Systemic mechanisms of individual reproductive life history in female Medflies

Electronic recording of lifetime locomotory activity patterns of adult medflies

Age-specific and diurnal patterns of locomotory activity, can be considered as biomarkers of aging in model organisms and vary across the lifetime of individuals. Τhe Mediterranean fruit fly (medfly), Ceratitis capitata, is a commonly used model-species in studies regarding demography and aging. In the present study, we introduce a modification of the automated locomotory activity electronic device LAM25system (Locomotory Activity Monitor)–Trikinetics, commonly used in short time studies, to record the daily locomotory activity patterns of adult medflies throughout the life. Additionally, fecundity rates and survival of adult medflies were recorded. Male and female medflies were kept in the system tubes and had access to an agar-based gel diet, which provided water and nutrients. The locomotory activity was recorded at every minute by three monitors in the electronic device. The locomotory activity of females was higher than that of males across the different ages. For both sexes locomotory rates were high during the first 20 days of the adult life and decreased in older ages. The activity of males was high in the morning and late afternoon hours, while that of females was constantly high throughout the photophase. Negligible locomotory activity was recorded for both sexes during the nighttime. Males outlived females. Fecundity of females was higher in younger ages. Our results support the adoption of LAM25system in studies addressing aging of insects using medfly as a model organism.

Distinctive egg-laying patterns in terminal versus non-terminal periods in three fruit fly species

The specific objective of this study was to use a logistic regression model for determining the degree to which egg laying patterns of individual females at the end of life (i.e., terminal segments) in each of three different fruit fly species could be distinguished from the egg-laying patterns over a similar period in midlife (i.e., non-terminal segments). Extracting data from large-scale databases for 11-day terminal and 11-day non-terminal segments in the vinegar fly (Drosophila melanogaster), the Mexican fruit fly (Anastrepha ludens) and the Mediterranean fruit fly (Ceratitis capitata) and organizing the model's results in a 2 × 2 contingency table, we found that: (1) daily egg-laying patterns in fruit flies can be used to distinguish terminal from non-terminal periods; (2) the overall performance metrics such as precision, accuracy, false positives and true negatives depended heavily on species; (3) differentiating between terminal and non-terminal segments is more difficult when flies die at younger ages; and (4) among the three species the best performing metrics including accuracy and precision were those produced using data on D. melanogaster. We conclude that, although the reliability of the prediction of whether a segment occurred at the end of life is relatively high for most species, it does not follow precisely predicting remaining life will also be highly reliable since classifying an end of life period is a fundamentally different challenge than is predicting an exact day of death.

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.

Female access and diet affect insemination success, senescence, and the cost of reproduction in male Mexican fruit flies Anastrepha ludens

Hypotheses exploring the influence of dietary conditions on the life history trade-off between survival and reproductive success are extensively tested in female insects, but are rarely explored in males. Here, the impact of dietary quality and female access on age-specific reproduction and survival of male Mexican fruit flies, Anastrepha ludens Loew (Diptera: Tephritidae), are examined. There is a clear cost of female access for males with access to dietary protein, measurable as a decrease in life expectancy, which is further influenced by the age when females are introduced. A protein deficient diet reduces the lifespan benefit of virginity and masks the detrimental effect of female access on male life expectancy. Dietary protein is not necessary for reproductive success, but access to protein at eclosion improves the lifetime reproductive success of males compared to when it is delayed. Overall, reproductive success diminishes as the male flies age, regardless of the dietary conditions, providing evidence for reproductive senescence in males. Delaying the males' access to a protein source fails to influence the negative effect of age on reproductive ability. Because age specific reproductive rates decline with age, regardless of diet, male fitness does not benefit from lifespan extension. Therefore, males can be expected to allocate available resources towards reproductive effort in favour of extended lifespan, regardless of mate and protein availability.

Polyandry in the medfly - shifts in paternity mediated by sperm stratification and mixing

Background

In the Mediterranean fruit fly (medfly), Ceratitis capitata, a highly invasive agricultural pest species, polyandry, associated with sperm precedence, is a recurrent behaviour in the wild. The absence of tools for the unambiguous discrimination between competing sperm from different males in the complex female reproductive tract has strongly limited the understanding of mechanisms controlling sperm dynamics and use.

Results

Here we use transgenic medfly lines expressing green or red fluorescent proteins in the spermatozoa, which can be easily observed and unambiguously differentiated within the female fertilization chamber. In twice-mated females, one day after the second mating, sperm from the first male appeared to be homogenously distributed all over the distal portion of each alveolus within the fertilization chamber, whereas sperm from the second male were clearly concentrated in the central portion of each alveolus. This distinct stratified sperm distribution was not maintained over time, as green and red sperm appeared homogeneously mixed seven days after the second mating. This dynamic sperm storage pattern is mirrored by the paternal contribution in the progeny of twice-mated females.

