When conducting aging studies, it is most important to control genetic and environmental factors that affect lifespan. Uncontrolled variables can lead to erroneous interpretation of the results. The information below should be used both as a guide for variables that are particularly important to control, and as a primer on lifespan-related variables for further exploration.
Intrinsic/genetic factors Genetic background plays a major role in lifespan. It is important that the effects of mutant gene actions or any experimental manipulations be studied on a standard genetic background with an appropriate control group. However, because genetic influences on longevity are described in detail in Chapter 25 of this volume, no more will be said here.
Adult survival and fecundity show significant inbreeding depression, presumably because deleterious recessive mutations present are more likely to become homo-zygous and have deleterious phenotypic effects on the offspring of matings between relatives. Adult survival and fecundity evolve rapidly in laboratory culture. In a typical fly lab, the newly emerged adult flies are collected into new culture bottles, allowed to lay eggs, and then discarded. When this regime is applied to flies collected from nature, there is a rapid increase in early fecundity (which is selectively favored under this culture regime) and also a large decrease in adult survivorship as a correlated response to the early fecundity (Partridge and Pletcher, 2003).
There is no evidence to suggest sex differences in lifespan; however, males and females respond differently to interventions known to extend lifespan (e.g., dietary restriction (Magwere et al., 2004)). Physiological differences, for example, differences in nutrient demand, differences in resource allocation (females for egg production; males for activity and courtship), and differences in sensitivity to hormonal signaling pathways that are known to determine lifespan (such as insulin/insulinlike growth factor signaling (IIS) pathway) may be responsible.
Extrinsic factors The lifespan of Drosophila is also affected by factors unrelated to genetics (Table 22.3), many of which have
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