How an Individual Fecundity Pattern Looks inDrosophilaand Medflies

Age-related changes in the reproductive potential of the Queensland fruit fly

Despite the known negative impacts of aging on the reproductive potential of many insects, Bactrocera tryoni populations show a rapid increase in abundance from early to late spring when the population is composed of predominantly old individuals. While some aspects of how male and female reproductive potential are influenced by age for this species are known, no study investigates lifelong reproductive potential of either sex. We conducted a whole-of-life study in the laboratory to assess the effect of age and mating-partner age on reproductive potential of B. tryoni. The fertility of 70 individual females was directly measured by the number of eggs laid and hatched; while 70 individual males' fertility was assessed indirectly by measuring the hatch rate of eggs laid by a female partner. Half of the males and females had access to a same-age virgin mating partner, while the other half received a prime-age virgin partner (17-19 days old): in both groups mating partners were replaced weekly. Results showed that independent of the age of male mating partner, increasing age significantly reduced the fecundity and fertility of female B. tryoni after a peak at approximately 20 days of age. However, females mated with prime-age males showed higher egg hatch rates during early life than did females mated with a same-age mating partner. As indirectly measured through their partner's egg hatch rate, the fertility of B. tryoni males was also affected by the age of the male and their mating-partner's age. Males mated consistently with a prime-age partner showed an increasing trend in the egg hatch rate of their partner: indirect evidence of increasing fertility in males with increasing age. No such affect was seen when males were mated with a same-age female, possible because of the age-related changes in female fecundity and fertility. While fecundity is greatly reduced in old females, the whole-of-life data shows that the very old flies present in the field at the end of winter are physiologically capable of starting the new season's F₁ generation. Beyond getting it begun, old females are unlikely to further contribute to the new season's population as their fecundity does not increase even if mated with a prime-age, new generation male. In contrast, old males, if they have subsequent access to new generation females, have the capacity to help contribute to the rapid spring population growth which is observed in the field.

Late-life fecundity plateaus in Drosophila melanogaster can be explained by variation in reproductive life spans

Population trajectories of age-specific fecundity in Drosophila melanogaster typically decline with increasing age and then exhibit an upward inflection, or "plateau", at the oldest ages. This pattern has been interpreted as evidence of an evolved and physiologically distinct life history stage in late life. While low levels of fecundity are common in the last few days of life of individual flies, it is unclear that defining a single age as the beginning of a period of low fecundity for the entire cohort is useful, since reproductive life spans vary substantially from fly to fly. Here I analyze published data on survival and reproduction of individual female flies and show that non-linearities in late-life fecundity trajectories arise from a type of demographic selection that occurs when sub-groups with different reproductive life spans (RLS) are mixed. For groups of flies stratified by RLS late-life fecundity declines linearly with age. A simulation incorporating strictly linear decline of individual fecundities and realistic levels of variation in RLS produces late-life plateaus similar to those observed in experiments. Existing population heterogeneity is a sufficient explanation, and no special evolutionary argument is required. For these data survival and reproduction are governed by positive correlations.

Date of eclosion modulates longevity: insights across dietary-restriction gradients and female reproduction in the mexfly Anastrepha ludens

We use unique experimental data on daily reproduction and survival of individual fruit flies from eight cohorts eclosed at different dates in 2004 and 2005 who were treated with varying proportions of sugar and yeast and subject to different caloric restrictions (CR). We investigate the relationship between eclosion date and longevity across diets and reproduction in Anastrepha ludens. We show that eclosion date can be associated with uncontrolled external or internal factor(s) which can modulate longevity of males and females independently of diet and reproduction to the extent similar to the effect of diet on longevity. The effect of diet manipulation on longevity is sensitive to date of eclosion with the role of CR in life extension ranging from beneficial to harmful. Interaction of date of eclosion with compositional changes of sugar and yeast but not with CR is responsible for life extension. Highly protein-enriched diets reliably maximize reproduction but not life span. Decreased longevity of flies treated with high-protein diets may be associated with harmful consequences of protein ingestion but is unlikely a result of high reproduction rates. We present evidence for the presence of two frailty-sensitive weakly interacting mechanisms of longevity in female flies associated with differences in predisposed fitness.

Quantitative trait loci with age-specific effects on fecundity in Drosophila melanogaster

Life-history theory and evolutionary theories of aging assume the existence of alleles with age-specific effects on fitness. While various studies have documented age-related changes in the genetic contribution to variation in fitness components, we know very little about the underlying genetic architecture of such changes. We used a set of recombinant inbred lines to map and characterize the effects of quantitative trait loci (QTL) affecting fecundity of Drosophila melanogaster females at 1 and 4 weeks of age. We identified one QTL on the second chromosome and one or two QTL affecting fecundity on the third chromosome, but these QTL affected fecundity only at 1 week of age. There was more genetic variation for fecundity at 4 weeks of age than at 1 week of age and there was no genetic correlation between early and late-age fecundity. These results suggest that different loci contribute to the variation in fecundity as the organism ages. Our data provide support for the mutation accumulation theory of aging as applied to reproductive senescence. Comparing the results from this study with our previous work on life-span QTL, we also find evidence that antagonistic pleiotropy may contribute to the genetic basis of senescence in these lines as well.

Functional senescence in Drosophila melanogaster

The fruit fly Drosophila melanogaster is one of the principal model organisms used for studying the biology of aging. Flies are well suited for such studies for a number of reasons. Flies develop to adulthood quickly, have a relatively short life span, and are inexpensive to house. Most of the fly genome has been sequenced, powerful genetic tools are available to manipulate it, and most fly genes have obvious homologues in mammals. While the majority of aging studies in flies have focused on regulation of life span, the fly is emerging as a powerful model system for investigating the biology that underlies age-related functional decline. Key to the use of flies in this way is the striking number of parallels between functional senescence in Drosophila and humans. Here, we review age-related functional declines in Drosophila, human correlates of these age-related declines, and common mechanisms that influence longevity and specific aspects of functional senescence in flies.