Conclusions

Polyandrous medfly females, unlike Drosophila, conserve sperm from two different mates to fertilize their eggs. From an evolutionary point of view, the storage of sperm in a stratified pattern by medfly females may initially favour the fresher ejaculate from the second male. However, as the second male's sperm gradually becomes depleted, the sperm from the first male becomes increasingly available for fertilization. The accumulation of sperm from different males will increase the overall genetic variability of the offspring and will ultimately affect the effective population size. From an applicative point of view, the dynamics of sperm storage and their temporal use by a polyandrous female may have an impact on the Sterile Insect Technique (SIT). Indeed, even if the female's last mate is sterile, an increasing proportion of sperm from a previous mating with a fertile male may contribute to sire viable progeny.

Population-specific demography and invasion potential in medfly

Biological invasions are constantly gaining recognition as a significant component of global change. The Mediterranean fruit fly (medfly) constitutes an ideal model species for the study of biological invasions due to its (1) almost cosmopolitan geographic distribution, (2) huge economic importance, and (3) well-documented invasion history. Under a common garden experimental set up, we tested the hypothesis that medfly populations obtained from six global regions [Africa (Kenya), Pacific (Hawaii), Central America (Guatemala), South America (Brazil), Extra–Mediterranean (Portugal), and Mediterranean (Greece)] have diverged in important immature life-history traits such as preadult survival and developmental times. We also tested the hypothesis that medfly populations from the above regions exhibit different population growth rates. For this purpose, data on the life history of immatures were combined with adult survival and reproduction data derived from an earlier study in order to calculate population parameters for the above six populations. Our results clearly show that medfly populations worldwide exhibit significant differences in preadult survival, developmental rates of immatures and important population parameters such as the intrinsic rate of increase. Therefore, geographically isolated medfly populations may share different invasion potential, since population growth rates could influence basic population processes that operate mostly during the last two stages of an invasion event, such as establishment and spread. Our findings provide valuable information for designing population suppression measures and managing invasiveness of medfly populations worldwide.

Coping with an unpredictable and stressful environment: the life history and metabolic response to variable food and host availability in a polyphagous tephritid fly

The way energy resources are used under variable environmental conditions lies at the heart of our understanding of resource management and opportunism in many organisms. Here we sought to determine how a time-limited, synovigenic and polyphagous insect with a high reproductive-potential (Anastrephaludens), copes behaviourally and metabolically with environmental unpredictability represented by constant and variable regimes of host availability and variation in food quality. We hypothesized that an adaptive response to a windfall of nutritious food would be the rapid accumulation of energy metabolites (whole body lipids, glycogen and proteins) in the female. We also studied patterns of oogenesis as an indicator of egg-reabsorption under stressful environmental conditions. As predicted, patterns of energy metabolites were mainly driven by the quality and temporal pattern of food availability. In contrast, patterns of host availability had a lower impact upon metabolites. When given constant access to high quality nutrients, after an initial increase early in life, whole body lipids and glycogen were regulated downward to a steady-state level and somatic protein levels did not vary. In contrast, when food uncertainty was introduced, whole body lipid, glycogen and protein oscillated sharply with peaks associated with pulses of high-quality food. Production of eggs was highest when offered continuous access to hosts and high quality food. Importantly, females fully recovered their reproductive capacity when fruit became available following a period of host deprivation. With no evidence of egg resorption and high levels of egg dumping, it appears that egg dumping may favour the continuous production of eggs such that the female's reproductive tissues are ready to respond to rapid changes in the availability of hosts. Our results exemplify the capacity of insects to maximize reproduction under variable and stressful environmental conditions.

Sperm storage and use in polyandrous females of the globally invasive fruitfly, Ceratitis capitata

The medfly, Ceratitis capitata, is an invasive species in which polyandry, associated with sperm precedence, is a common behaviour in the wild. In this species, characterized by internal fertilization, we disclose how the sperm from two males are stored in the female storage organs and how they are used in terms of paternity outcome. The experiments were designed to furnish comparable and unbiased estimates of sperm numbers and progeny in twice-mated females. Results are incorporated in a model through which it is possible to relate the amount of stored sperm with the progeny of twice-mated females. The results show that polyandrous medfly females conserve equal amounts of sperm from the two males to fertilize their eggs. However, we observed a clear advantage of the second male's sperm in siring progeny, which interestingly decreases in favor of the first male as ovipositions progress. The results enable us to exclude differential sperm mortality and suggest that it is the mechanics governing the storage organs which causes the initial, but decreasing second male sperm precedence during the female reproductive life. These outcomes allow us to correlate sperm use in polyandrous females with the mating strategies and invasiveness of this fly.

