Testing the heterogeneity theory of late-life mortality plateaus by using cohorts of Drosophila melanogaster

Variation among individuals in robustness has been posed as a general expla nation for the lack of increase in late-life mortality rates. Here, we test corollaries of this heterogeneity theory. One is that populations that hav e undergone strong laboratory selection for differentiated stress resistanc e should show significant differences in their late-life mortality schedule s. To test this corollary, we employed 40 410 flies from three groups of Dr osophila melanogaster populations that differ substantially in their resist ance to starvation. No significant differences between these groups were fo und for late-life mortality. Another corollary of the heterogeneity theory is that there should be late-life plateaus in stress resistance that coinci de with the plateau stage of the mortality curve. In 20 994 flies from six replicate outbred laboratory populations, we measured mortality rates every other day and starvation and desiccation resistance every 7 days. Both mal e and female starvation and desiccation resistance clearly decreased with t ime overall. There was no late-life plateau in male desiccation resistance. A late-lie plateau in male starvation resistance may exist, however. Toget her, these two experiments generally constitute evidence against heterogene ity as a major contributor to the phenomenon of late-life mortality plateau s. (C) 2000 Elsevier Science Inc. All rights reserved.