Summary
In a group of organisms which are in competition with one another each individual plays a dual part: it will be exerting competitive pressure on its fellows and at the same time it will be responding to competitive pressure from them. The kind of competition experiment described by Mather and Caligari (1981) and Mather et al. (1982) allows differences in the pressure exerted by individuals of a given genotype (as the associate or aggressor) to be quantified separately from the response of that same genotype (as indicator or responder) to the pressure it experiences. Differences in pressure, or aggressiveness, and in response can be measured in terms of parameters a and r respectively and a value of each parameter, expressed as a deviation from the mean of the experiment, can be assigned to each genotype. The inter-relation of a and r can be measured by the interaction parameter i. The values of a, r and i are derived from the competition values (c) of the various pairs of genotypes, which in turn are derived from the regressions of character expression on density of the indicator genotype in mono- and duo-cultures.
The analysis is applied to data from the Lolium experiment by Mather et al., where the character is mean yield of dry matter per individual, and from four Drosophila experiments (T,L, A,C, C,H and X) using two characters, pa (the proportion of eggs developing successfully into adult flies) and w (the mean weight of the adults so obtained). In two of the Drosophila experiments (A,C and C,H) y2 was used as a marker gene and it is shown that of itself this gene had no significant effect on competitive ability. In a third experiment (X) involving three genotypes, both y2 and wa were used as markers.
In all three Drosophila experiments (T,L, A,C and C,H) where character w was followed no difference is found in response as measured by r, in respect of this character, though in all cases differences are found in the pressure parameter, a. The same is true for character pa in experiments T,L and A,C but in the X experiment variation is observed among the three genotypes for both response and pressure. The a and r appear to be independent in their variation and they can be taken as combining additively in their contributions to the competitive value of the genotype. This experiment also suggests that there may be a relatively simple genetical basis for competitive ability as observed in it. In the remaining Drosophila experiment (C,H) significant values are found for both a and r but a strong interaction, i, between them is found also. This stems from the fact that one of the genotypes (H,I), far from competing with the other (C,D), facilitates its successful development into adults. C,D however still competes with H,I (fig. 1).
In the Lolium experiment both a and r vary significantly among the three genotypes, in what appears to be a correlated fashion. They combine in a multiplicative rather than an additive way. The a values vary more among the genotypes than do the r values. The absolute values of a and r are dependent on the cutting regime, but the relative values of r stay constant among the genotypes. The relative values of a do, however, change with the regime.
The results are discussed in relation to the different mechanisms of competition which flies and grass plants must be expected to show.
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Mather, K., Caligari, P. Pressure and response in competitive interactions. Heredity 51, 435–454 (1983). https://doi.org/10.1038/hdy.1983.54
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DOI: https://doi.org/10.1038/hdy.1983.54
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