The role of the use of different host plants in the maintenance of the inversion polymorphism in the cactophilic Drosophila buzzatii

Inversion polymorphisms often have been associated with fitness variation. Cactophilic Drosophila buzzatii has been used widely for the study of the m aintenance of chromosomal variation. The purpose of this paper is to addres s the relative importance of variable selection regimes associated with the use of three different host cacti and antagonistic pleiotropy in the maint enance of chromosomal variation. Using homokaryotypic stocks derived from s everal lines homozygous for four second-chromosome arrangements, we show th at inversions significantly affect first-instar larva to adult viability (V T), developmental time (DT) and adult thorax length (TL). We also show that the effects of inversions on DT and VT are dependent on the cactus rearing media. The effects of polymorphic gene arrangements on life-history traits suggest the existence of trade-offs between early and late fitness compone nts. The dosage of arrangement 2st, the ancestral gene order, was negativel y correlated with DT and TL, whereas flies carrying the derived arrangement s 2j and 2jq(7) had longer DTs and larger TLs. Arrangements 2st and 2jq7 in creased viability, at least in one of the cactus media tested. Our results suggest that environmental heterogeneity, as represented by the use of diff erent cactus hosts and the trade-off between DT and TL, may be involved in the maintenance of the polymorphism. In addition, our data suggest that the chromosomal phylogeny may be decoupled from the evolution of the genes aff ecting life-history traits linked to the inversion system.