A GENETIC-ANALYSIS OF SOME COMPONENTS OF REPRODUCTIVE ISOLATION BETWEEN 2 CLOSELY-RELATED SPECIES, SPODOPTERA-LATIFASCIA (WALKER) AND SPODOPTERA-DESCOINSI (LALANNE-CASSOU AND SILVAIN) (LEPIDOPTERA, NOCTUIDAE)

The two closely related noctuid moths Spodoptera latifascia and S. des coinsi have been found in sympatry in some parts of French Guiana. The y differ by i) ecological preferences? ii) some anatomical features of the genital tract, iii) the relative amounts of the two main componen ts of the female pheromonal blend, and iv) the temporal pattern of sex ual behaviour. Viable and fertile interspecific hybrids can be obtaine d in the laboratory, thus permitting a genetic study of some of these traits. The anatomical differences cause perturbations in mating effic iency. Experimental results show that these differences are hereditari ly transmitted, but their exact mode of inheritance remains unknown. T he timing of initiation of female calling activity is very likely to b e under polygenic control. Conclusive evidence showed that a pair of m ajor autosomal alleles determined the pheromone composition. Moreover, some of the data suggest that the descoinsi allele would become letha l in certain mosaic latifascia/descoinsi genomic environments. The eco logical factor is likely to be the major component of reproductive iso lation between the two species in natural conditions. However in conta ct zones, the differences in pheromone composition and in timing of se xual activity are probably involved in preventing interspecific hybrid ization. Two speciation scenarios are proposed, both sympatric. In one of them, the primary event would be a fragmentation of the tropical f orest followed by disruptive selection associated with habitat diversi fication. In the other, the speciation process might have been initiat ed by the occurrence of mutations acting on the pheromone mediated mat e recognition system.