Density and frequency-dependent selection on the singed locus of Drosophila melanogaster

Abstract

Larva-to-adult viability of tile progeny of four different crosses involving genotypes of the singed locus (SII+/Sll + X sn+/Y, sn+/sn + x snqr/y, snqr/snq r x sn+/Y and snqr/snqr x snqr/y) under optimum and high competition densities, has been studied. The fact that at high density the snqr/sn qr x sn+/Y and sn+/sn + x snqr/y crosses show the highest and lowest viabilities, respectively, suggested the possibility of some type of facilitation between wild-type and mutant phenotypes.This hypothesis was checked in an experiment similar to the previous one, but in which media conditioned either by wild-type larvae or by mutant larvae, were used. It was found that in each conditioning the cross that showed the lowest viability was the one that produced larvae of the same phenotype as the conditioning larvae.In order to check if a conditioning effect of this sort could form the basis for a frequency-dependent selection mechanism, an experiment of direct competition between wild-type and mutant larvae at different frequencies and at two densities, was carried out. At the higher density, the results showed that when the frequency of either of the two genotypes is low its viability increases. On the other hand, when the frequency of either of these genotypes is lfigh its viability decreases. These results suggest the existence of a frequency-dependent selection mechanism in whicli an inverse relationslfip between frequency and adaptive value is established. It also demonstrates tile important effect of density on frequency-dependent selection.

Adaptive significance of amylase polymorphism in Drosophila . XII. density- and frequency-dependent selection at the Amy locus in Drosophila subobscura reared on media with different carbohydrate composition

“…In LD conditions the effect of relative frequency of Amy genotypes of D. subobscura on their egg-to-adult viability is smaller than in HD conditions, as was observed in some previous experiments (Alavarez et al 1979;Nassar 1979;Tosˇicá nd Ayala 1981). This could be the result of two situations.…”

“…From the regression analysis of the data obtained it is also clear that the viability of Amy locus heterozygotes (S/F) of D. subobscura under these experimental conditions are not affected by a selection that would stem from the effect of frequency of that genotype on its egg–adult viability. The lower sensitivity of the heterozygote genotype to the density of experimental cultures and/or relative frequency of genotypes in them, could be the consequence of heterozygote advantage (Kojima and Tobari 1969b; Alvarez et al 1979). As heterozygote advantage represents one mode of balanced selection, its effect is probably not of the same intensity in all conditions of changing larval density and relative frequency of different genotypes.…”

“…It is not known how the conditioning of the media by one genotype makes the media less unlivable for the same genotype. The medium may be depleted in some nutrients, or some byproducts may be left that could reduce the chance of survival of its own genotype (Yamazaki 1971; Birley and Beardmore 1977; Alvarez et al 1979; Mueller 1995). Some authors argue that qualitative and quantitative differences in biotic residues secreted by different genotypes may be a factor contributing to frequency‐dependent viability, but not necessarily.…”

Selection at sex-linked loci. I. A method of estimating total fitnesses

“…Early methods on fitness estimation as Wright's one (in Wright and Dobzhansky, 1946), and also DuMouchel and Anderson's one (1968), are based on this supposition, but its validity has been called into question since the discovery of a great deal and diversity of frequencydependent selection mechanisms (Kojima, 1971;Ayala and Campbell, 1974). This is specially true for sex-linked loci, because many cases of frequency-dependent fitness have been demonstrated to occur for morphological (Petit, 1951(Petit, , 1954Alvarez et a!., 1979;Alvarez and Fontdevila, 1981) as well as for allozyme loci (Fontdevila and Méndez, 1979) located on the X chromosome. In this context, the method presented here is sensitive to any form of non constant fitnesses.…”