Recurrent selection programmes for both high response and low aggression have been employed in the Texas population of Drosophila melanogaster. Five main points have emerged from this investigation. First, the population exhibits extensive genetic variation for the aggression and response components of competitive interactions which take place in genetically heterogeneous cultures. Secondly, the two components of such interactions, namely aggression and response, can be adjusted by the selection of particular groups of genes. Thirdly, the rapid change in the selected components in the early generations suggests that a considerable amount of additive genetic variation is involved in the control of aggression and response. This is further supported by estimates of the realised heritability taken over the whole selection programme which amount to O•79 and O•74 for the low aggression and high response selections respectively. Fourthly, the rapidity with which changes in aggression and response approached plateaux suggests that such changes in the earlier generations are primarily due to the assortment of major chromosomes as units. Fifthiy, it was concluded that aggression and response do not behave entirely independently or dependently. The results suggest that a single array of genes might be responsible for the determination of both characters. The observed results would then be consistent with certain of these genes having a much larger effect on aggression than they do on response.

A logistic model of the competition diallel is presented based on two linear parameters for the exploitation component of competition, namely the acquisition rate (f) and utilization efficiency (u), and one linear parameter for the interference component of competition (i). This interference component encompasses all phenomena that are uniquely related to duocultures, such as resource partitioning, mutual stimulation, inhibition and complementation. The model uses yield-density regression coefficients (c-values), but could be adapted to suit other variates that account for both competitor density and relative frequency. In Drosophila larval competition most interference is negative and depresses the performance of duocultures with respect to monocultures, over and above that expected from shared exploitation of a common resource. Even in the closely controlled competitive conditions of these experiments this interference accounts for a considerable proportion of the total variation. The isolation of a general, and therefore predictable, interference component may prove useful in agriculture when assessing the relative importance of mixture effects to the yield potential of different crops.

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 melanogas fee. 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. melanogasfer. 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-ailelic interactions. Heterosis was evident, determined both by the progeny's own genotype and by one of the F1 maternal genotypes. All dominance and heterosis was directed towards reduced inter-genotypic sensitivity or, iii other words, superior competitive ability. The analysis of maternal effect components for inter-genotypic competitive pressure could not be accommodated for reasons described in the text, although the data provided evidence for their involvement. The fitting of a simplified model revealed significant additive and dominance components of similar magnitude together with heterosis determined by the progeny's own genotype. There was no evidence of non-allelic interaction. As before all dominance and heterosis was directed towards superior competitive ability (i.e., increased inter-genotypic pressure). Throughout the experiment, there was no evidence for sex-linkage in the determination of competitive parameters. This is thought to be a prerequisite for stability of the sex ratio in the intense competitive environment of natural populations. Possible interpretations of the genetical architecture of competition are discussed in the light of these results.

Interference, which is one of two aspects of the process of competition which take place in genetically heterogeneous mixtures has been studied in the Texas population of Drosophila melanogaster. Both survival and mean adult weight were investigated in the population itself (which displays high levels of aggression and little response) and in LA, a genotype derived from the population (which displays low aggression and high levels of response) in both homotypically and heterotypically conditioned media. The results presented here show that the competitive effects of conditioning depend not only on the concentration of the conditioned medium but also on the genotype of the larvae which conditioned the medium and that of the flies which respond to such media. It was also concluded that medium conditioning is one of a range of biological parameters involved in the determination of the aggression and response components of the competitive interaction among Drosophila larvae. Thus the competitive fitness of a genotype of D. melanogaster is related not only to genetic variation for aggression and response but also to genetic variation in the ability to condition media and the sensitivity to such media.

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.