How an individual fecundity pattern looks in Drosophila and medflies.

Reproduction usually is characterized by a mean-population fecundity pattern. Such a pattern has a maximum at earlier ages and a subsequent gradual decline in egg production. It is shown that individual fecundity trajectories do not follow such a pattern. In particular, the regular individual fecundity pattern has no maximum so that experimentally observed maximums are average-related artifacts. The three-stage description of individual fecundity, which includes maturation, maturity, and reproductive senescence, is more appropriate. Data are presented for Drosophila and Mediterranean fruitfly females that clearly confirm this hypothesis. A systematic error between egg-laying scores and the regular individual pattern allows for evaluation of how close the random scores are to the pattern. The first finding of the analysis of the systematic errors is that they are consistent with the three-stage hypothesis and do not contradict the absence of the maximum in the regular individual pattern. The other finding is the existence of obvious dynamic properties of the systematic error. The slow decrease in egg-laying at the maturity stage might be the result of a cost of mating. It can also be a consequence of "structural" senescence, that is, a slow rate accumulation of oxidative damage in the gonads.

Systemic mechanisms of individual reproductive life history in female Medflies.

This paper is the second one in a series of two papers hypothesizing and testing systemic grounds of reproductive life history in the female fruit fly. In the first paper, we analyzed mechanisms of individual fecundity scheduling and have drawn the following conclusions. Individual fecundity in female flies is endowed as a flat pattern with a steady-state period of a constant rate of egg-laying. An individual female reveals three stages in her adult life history: maturation, maturity, and senescence. The first stage is a transient period of achieving a steady state at maturity, which can be maintained until the senescence stage. Thus, an individual fecundity pattern has no maximum. The maximums observed experimentally are averaging-caused artifacts. Two natural causes of deaths exist in flies, senescence-caused ones and premature deaths, probably due to a reproductive overload. In this paper, to confirm these findings, we use individual daily scores of egg-laying in four populations of Mediterranean fruit flies. Based on fecundity scores, we divide each Medfly population into four classes, namely zero-egg, short-, medium- and long-lived egg-layers. We demonstrate that, indeed, the three above findings definitely exist in Medflies. Our procedure allows the efficient storage of individual fecundity in parametric form, with only five numbers for each fly. Finally, this protocol will allow a more precise analysis of fecundity-energy trade-offs in flies carrying appropriate longevity mutations.

What does a fly's individual fecundity pattern look like? The dynamics of resource allocation in reproduction and ageing.

Reproduction is usually characterised by an average fecundity pattern having a maximum at earlier ages and a subsequent gradual decline later on. An individual fecundity trajectory does not follow such a pattern and has no maximum. A three-stage pattern, which includes maturation, maturity and reproductive senescence, is a more appropriate description. An analysis of the power balance of an adult female fly during its life course allows us to predict two critical periods in an individual life history. The first crisis occurs at early ages when the increasing power demand becomes greater than the power supply. It often results in premature death. The surviving flies enjoy maturity and lay eggs at a presumably constant rate. The second critical period at advanced ages ends up in a senescence-caused death. Our approach predicts that there will be a bimodal death time distribution for a population of flies.

Evolutionary optimality applied to Drosophila experiments: hypothesis of constrained reproductive efficiency.

The general purpose of the paper is to test evolutionary optimality theories with experimental data on reproduction, energy consumption, and longevity in a particular Drosophila genotype. We describe the resource allocation in Drosophila females in terms of the oxygen consumption rates devoted to reproduction and to maintenance. The maximum ratio of the component spent on reproduction to the total rate of oxygen consumption, which can be realized by the female reproductive machinery, is called metabolic reproductive efficiency (MRE). We regard MRE as an evolutionary constraint. We demonstrate that MRE may be evaluated for a particular Drosophila phenotype given the fecundity pattern, the age-related pattern of oxygen consumption rate, and the longevity. We use a homeostatic model of aging to simulate a life history of a representative female fly, which describes the control strain in the long-term experiments with the Wayne State Drosophila genotype. We evaluate the theoretically optimal trade-offs in this genotype. Then we apply the Van Noordwijk-de Jong resource acquisition and allocation model, Kirkwood's disposable soma theory. and the Partridge-Barton optimality approach to test if the experimentally observed trade-offs may be regarded as close to the theoretically optimal ones. We demonstrate that the two approaches by Partridge-Barton and Kirkwood allow a positive answer to the question, whereas the Van Noordwijk-de Jong approach may be used to illustrate the optimality. We discuss the prospects of applying the proposed technique to various Drosophila experiments, in particular those including manipulations affecting fecundity.

A homeostatic model of oxidative damage explains paradoxes observed in earlier aging experiments: a fusion and extension of older theories of aging.

The Rate of Living and the Threshold Theories of Aging are two contradicting approaches used to explain experimental facts about aging in fruit flies. In this paper we suggest an approach that unifies these theories and removes the contradiction. The approach involves quantitative description of the oxidative stress theory of aging, which is presented in the form of a mathematical homeostatic model. The crucial variable in the model is called 'homeostatic capacity', which is analogous to the classical notion of vitality. We model the process of aging as the age-related accumulation of damage produced by oxidative stress, which reduces the homeostatic capacity of the organism. The model is tested with the experimental data obtained in the classical experiments by Maynard Smith in 1958-1963. Our homeostatic model explains the well-known results of these experiments more accurate than any one of the early theories of aging. We form an hypothesis about the mechanisms underlying the results observed in the experiments and analyze a possible interplay of these mechanisms. Our virtual replication of Maynard Smith's classical experiments demonstrates that mathematical modeling can be a powerful tool to reveal and investigate the inherent genetic and physiological processes underlying the data observed in complicated insect experiments.

Ageing and survival after different doses of heat shock: the results of analysis of data from stress experiments with the nematode worm Caenorhabditis elegans.

Stress experiments performed on a population of sterilised nematode worms (Caenorhabditis elegans) show a clear hormesis effect after short exposure and clear debilitation effects after long exposure to heat shock. An intermediate duration of exposure results in a mixture of these two effects. In this latter case the survival curves for populations in the stress and control groups intersect. In this paper we develop an adaptation model of stress and apply it to the analysis of survival data from three such stress experiments. We show that the model can be used to explain empirical age-patterns of mortality and survival observed in these experiments. We discuss possible biological mechanisms involved in stress response and directions for further research.

Anticipation of oxidative damage decelerates aging in virgin female medflies: hypothesis tested by statistical modeling.

Empirical analysis of survival data obtained from large samples of Mediterranean fruit flies shows that the trajectory of the mortality rate for virgin females departs from that for females maintained in mixed sex cages. It increases, decelerates, reaches its maximum, declines and then increases again within the reproductive interval. Non-virgin females, however, display an early-age plateau instead of this dip. We assume that these deviations are produced by the interplay between changes in oxygen consumption associated with reproductive behavior and the antioxidant defense that acts against anticipated oxidative damage caused by reproduction. Since there are no data on antioxidant mechanisms in medflies available that explain the observed patterns of mortality, we develop a model of physiological aging based on oxidative stress theory, which describes age-related changes in oxygen consumption and in antioxidative capacity during the reproductive period. Using this model, we simulate virtual populations of 25,000 virgin and non-virgin flies, calculate the respective mortality rates and show that they practically coincide with those of experimental populations. We show that the hypothesis about the biological support of reproduction used in our model does not contradict experimental data. The model explains how the early-age dip and plateau might arise in the mortality rates of female medflies and why the male mortality pattern does not exhibit such deviations.