Selection for greater dispersal in early life leads to increased rate of age-dependent decline in locomotor activity and shorter lifespan

Locomotor activity is one of the major traits that is affected by age. Greater locomotor activity is also known to evolve in the course of dispersal evolution. However, the manner in which dispersal evolution genetically alters the aging profile of locomotor activity, and whether the nature of this profile varies in the two sexes of sexually dimorphic organisms are largely unknown. Moreover, little is known regarding the fate of the evolved dispersal-associated traits, when the selection pressure for dispersal ceases to exist for several generations. We addressed these knowledge gaps using large outbred populations of Drosophila melanogaster, which evolved greater locomotor activity in their early life as a correlated response to early life selection for increased dispersal. For this, we maintained the selected and ancestry-matched control populations under identical conditions for nine generations in absence of dispersal selection. Subsequently, we tracked locomotor activity of individual flies of both sexes at regular intervals until a late age. Longevity of these flies was also recorded. Interestingly, we found that locomotor activity declines with age in general, but dispersal selected populations show a greater level of activity at all ages compared to the controls. More importantly, the rate of age-dependent decline in activity of the dispersal selected population was found to be much larger; while the two sexes perform similarly with respect to the ageing profile of activity. Dispersal selected populations were also found to have a shorter lifespan as compared to the control, a likely cost of elevated level of activity throughout their life. These results are crucial in the context of invasion biology as contemporary climate change, habitat degradation, and destruction provide congenial conditions for dispersal evolution. Such controlled and tractable studies investigating the ageing pattern of important functional traits are important in the field of biogerontology as well, which looks into improving the biological quality of life during late-life by postponing the onset of senescence.