GENETIC HETEROGENEITY INCREASES VIABILITY IN COMPETING GROUPS OF DROSOPHILA HYDEI

Nonadditive indirect effects of group genetic diversity on larval viability in Drosophila melanogaster imply key role of maternal decision-making

Genetic variation can have important consequences for populations: high population genetic diversity is typically associated with ecological success. Some mechanisms that account for these benefits assume that local social groups with high genetic diversity are more successful than low-diversity groups. At the same time, active decision-making by individuals can influence group genetic diversity. Here, we examine how maternal decisions that determine group genetic diversity influence the viability of Drosophila melanogaster larvae. Our groups contained wild-type larvae, whose genetic diversity we manipulated, and genetically marked 'tester' larvae, whose genotype and frequency were identical in all trials. We measured wild-type and tester viability for each group. Surprisingly, the viability of wild-type larvae was neither augmented nor reduced when group genetic diversity was altered. However, the viability of the tester genotype was substantially depressed in large, high-diversity groups. Further, not all high-diversity groups produced this effect: certain combinations of wild-type genotypes were deleterious to tester viability, while other groups of the same diversity-but containing different wild-type genotypes-were not deleterious. These deleterious combinations of wild-type genotypes could not be predicted by observing the performance of the same tester and wild-type genotypes in low-diversity groups. Taken together, these results suggest that nonadditive interactions among genotypes, rather than genetic diversity per se, account for between-group differences in viability in D. melanogaster and that predicting the consequences of genetic diversity at the population level may not be straightforward.

Intraspecific genetic variation and competition interact to influence niche expansion

Theory and empirical evidence show that intraspecific competition can drive selection favouring the use of novel resources (i.e. niche expansion). The evolutionary response to such selection depends on genetic variation for resource use. However, while genetic variation might facilitate niche expansion, genetically diverse groups may also experience weaker competition, reducing density-dependent selection on resource use. Therefore, genetic variation for fitness on different resources could directly facilitate, or indirectly retard, niche expansion. To test these alternatives, we factorially manipulated both the degree of genetic variation and population density in flour beetles (Tribolium castaneum) exposed to both novel and familiar food resources. Using stable carbon isotope analysis, we measured temporal change and individual variation in beetle diet across eight generations. Intraspecific competition and genetic variation acted on different components of niche evolution: competition facilitated niche expansion, while genetic variation increased individual variation in niche use. In addition, genetic variation and competition together facilitated niche expansion, but all these impacts were temporally variable. Thus, we show that the interaction between genetic variation and competition can also determine niche evolution at different time scales.

Resource subdivision and the advantage of genotypic diversity in Drosophila

It is noted that maintenance of genetic heterogeneity in natural populations of Drosophila, through the effect that variation in genotypic diversity across breeding sites may have on their productivity, depends not only on the effective number of parents contributing gametes to a site, as previously shown by other authors, but also on the number of loci underlying the variation in fitness. Using Monte Carlo simulation, it is found that as the number of loci increases, the effect of resource subdivision on the establishment of an initially rare allele introduced into the population becomes virtually indistinguishable from the pure drift case. It seems unlikely that this mechanism can explain the maintenance of a significant proportion of genetic variation in natural populations of Drosophila, although it may still be important in preserving linked gene complexes such as inversions.