THE CHROMOSOMAL BASIS OF VIABILITY IN INTERSPECIFIC HYBRIDS BETWEENDROSOPHILA ARIZONENSISANDDROSOPHILA MOJAVENSIS

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Abstract
The chromosomal effects on differences in viability among progeny from interspecific crosses was studied in the interfertile pair, D. arizonensis and D. majavensis. Interspecific crossing-over was avoided by crossing hybrid males to pure-species females, and chromosomal identification in backcross progeny was possible by electrophoretic markers. One chromosome supresses viability when in the heterospecific state, this being mainly so when the rest of the genotype is predominantly of mojavensis type; the other chromosomes show occasional interspecific heterosis, but are neutral in the majority of cases; interactions are not significant, except in 1 pair of chromosomes within a mojavensis background; there is no correlation between numbers of heterospecific chromosomes and viability scores. Hybrid and backcross progeny inviability is not a very potent mechanism for keeping these species apart. Should ethological and ecological barriers break down, introgressive hybridization is a real possibility. The findings are compared with those from studies concerning hybrid sterility and mating behavior in these 2 spp. There is no common chromosomal basis for these phenomena. The role of interspecific inversions to speciation remains obscure.