Genetic analysis by interspecific crosses of the tolerance of Drosophila sechellia to major aliphatic acids of its host plant

Toxicity of hexanoic and octanoic acid, ie, the two major aliphatic acids found in ripe fruits of Morinda citrifolia, was measured on adult flies of Drosophila sechellia, D simulans, F1 hybrids and backcrosses. With both acids, tolerance was much higher in D sechellia than in D simulans while F1 and backcross progeny exhibited intermediate characteristics. Tolerance to these two acids in D sechellia appears to be a major mechanism for understanding the ecological specialization of that species to the toxic morinda. Significant differences in tolerance were found between sexes, especially in F1 hybrids. The role of X-linked tolerance genes was, however, not obvious from the backcross generation, and most of the interspecific difference seems to be autosomal and polygenic. Attempts were made to introgress the tolerance of D sechellia into D simulans by selecting with either morinda fruit or pure octanoic acid. Both techniques proved to be unsuccessful. Introgressed genotypes progressively returned to the apparently pure D simulans phenotype and tolerance regressed to the low value typical of that species. This barrier against introgression seems quite similar to the barrier observed in hybrid zones of various animal species.