Reproductive aging in tephritid fruit flies

The broad objective of this paper is to present an overview and synthesis of selected studies on reproduction and aging in two model tephritid fruit fly species including the Mediterranean fruit fly, Ceratitis capitata, and the Mexican fruit fly, Anastrepha ludens. We summarize the research findings from empirical studies and modeling investigations involving reproduction in the two tephritid species. At the end we identify and discuss four general principles regarding reproductive aging in tephritids including reciprocity of reproductive and aging costs, qualitative tradeoffs, plasticity of lifespan and reproduction, and life history constraints and determinacy.

Social interactions regulate resource utilization in a Tephritidae fruit fly

Previous studies have suggested that social interactions (e.g., the actions and reactions elicited by the interaction of co-specific individuals) induce individual fruit flies (Tephritidae) to ingest more food, especially protein-rich food. Changes in feeding behavior related to social interactions have been associated with reproduction (e.g., when different sexes are present), reproductive facilitation (e.g., when two females interact) and stress and aggression (e.g., flies of the same sex, or crowdedness). The present study investigated the effect of social interaction on the feeding, longevity and resource management of the Ethiopian fruit fly, Dacus ciliatus. Single flies and pairs of flies (of the same or different sexes) were confined to a small arena (the PUB system), in which we measured the amount of liquid food ingested daily by each fly. In addition, we sampled flies of different ages, extracted and quantified their lipid and protein contents, and related individual metabolic contents to the ingestion of a fructose and protein hydrolysate solution. Results showed that individual ingestion was significantly higher in flies maintained in pairs than in flies kept as solitary individuals. The highest intake rates were observed for the female-female pairs. In general, females ingested significantly greater volumes than males. Lipid contents tended to decrease progressively with age in flies kept as solitary individuals, especially in female flies, while lipid levels decreased and then increased in flies maintained in pairs. Protein trends were similar, although less pronounced than the patterns observed for the lipids. The flies kept as solitary individuals lived significantly longer than those kept in pairs. A resource-management analysis points to a decreased metabolic rate in flies kept as solitary individuals, as compared to paired flies. Results are discussed in view of theories of resource management and survival strategies.

Adult diet affects lifespan and reproduction of the fruit-feeding butterfly Charaxes fulvescens

Fruit-feeding butterflies are among the longest lived Lepidoptera. While the use of pollen-derived amino acids by Heliconius butterflies has been interpreted as important for the evolution of extended lifespans, very little is known about the life-history consequences of frugivory. This issue is addressed by investigating effects of four adult diets (sugar, sugar with amino acids, banana, and moistened banana) on lifespan and reproduction in the fruit-feeding butterfly Charaxes fulvescens Aurivillius (Lepidoptera: Nymphalidae). Female butterflies were collected from Kibale National Park, Uganda, and kept individually in cages near their natural habitat and data were collected on lifespan, oviposition, and hatching of eggs. Lifespan in captivity was longer for the sugar and the amino acid cohort, than for the banana cohorts. The longitudinal pattern of oviposition was erratic, with many days without oviposition and few periods with high numbers of eggs laid. Butterflies typically did not lay eggs during their 1st week in captivity and the length of the period between capture and first reproduction was significantly shorter for butterflies fed moistened banana. The length of the reproduction period (first reproduction-last reproduction in captivity) and the reproduction rate (total number of eggs/length of the reproduction period) did not differ significantly between the diet treatments. Those fed with amino acid and moistened banana had significantly higher egg hatchability than those fed with sugar and banana. We found no evidence for a lifespan cost of reproduction. Our results show that (1) female C. fulvescens can use amino acids in their diet for laying fertile eggs, (2) more wing-wear does correlate with lower survival in captivity (indicating aging in the wild), but not with intensity of reproduction (providing no evidence for reproductive aging), and (3) fruit-feeding butterflies may be dietary restricted in the field.

Globalization and fruitfly invasion and expansion: the medfly paradigm

The phytophagous insects of the Tephritidae family commonly referred to as "true fruit flies" offer different case histories of successful invasions. Mankind has played an important role in altering the distributions of some of the more polyphagous and oligophagous species. However, the question arises why only a few species have become major invaders. The understanding of traits underlying adaptation in different environments is a major topic in invasion biology. Being generalists or specialists, along the K-r gradient of the growth curve, make a difference in term of food resources exploitation and interspecies competition and displacement. The species of the genus Ceratitis are good examples of r-strategists. The genetic and biological data of the most notorious Ceratitis species, the Mediterranean fruit fly Ceratitis capitata (medfly), are reviewed to investigate the traits and behaviours that make the medfly an important invader. It can be learnt from medfly, that invasions in a modern global trade network tend to be due to multiple introductions. This fact allows a maintenance or enhancement of genetic variability in the adventive populations, which in turn increases their potential invasiveness. Our current knowledge of the medfly genome opens the way for future studies on functional genomics.

