ESTIMATING HERITABILITY IN A THRESHOLD TRAIT - HEAT-SHOCK TOLERANCE IN DROSOPHILA-BUZZATII

Stress tolerance is often measured as a threshold trait, the proportio n of a group that survives a defined stress regime. Requirements of la rge 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 t o heat shock in pretreated Drosophila buzzatii adults. Highly signific ant levels or genetic variation were found in males and females at the third generation of laboratory rearing, and the intraclass correlatio ns were estimated to be about 0.2 for four independent sets of 25 isof emale lines. The proportion of the variance explained within lines amo ng same-sex replicates, however, was larger than that between replicat es of males and females. Because genetic variation was estimated from groups, the error variation required factoring by the group size to es timate 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 degr ees C. Survival in the 13th-13th generations was positively and signif icantly associated with survival at generation 3. These comparisons in dicate the high repeatability of measurements of heat-shock tolerance.