Academic journal article Demographic Research

Heterogeneity's Ruses: How Hidden Variation Affects Population Trajectories of Age-Dependent Fecundity in Drosophila Melanogaster

Academic journal article Demographic Research

Heterogeneity's Ruses: How Hidden Variation Affects Population Trajectories of Age-Dependent Fecundity in Drosophila Melanogaster

Article excerpt

1. Introduction

One goal of biodemography is to understand the forces that shape trajectories of survival and reproduction in laboratory populations of experimental organisms. Here we examine original and published life history data from five populations of the fruit fly Drosophila melanogaster. Our objective is to determine whether population trajectories of age-specific fecundity, which plateau late in life, accurately reflect the pattern of reproductive senescence in individual flies, or whether plateaus are artifacts of population heterogeneity.

Examples of population fecundity trajectories are shown in Figure 1. The reproductive chronology starts when females mate, store sperm, and begin laying eggs, one or two days after emergence from the puparium. Flies typically reach peak daily fecundity within the first two weeks of adult life, and then exhibit declining fecundity in old age. At some point in mid- to late-life, the decline moderates and population trajectories inflect upward, producing fecundity plateaus. The plateaus could reflect real biological change in individuals, or they could be an example of heterogeneity's ruses (Vaupel and Yashin 1985). The alternative explanations are analogous to discussions surrounding mortality plateaus, which are well documented in this species (Curtsinger et al. 1992; Vaupel et al. 1998, Khazaeli, Pletcher, and Curtsinger 1998; Wachter 1999; Drapeau et al. 2000; Service 2000; Curtsinger, Gavrilov, and Gavrilova 2006).

1.1 The argument for late-life fecundity plateaus

Based on study of three genetically heterogeneous, lab-adapted populations of D. melanogaster, Rauser et al. (2003; 2005a, 2005b; 2006) argued that reproductive senescence is best understood as a two-stage process: rapid linear decline until a breakday age, followed by a fecundity plateau with low, relatively constant levels late in life. The evidence came primarily from fitting two-stage regression models to complete data on survival and reproduction of individual flies, and also from testing and rejecting a heterogeneity model that involved trade-offs between survival and reproduction (Rauser et al. 2005a). Subsequent research has led to speculations about the cessation of aging, immortality, and the evolution of life history characteristics that are special to late life (Rose, Rauser, and Mueller 2005; Mueller, Rauser, and Rose 2011).

1.2 Counterarguments

Other analyses are not consistent with the hypothesis that late-life fecundity reaches a plateau. Novoseltsev et al. (2004) analyzed individual fecundity records in D. melanogaster and in Medflies and found evidence for three stages: maturation, followed by a period of high, relatively constant fecundity in the prime of life, and then a long period of senescent decline with no clear break point marking a late-life plateau. Klepsatel et al. (2013a) studied lifetime survival and reproduction in recently collected stocks of D. melanogaster and suggested a four-stage model: maturation to a peak, followed by linear and then exponential decline, and then a post-ovipository period. The specific model that best fit the data was not consistent with late-life fecundity plateaus.

Curtsinger (2013) reanalyzed the data of Rauser et al. (2005a) and found that late- life inflections in the population fecundity trajectories can be explained by a type of population heterogeneity that had not previously been investigated. The argument consisted of three parts. First, in subgroups stratified by reproductive life span (RLS), which is defined for each individual fly as the duration from emergence until the age when the last egg is laid, the decline of fecundity was generally linear with age. Second, pooling data from subgroups with different RLS produced upward inflections in the mixture trajectory. Third, a simulation of the dynamics of reproductive senescence that assumed linear decline of fecundity in individual flies and realistic levels of variation in RLS produced population trajectories that were very similar to those observed in experiments, including the late-life upward inflections. …

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