Is inbreeding depression more severe in a stressful environment?

Successful reintroduction of endangered species depends in part on their ability to respond to changing environmental conditions. Population genetics theory suggests that inbred populations lacking genetic variability may be unable to respond effectively to environmental stress. There have been very few studies designed explicitly to investigate the phenomenon of inbreeding depression under environmental stress, particularly in the context of conservation genetics. Three separate experiments using Drosophila melanogaster were designed to explore this issue. No increase in the magnitude of inbreeding depression was detected in laboratory lines subjected to three generations of continuous full‐sib mating under temperature stress (28°C), lead stress (medium contaminated with 400 ppm Pb), or a combination of these stresses. Individual isofemale lines from a different population, however, did show significant increases in inbreeding depression when exposed to temperature stress for one generation of full‐sib mating following three generations of full‐sib inbreeding at 25°C. Further, chromosome‐2 homozygotes showed, on average, a significant increase in inbreeding depression under lead stress when in competition with corresponding chromosome‐2 heterozygotes compared to the same lines in a benign environment. Taken together, these results suggest that inbreeding depression is more severe under conditions of environmental stress and is more likely to be realized in an inter‐ or intraspecific competitive situation as can be experienced in the wild. Therefore, it is likely that reduced genetic variability through inbreeding is a much greater problem for recently reintroduced populations than it is for populations in a relatively benign zoo environment. © 1994 Wiley‐Liss, Inc.