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 explanation for the lack of increase in late-life mortality rates. Here, we test corollaries of this heterogeneity theory. One is that populations that have undergone strong laboratory selection for differentiated stress resistance should show significant differences in their late-life mortality schedules. To test this corollary, we employed 40 410 flies from three groups of Drosophila melanogaster populations that differ substantially in their resistance to starvation. No significant differences between these groups were found for late-life mortality. Another corollary of the heterogeneity theory is that there should be late-life plateaus in stress resistance that coincide 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 male and female starvation and desiccation resistance clearly decreased with time overall. There was no late-life plateau in male desiccation resistance. A late-life plateau in male starvation resistance may exist, however. Together, these two experiments generally constitute evidence against heterogeneity as a major contributor to the phenomenon of late-life mortality plateaus.