Lifelong heterogeneity in fecundity is insufficient to explain late-life fecundity plateaus in Drosophila melanogaster

Previous studies have demonstrated that fecundity, like mortality, plateaus at late ages in cohorts of Drosophila melanogaster. Although evolutionary theory can explain the decline and plateau in cohort fecundity at late ages, it is conceivable that lifelong heterogeneity in individual female fecundity is producing these plateaus. For example, consistently more fecund females may die at earlier ages, leaving only females that always laid a low number of eggs preponderant at later ages. We simulated fecundity within a cohort, assuming the two phenotypes described above, and tested these predictions by measuring age of death and age-specific fecundity for individual females from three large cohorts. We statistically tested whether there was enough lifelong heterogeneity in fecundity to produce a late-life plateau by testing whether early female fecundity could predict whether that female would live to lay eggs after the onset of the population fecundity plateau. Our results indicate that heterogeneity in fecundity is not lifelong and thus not likely to cause late-life fecundity plateaus. Because lifelong heterogeneity models for fecundity are based on the same underlying assumptions as heterogeneity models for late-life mortality rates, our test of this hypothesis is also an experimental test of lifelong heterogeneity models of late life generally.

Individual fecundity and senescence in Drosophila and medfly

Evolutionary theory postulates that there should be a robust relationship between fecundity and longevity. Prior work has generally supported this concept, but has not shed much light on the mechanisms at play. In preceding work, we have developed and verified a mathematical model of Drosophila melanogaster female fecundity based on the analysis of empirical studies independently done by several different laboratories. Then we applied this technique to Mediterranean fruit fly (medfly) populations. In this article we analyze associations between individual longevity and the parameters of individual fecundity pattern in Drosophila and medfly. We cluster both Drosophila and medfly individuals by life span and discuss the differences. It allows us to demonstrate that only one fecundity-related parameter is associated with longevity in Drosophila, whereas two such parameters can be found in medflies. This difference demonstrates different ways of aging in various Diptera species. Finally, we discuss the possible implications of this finding.

Biodemography of a long-lived tephritid: reproduction and longevity in a large cohort of female Mexican fruit flies, Anastrepha ludens

Age of sexual maturity, daily and lifetime reproductive rates, and life span were recorded in a laboratory cohort of Mexican fruit flies consisting of over 1100 females maintained individually. The results revealed that, relative to the medfly, the Mexfly is slower maturing (14 vs 17 days), more fecund (1400 vs 650-1100 eggs/female), and longer lived (50 vs 35 days). The results reinforced the generality of several earlier findings on the medfly including the deceleration of mortality at older ages and the weakness of the correlation between the rate of egg laying at early ages and both subsequent reproduction and remaining longevity. Discussion includes perspectives on the role of artificial selection in shaping the demographic traits of the mass-reared strain of Mexfly used in this study, as well as the overall significance of large scale biodemographic studies in understanding aging and longevity.

Testing whether male age or high nutrition causes the cessation of reproductive aging in female Drosophila melanogaster populations

Fecundity seems to stop declining and plateaus at low levels very late in Drosophila melanogaster populations. Here we test whether this apparent cessation of reproductive aging by a population, herein referred to as fecundity plateaus, is robust under various environmental influences: namely, male age and nutrition. The effect of male age on late age fecundity patterns was tested by supplying older females with young males before average population fecundity declined to plateau levels. The second possible environmental influence we tested was nutrition and whether late-life fecundity plateaus arise from a decline in the calories available for reproduction. This hypothesis was tested by comparing average daily female fecundity with both low- and high-lifetime nutrition. Both hypotheses were tested by measuring mid- and late-life fecundity for each cohort under the various environmental influences, and statistically testing whether fecundity stops declining and plateaus at late ages. These experiments demonstrate that mid- and late-life population fecundity patterns are significantly affected by the age of males and nutrition level. However, male age and nutrition level did not affect the existence of late-life fecundity plateaus, which demonstrates the robustness of our earlier findings. These results do not address any issue pertaining to the possible role, if any, of lifelong inter-individual heterogeneity in Drosophila fecundity.

How an Individual Fecundity Pattern Looks inDrosophilaand 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.