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

Abstract

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°C. Survival in the l3th-l4th generations was positively and significantly associated with survival at generation 3. These comparisons indicate the high repeatability of measurements of heat-shock tolerance.

Levels of Variation in Stress Resistance in Drosophila Among Strains, Local Populations, and Geographic Regions: Patterns for Desiccation, Starvation, Cold Resistance, and Associated Traits

“…In all cases, correlations between means of strains tested at different times were positive and exceeded 0.50, reflecting the fact that rankings of isofemale strains tend to be fairly consistent across generations of laboratory rearing (see also Krebs and Loeschcke 1997).…”

“…These estimates need to be treated cautiously because there can be a nongenetic correlation among individuals within the same group. Krebs and Loeschcke (1997) provide a way of overcoming this problem when groups of different sizes are available, but this information was unavailable at the time of the present study. Isofemale heritabilities also do not correct for cross generation effects and common environment effects can confound the results when flies representing a strain are reared in the same container.…”

“…We also computed intraclass correlations and isofemale heritabilities following Hoffmann and Parsons (1988) and Krebs and Loeschcke (1997) to examine levels of variation within geographic regions. Isofemale heritabilities (H) are computed from the variance components within and between strains adjusted for group size where appropriate, using Hoffmann and Parsons 1988).…”

Isofemale lines in Drosophila: an empirical approach to quantitative trait analysis in natural populations

“…Physiological or behavioral traits are, however, far more difficult to standardize, and more sensitive to accidental variation. This does not mean that such traits are not genetically repeatable, and indeed repeatability has been demonstrated for heat tolerance (Krebs and Loeschcke, 1997) or sexual behavior (Ritchie and Kyriacou, 1994). Other traits may be extremely variable under the same laboratory conditions.…”

“…Another convincing method to demonstrate a genetic basis to isofemale line variations is to investigate their mean values in different generations (Krebs and Loeschcke, 1997;Gibert et al, 1998a;Karan et al, 1998). Such experiments have shown a positive correlation between mean values in different generations.…”