Environmental dependence of inbreeding depression and purging in Drosophila melanogaster

Elimination or reduction of inbreeding depression by natural selection at t he contributing loci (purging) has been hypothesized to effectively mitigat e the negative effects of inbreeding in small isolated populations. This ma y, however, only be valid when the environmental conditions are relatively constant. We tested this assumption using Drosophila melanogaster as a mode l organism. By means of chromosome balancers, chromosomes were sampled from a wild population and their viability was estimated in both homozygous and heterozygous conditions in a favourable environment. Around 50% of the chr omosomes were found to carry a lethal or sublethal mutation, which upon inb reeding would cause a considerable amount of inbreeding depression. These d etrimentals were artificially purged by selecting only chromosomes that in homozygous condition had a viability comparable to that of the heterozygote s (quasi-normals), thereby removing most deleterious recessive alleles. Nex t, these quasi-normals were tested both for egg-to-adult viability and for total fitness under different environmental stress conditions: high-tempera ture stress, DDT stress, ethanol stress, and crowding. Under these altered stressful conditions, particularly for high temperature and DDT, novel rece ssive deleterious effects were expressed that were not apparent under contr ol conditions. Some of these chromosomes were even found to carry lethal or near-lethal mutations under stress. Compared with heterozygotes, homozygot es showed on average 25% additional reduction in total fitness. Our results show that, except for mutations that affect fitness under all environmenta l conditions, inbreeding depression may be due to different loci in differe nt environments. Hence purging of deleterious recessive alleles can be effe ctive only for the particular environment in which the purging occurred, be cause additional load will become expressed under changing environmental co nditions. These results not only indicate that inbreeding depression is env ironment dependent, but also that inbreeding depression may become more sev ere under changing stressful conditions. These observations have significan t consequences for conservation biology.