Heterogeneity's ruses: How hidden variation affects population trajectories of age-dependent fecundity in Drosophila melanogaster

Terminal life history: late-life fecundity and survival in experimental populations of Drosophila melanogaster

There are two life history landmarks that can be used to define the terminal period in individual Drosophila melanogaster females: the cessation of daily oviposition, which defines the start of the retired stage, and final oviposition, which defines the start of post-ovipository survival. The terminal period is a substantial component of D. melanogaster life history. Analysis of published data on the daily fecundity and survival of 3971 individually maintained, mated female flies reveals that the terminal period is far more variable within populations than other life history components, including total adult life span. It has been reported that there is a negative correlation between fecundity and duration of the terminal state in recently collected wild stocks. Here I show that the negative correlation occurs in multiple inbred and outbred lab-adapted populations as well. In terms of proportion of adult life, lower fecundity flies spend on average twice as much time in the terminal stage as higher fecundity flies from the same population. Both high and low fecundity flies experience end-of-life plateaus in mortality, with the former exhibiting higher plateau levels. The negative correlation between fecundity and terminal survival is of sufficient magnitude to create heterogeneity among the oldest old: the final 10% of survivors are predominately flies with a history of high fecundity, but about one in five is a low fecundity fly with long terminal stage.

Reproductive Homeostasis and Senescence in Drosophila melanogaster

The homeostatic properties of reproduction in aging female Drosophila melanogaster are investigated. Classic studies based on cohort analysis suggest that homeostatic capacity declines gradually as daily oviposition rates decline in aging flies. Analysis at the level of individuals gives a very different picture: reproductive homeostasis remains relatively constant for most of adult life until a critical point when oviposition either ceases entirely or continues in dysregulated fashion. The collapse of homeostatic capacity is abrupt. Enhanced homeostasis is associated with increased lifetime fecundity and improved prospects for survival. The fractal concept of lacunarity can be used to parameterize the "roughness" of individual fecundity trajectories and is inversely related to homeostatic capacity.

The death spiral: predicting death in Drosophila cohorts

Drosophila research has identified a new feature of aging that has been called the death spiral. The death spiral is a period prior to death during which there is a decline in life-history characters, such as fecundity, as well as physiological characters. First, we review the data from the Drosophila and medfly literature that suggest the existence of death spirals. Second, we re-analyze five cases with such data from four laboratories using a generalized statistical framework, a re-analysis that strengthens the case for the salience of the death spiral phenomenon. Third, we raise the issue whether death spirals need to be taken into account in the analysis of functional characters over age, in aging research with model species as well as human data.

The Retired Fly: Detecting Life History Transition in Individual Drosophila melanogaster Females

Life history observations at the level of individual model organisms are relatively scarce, but highly informative. Here I analyze published data on the survival and lifetime fecundity of 3,971 individually housed, mated Drosophila melanogaster females from nine experimental populations. Data were collected from four laboratories and include counts of over 4.6 million eggs. Individual fecundity records are dominated by zero-egg-days (ZEDs). I show that the timing of ZEDs is informative about the survival and reproduction of individual flies. The first postmaturation ZED divides adult life into two functional stages: working and retired. The working stage is characterized by relatively high levels of oviposition and survival, while the retired stage is characterized by low levels of oviposition and reduced survival. The retired stage typically lasts one quarter of the total adult life span. The age of transition varies between flies; consequently age-synchronized cohorts will generally contain a mixture of working and retired flies, possibly influencing responses to experimental treatments. ZED can be used as a nonintrusive, real-time biomarker to distinguish live flies in the prime of life from those in a terminal state.

On the analysis and interpretation of late-life fecundity in Drosophila melanogaster

Late-life plateaus have been described in both cohort and individual trajectories of fecundity in Drosophila melanogaster females. Here I examine life history data recently analyzed by Le Bourg and Moreau (2014) and show that non-linearity in the cohort trajectory of fecundity is largely explained by heterogeneity in the duration of reproductive life spans. A model specifying linear post-peak decline of fecundity in individual flies provides a better fit to the data than one that combines linear decline with late-life fecundity plateaus. Using repeated measures analysis of variance, I show that age-dependent trends in individual fecundity are mostly linear, while among the most longevous individuals up to 20% of the variation in trends is non-linear. Plateaus in individual trajectories might be explained by evolutionary processes or by random environmental variation. The dominant role of environmental variation is supported by several observations, including the high variability of late-life fecundity, the occurrence of occasional individual plateaus in inbred lines, and the observation of plateaus in only a fraction of the population. Plateau and non-plateau flies identified by Le Bourg and Moreau (2014) have, on average, the same total fecundity and the same fecundity trajectories. The available evidence suggests that the environmental variance for late-life fecundity is sufficiently large to produce occasional individual trajectories that resemble plateaus but are not heritable.

Individual late-life fecundity plateaus do exist in Drosophila melanogaster and are very common at old age

Several authors have discussed the existence of late-life fecundity plateaus in Drosophila melanogaster. However, all these studies have pooled flies to show such plateaus. Here, we have reanalyzed previously published fecundity results to know whether these plateaus exist at the individual level. We found that these plateaus are observed in ca 20% of females and in more than 50% if only the longer-lived flies are taken into account. We conclude that late-life fecundity plateaus are not a rare phenomenon when considering a whole cohort of flies and are very common in oldest flies.