Analysis of competitive interactions in triocultures of Drosophila melanogaster

Abstract

Competitive interactions in complex mixtures of genotypes have rarely been studied despite their obvious importance in both natural and commercial populations. Here, we describe a procedure for the analysis of competition in tripartite mixtures of Drosophila melanogaster genotypes. We have utilised a substitution design coupled with a yield-density regression analysis which describes intra- and inter-genotypic competitive effects in terms of simple linear parameters. The experimental design allows any of the competitors to be considered as the primary or indicator genotype and also incorporates variation in the relative proportions of the two associate competitors. The regression parameters are used to derive estimates of the competitive pressure exerted by each associate on the indicator genotype and also the response or sensitivity of the indicator to the competitive pressure which it faces in mixed culture. The results indicate that the joint pressure exerted by the paired associate genotypes in trioculture is equal to the sum of the individual pressures of those associates. This additive relationship holds for a variety of indicator genotypes isolated from the Texas population and appears to be a general property of Drosophila competition. We identified one indicator genotype which consistently departed from this relationship although additivity of joint pressures could be restored in combination with particular associate genotypes. The possible role of larval interference in the determination of these interactions is discussed.

The Maintenance of Heritable Variation Through Social Competition

“…Thus, we find the surprising outcome that competition can lead to long‐term maintenance of heritable variation for traits under constant directional selection, but those traits may never show an actual evolutionary response to selection. We suggest that competition dependence of fitness‐related traits may be common in nature because of evidence for both genetic interactions for competitive success (e.g., Hemmat and Eggleston 1989; De Miranda et al 1991; Colegrave 1993) and condition‐dependent expression of traits (e.g., Bakker et al 1999; David et al 2000; Holzer et al 2003). However, empirical investigation is required to confirm this.…”

“…However, the degree to which such interactions are prevalent in natural populations remains an empirical problem and the generality of whether competitive hierarchies evolve to be transitive or nontransitive in nature is not well known. For example, there is experimental evidence that three‐way genetic competitive interactions can be significant and complex (e.g., Castro et al 1985; Hemmat and Eggleston 1989), and may exhibit negative frequency dependence (Adell et al 1989). In contrast, there is a much larger body of literature examining competitive transitivity in interspecific interactions, which has demonstrated that such a pattern of nontransitivity may be relatively common, but it is obviously unclear the extent to which patterns from interspecific competition are mirrored at the intraspecific level.…”