BIOSYSTEMATICS OF THE GENUSEUXOA(LEPIDOPTERA: NOCTUIDAE). XVIH. COMPARATIVE BIOLOGY AND EXPERIMENTAL TAXONOMY OF THE SIBLING SPECIESEUXOA RIDMGSIANA(GRT.) ANDEUXOA MAIMES(SM.)

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Abstract
The species previously recognized asEuxoa ridingsiana(Grt.) is shown to be composed of a sympatric pair of sibling species,Euxoa ridingsiana(Grt.) andEuxoa maimes(Sm.), which in the laboratory will produce viable F1hybrids but no F2. Results of F1sib and backcrosses show that the F1males are fertile and the F1females are infertile. In mating-bias tests conducted in laboratory cages, 74% of matings were conspecific and 26% interspecific. Differences in the diel periodicities of mating, which are about 2 h out of phase, may account for the mating bias. The duration of development ofE. ridingsianain the laboratory and its seasonal flight period in the field are about 2 weeks in advance of that ofE. maimes. However, there is considerable overlap of the flight periods and, with the tendency of females of both species to mate several times, it is unlikely that the difference in seasonal emergence is enough to effect reproductive isolation. It is evident that, under natural conditions, reproductive isolation can be maintained entirely by species-specific sex pheromones. This mechanism of reproductive isolation is, however, apparently ineffective when moths are confined in cages in the laboratory.Biogeographic considerations suggest that the differences in life-cycle timing and mating periodicities might have been adaptations to adjust development and reproduction to prevailing ancestral environments. If the initial differentiation of the 2 species occurred in isolation and included at least an incipient shift in the pheromonal mate-recognition system, it is possible that upon reestablishment of contact between ancestral populations the differences in life-cycle timing and mating periodicities acting in concert could have effected substantial, albeit incomplete, reproductive isolation. Subsequent selection to reinforce assortative mating to preserve coadapted gene complexes could then have resulted in differentiation of discrete pheromonal systems and attainment of species status.

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