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

Abstract

SUMMARYGenetic incompatibilities between two races of the grasshopper Podisnia pedestris, 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.

Differences in DNA content between two chromosomal races of the grasshopper Podisma pedestris

“…4). Though marginally significant, this pattern is analogous to other phenomena in Podisma: there is a 5-fold increase in the frequency of rare electrophoretic variants at two loci (Barton, Halliday and Hewitt, 1983), and a 2-fold increase in the frequency of chromosomal aberrations in hybrid embryos (Barton, 1980) (though the overall frequency of these latter aberrations is much too low to account for the variation in Feulgen staining). Such phenomena are frequently found in other hybrid zones (Barton and Hewitt, 1985).…”

“…First, there is a reduced hatch rate in Fl's from crosses between pure populations, and there is no consistent difference between reciprocal crosses. Second, although the proportion of haploid pre-diapause embryos is somewhat increased in hybrids, it is still not high (Barton, 1980), suggesting that failure of fertilisation does not cause much mortality. Third, there is significant mortality in the first week of nymphal life, as well as at all the preceding stages of embryonic development (Barton and Hewitt, 1981b, and unpublished results).…”

“…Such phenomena are frequently found in other hybrid zones (Barton and Hewitt, 1985). The most important biological question raised by these results is their relation to the 50 per cent reduction in hatchability observed in hybrids (Barton, 1980;Barton and Hewitt, 1981b), and thus to the development of isolation between the two diverging taxa. The variance between spermatids suggests that some of the reduction in hatchability may be due to male sterility.…”

“…800 m), but is still much wider than the dispersal range. This implies that the inviability cannot result from strong selection at a single locus, but is due instead to incompatibilities at many loci, each of which is only weakly selected (Barton, 1980;Barton and Hewitt, 1981b).…”

Multivalents resulting from monobrachial homologies within a hybrid zone in Dichroplus pratensis (Acrididae): meiotic orientation and segregation

“…Mouse hybrids between laboratory stocks and natural populations, heterozygous for single fusions, show heterogeneity in trivalent segregation (Gropp & Winking, 1981). The sex trivalent of female hybrids of Podisma pedestris segregates efficiently (Barton, 1980). In contrast, natural hybrids between two Trimerotropis species that differ for a fusion, show a high incidence of aberrant meiotic behaviour .…”