Competition and genotype-by-environment interaction in natural breeding substrates of Drosophila

Although empirical studies frequently suggest that genotype-by-environment (G x E) interaction can maintain genetic variation, very few data are avail able to test for the specific conditions necessary for the existence of a p rotected polymorphism (i.e., the property of persistence of an allele even when initially rare). Drosophila species live in patchy environments and th eir local population structure may be characterized to some extent by Leven e's migration pattern, namely by a single pool of individuals that presumab ly mate at random and breed on discrete and ephemeral resources. We present here a field experiment that links Drosophila ecology and population genet ics, which used the alcohol dehydrogenase (Adh) and alpha-glycerophosphate dehydrogenase (alpha Gpdh) polymorphic loci in D. melanogaster flies raised from Opuntia ficus-indica fruits (prickly pears). The results show that th ere is density-dependent mortality in those fruits with a relatively high n umber of larvae (i.e., selection is "soft") and suggest that there is diffe rential viability for alpha Gpdh genotypes. Additionally, a pattern of G x E interaction for fitness values, which is fully compatible with the theore tical conditions required for the existence of a protected polymorphism, wa s found after weighting the fitness estimates by the relative contribution that each fruit makes to the total adult population. The strong association between Adh(S) and alpha Gpdh(F) alleles suggests that the occurrence of t he common cosmopolitan inversion In(2L)t in the population might be respons ible for the negative frequency-dependent selection predicted by Levene's m odel when genetic variation persists in heterogeneous environments.