Competitive interactions in Drosophila melanogaster IV. Chromosome assay

On the analysis of viability data: an example with Drosophila

Larval competition experiments involving two wild type and eight mutant strains of Drosophila melanogaster have been carried out following the substitution procedure proposed by Mather and Caligari (1981). Our main goal has been to compare the competitive abilities of two phenotypically indistinguishable strains (wild and Oregon-R) by means of their responses with eight different mutants. Prior to the analyses of viability data, we have studied the normalizing effect of several transformations in order to determine which was best suited for the analyses. The differences found among the five transformations tested and the untransformed data were not very great. The folded power transformation (Mosteller and Tukey, 1977) was finally chosen. No constant pattern in the responses of the two wild type strains to the mutant competitors was detected. This leads us to conclude that the nature of the competition between the two wild type strains cannot be predicted from a knowledge of their competition with other strains.

The biometrical genetics of competitive parameters in Drosophila melanogaster

Despite the importance of competition as an evolutionary determinant in natural populations there have been few studies of the genetical control of competitive ability. Here, we report the results of a biometrical analysis of four continuously varying traits which, between them, describe the competitive interactions in mixed cultures of Drosophila melanogaster. The analysis involved the parental, F1, F2 and backcross generations (including all reciprocals) derived from crosses between two highly inbred lines isolated from the Texas population of D. melanogaster. The competitive performance of each genotype in monoculture and in duoculture with a phenotypically distinct tester were assessed using a yield-density regression analysis. Appropriate genetic models were fitted using a variance weighted least squares procedure and the resulting genetic components of the generation means used to define the genetical architecture of competition. Of the four competitive parameters investigated here the e-value, which describes the competitive performance of the indicator genotype at a fixed reference density, was found to be determined by simple additive genetic effects with no evidence of significant dominance. Conversely, competitive performance in monoculture (intra-genotypic competition) did display a significant net dominance component and the observed values in the F1 and parental generations indicated some degree of heterosis. Of the two competitive parameters determining performance in duoculture (inter-genotypic sensitivity and inter-genotypic pressure) the former was found to have a complex genetic determination involving not only additive and dominance components of the progeny's own genotype but also dominance components of the F1 maternal genotypes. There were also additive-dominance and dominance-dominance non-allelic interactions. Heterosis was evident, determined both by the progeny's own genotype and by one of the F1 maternal genotypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of competitive interactions in triocultures of Drosophila melanogaster

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.

Analysis of dominance for competitive ability in Drosophila melanogaster

In these experiments the genetic basis of larval competition in Drosophila melanogaster was investigated. Competitive ability was defined by a series of regression coefficients relating larval performance to their mono- and duo-culture densities. Sixteen inter-related F1 hybrids were individually compared with their parents, revealing the presence of large amounts of dominance and heterosis for the various competitive parameters, all directed towards improved competitive ability. Analysis of the F1 hybrids amongst themselves revealed that most of the heterosis was due to either interchromosomal interaction, or the complementing action of haploid autosomes and relatively little was due to any specific interaction between the homologues. The relevance of these results to the current understanding of heterosis is discussed.