from both types of cross gave no evidence for non-disjunction of the sex chromosome trivalent in heterozygous females; an upper limit of 16 per cent can be set on the non-disjunction rate. These findings suggest that the karyotypic difference is only a weakly selected marker for more fundamental changes in genotype.