The Role of the Use of Different Host Plants in the Maintenance of the Inversion Polymorphism in the Cactophilic Drosophila Buzzatii

Abstract

Abstract. Inversion polymorphisms often have been associated with fitness variation. Cactophilic Drosophila buzzatii has been used widely for the study of the maintenance of chromosomal variation. The purpose of this paper is to address the relative importance of variable selection regimes associated with the use of three different host cacti and antagonistic pleiotropy in the maintenance of chromosomal variation. Using homokaryotypic stocks derived from several lines homozygous for four second-chromosome arrangements, we show that inversions significantly affect first-instar larva to adult viability (VT), 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 components. The dosage of arrangement 2st, the ancestral gene order, was negatively correlated with DT and TL, whereas flies carrying the derived arrangements 2j and 2jq 7 had longer DTs and larger TLs. Arrangements 2st and 2jq 7 increased viability, at least in one of the cactus media tested. Our results suggest that environmental heterogeneity, as represented by the use of different 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 affecting life-history traits linked to the inversion system. Key words. Antagonistic pleiotropy, cactus hosts, Drosophila, inversion polymorphism, life-history traits.Received May 19, 1999. Accepted March 7, 2000 Maintenance of genetic variation may have multiple causes (Barton and Turelli 1989;Roff 1997) such as heterozygote advantage (overdominance), antagonistic pleiotropy (Rose 1982), epistasis (Gimelfarb 1989), and spatial or temporal variation in selection coefficients (Hedrick et al. 1976;Hedrick 1986). However, it is difficult to evaluate their relative importance. Convincing cases of single locus overdominance are rare. It is even debatable whether the best known example, sickle cell anemia, can be better interpreted as a case of antagonistic pleiotropy with reversal of dominance (Curtsinger et al. 1994). Recent theoretical studies on antagonistic pleiotropy have shown that the conditions for stable polymorphisms are very restrictive (Curtsinger et al. 1994). The role of epistasis in maintaining genetic variation is difficult to evaluate due to the limited evidence available. An impressive body of theoretical and experimental work has been devoted to determining the necessary conditions for multiple niche selection. Examples are genotype-environment interactions associated with spatial heterogeneity (Levene 1953;Dempster 1955;Hedrick et al. 1976;Hedrick 1986), regulation of population density by soft selection (Wallace 1968(Wallace , 1975, and genotype-specific habitat selection (Barker 1990).Several approaches can be employ...

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