Abstract
Genetic incompatibilities between two races of the grasshopper Podisnia pedes-
tris, which differ by a Robertsonian fusion involving the sex chromosome, have
been investigated. Crosses were made between four populations spanning the
chromosomal dine; Fl 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 dine supported
this finding. Crosses between grasshoppers from a mixed population in the dine
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.
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