Estimating heritability in a threshold trait: heat-shock tolerance in Drosophila buzzatii.

Stress tolerance is often measured as a threshold trait, the proportion of a group that survives a defined stress regime. Requirements of large offspring numbers coupled with fitness variation in the surviving cohort limit the use of some standard genetic analyses for estimating heritability. Therefore, we present an isofemale line analysis, which is a modified full-sib design, to estimate heritability of tolerance to heat shock in pretreated Drosophila buzzatii adults. Highly significant levels of genetic variation were found in males and females at the third generation of laboratory rearing, and the intraclass correlations were estimated to be about 0.2 for four independent sets of 25 isofemale lines. The proportion of the variance explained within lines among same-sex replicates, however, was larger than that between replicates of males and females. Because genetic variation was estimated from groups, the error variation required factoring by the group size to estimate heritability, which averaged 0.03. The four most tolerant, four least tolerant and four lines of average tolerance to heat stress in each set were reanalysed after 10-11 generations of rearing at 25 degrees C. Survival in the 13th-14th generations was positively and significantly associated with survival at generation 3. These comparisons indicate the high repeatability of measurements of heat-shock tolerance.