Stochastic dietary restriction using a Markov-chain feeding protocol elicits complex, life history response in medflies

Abstract

Lifespan in individually housed medflies (virgins of both sexes) and daily reproduction for females were studied following one of 12 dietary restriction (DR) treatments in which the availability of high-quality food (yeast-sugar mixture) for each fly was based on a Markov chain feeding scheme--a stochastic dietary regime which specifies that the future dietary state depends only on the present dietary state and not on the path by which the present state was achieved. The stochastic treatments consisted of a combination of one of four values of a 'discovery' parameter and one of three values of a 'persistence' parameter. The results supported the hypotheses that: (i) longevity is extended in most medfly cohorts subject to stochastic DR; and (ii) longevity is more affected by the patch discovery than the patch persistence parameter. One of the main conclusions of the study is that, in combination with the results of earlier dietary restriction studies on the medfly, the results reinforce the concept that the details of the dietary restriction protocols have a profound impact on the sign and magnitude of the longevity extension relative to ad libitum cohorts and that a deeper understanding of the effect of food restriction on longevity is not possible without an understanding of its effect on reproduction.

Fig. 1

Event-history diagrams (Carey et al., 1998) visualizing the demographic response of individual medflies subjected to one of 12 different stochastic dietary treatment regimes (see text). Each horizontal line denotes a female ‘life line’, the length of which is proportional to her lifespan. The age-specific egg laying intensity corresponds to the shading: light gray = zero eggs/day; medium gray = 1–30 eggs/day; black = > 30 eggs/day.

Fig. 2

Schematic graphs showing the stochastic food availability patterns in the 12 treatment cohorts of the medfly DR study. Each panel corresponds to the food availability patterns generated for 100 individuals represented by imaginary horizontal ‘lines’ moving left to right from day 0 through day 80. The density and pattern of black ticks (or series of ticks) represent the food-days that were generated with simulations using one of four values of the discovery parameter, p, and one of three values of the corresponding persistence parameter, q. For example, the upper-left panel (p = 0.05, q = 0.20) shows a sparse pattern of food availability with most patches available to individuals only one day at a time (i.e. non-persistent). In contrast, the lower-right panel (p = 0.20, q = 0.80) shows a more dense pattern of food availability with most patches available to individuals over a series of days (i.e. persistent). The insets containing p* in each panel give the predicted number of high-quality food-days (asymptotic probability) for each feeding regime calculated using Eq. (1). For example, p* = 0.200 indicates that individuals in a cohort received high-quality food an average of 20% of the time.