Adaptive egg size plasticity for larval competition and its limits in the seed beetle Callosobruchus chinensis

Life‐history theory predicts that females who experienced stressful conditions, such as larval competition or malnutrition, should increase their investment in individual offspring to increase offspring fitness (the adaptive parental hypothesis). In contrast, it has been shown that when females were reared under stressful conditions, they become smaller, which consequently decreases egg size (the parental stress hypothesis). To test whether females adjust their egg volume depending on larval competition, independent of maternal body mass constraint, we used a pest species of stored adzuki beans, Callosobruchus chinensis (L.) (Coleoptera: Chrysomelidae: Bruchinae). The eggs of females reared with competitors were smaller than those of females reared alone, supporting the parental stress hypothesis; however, correcting for female body size, females reared with competitors produced larger eggs than those reared in the absence of competition, supporting the adaptive parental hypothesis, as predicted. The phenotypic plasticity in females' investment in each offspring in stressful environments counteracts the constraint of body size on egg size.     

Reduced realised fecundity in the pine looper Bupalus piniarius caused by host plant defoliation

1. Whether the potential fecundity of herbivorous insects is realised or not may depend on female behaviour, which in turn may be influenced by host plant acceptability. Female Bupalus piniarius were observed to discriminate against needles growing out the year following defoliation (current‐year needles) of its host plant Pinus sylvestris. 2. It was hypothesised that the discriminatory behaviour was due to current‐year needles being less secure as a substrate. Field and laboratory experiments were designed to test this hypothesis and to estimate the discrepancy between potential and realised fecundity when females were offered defoliated branches. 3. In a laboratory oviposition experiment, B. piniarius females were exposed to branches bearing either current‐year needles only or both mature and current‐year needles. Daily oviposition rate, egg batch size, longevity, and mature eggs and fat retained at death were recorded for each female. In field experiments, the rate at which eggs dropped from expanding needles and the capacity of neonate larvae hatching from the dropped eggs to colonise a tree were assessed. 4. Significantly fewer eggs were laid when females were exposed to defoliated branches. 5. Twenty‐six and 16% of the eggs laid on current‐year needles dropped from the needles in 1998 and 1999 respectively, whereas no eggs dropped from mature needles in 1998 and only one egg (< 1%) dropped in 1999. 6. A very small proportion of larvae hatching on the forest floor (simulated egg drop) was able to recolonise host trees. 7. These results emphasise the importance of oviposition behaviour on realised fecundity when analysing insect population dynamics. In the case of B. piniarius, egg placement, although a minor detail during the normal course of events, became of key importance when defoliation deprived females of their preferred egg attachment site.     

Maternal adjustment of the sex ratio in broods of the broad-horned flour beetle, Gnathocerus cornutus

We report that females of the broad-horned flour beetle, Gnathocerus cornutus, can plastically adjust the sex ratio in their broods in response to environmental quality. Specifically, females reared in nutritionally poor environments produce broods that are 65% female, on average, with the degree of female-bias in some broods approaching 95%. In addition, females reared in nutritionally poor environments lay significantly more eggs than do females reared on standard medium, which produce broods with an even sex ratio. These effects of the mother's environment on size and sex ratio in broods are manifest even when oviposition occurs in the standard nutritional environment; indeed, the degree of female-bias increases with advancing female age despite the availability of nutritional resources to females at the time of egg laying. Our studies rule out sex-specific differences in viability early in larval development as the mechanism for the bias in sex-ratio of broods, since females reared in nutritionally poor environments have broods with hatchability and larval viability comparable to those of nonstressed females. Our studies also rule out an effect of the sire on the sex ratio in broods, since all male mates were reared on standard medium. We discuss our results in the context of theories for the evolution of plastic sex-ratios in the face of environmental deterioration and discuss how plasticity can resolve a long-standing question about the conditions underlying the evolution of biased sex ratios.     

Cooler butterflies lay larger eggs: developmental plasticity versus acclimation

We use a full factorial design to investigate the effects of maternal and paternal developmental temperature, as well as female oviposition temperature, on egg size in the butterfly Bicyclus anynana. Butterflies were raised at two different temperatures and mated in four possible sex-by-parental-temperature crosses. The mated females were randomly divided between high and low oviposition temperatures. On the first day after assigning the females to different temperatures, only female developmental temperature affected egg size. Females reared at the lower temperature laid larger eggs than those reared at a higher temperature. When eggs were measured again after an acclimation period of 10 days, egg size was principally determined by the prevailing temperature during oviposition, with females ovipositing at a lower temperature laying larger eggs. In contrast to widely used assumptions, the effects of developmental temperature were largely reversible. Male developmental temperature did not affect egg size in either of the measurements. Overall, developmental plasticity and acclimation in the adult stage resulted in very similar patterns of egg size plasticity. Consequently, we argue that the most important question when testing the significance of acclamatory changes is not at which stage a given plasticity is induced, but rather whether plastic responses to environmental change are adaptive or merely physiological constraints.     

Effect of host quality of Callosobruchus maculatus (Coleoptera: Bruchidae) on performance of the egg parasitoid Uscana lariophaga (Hymenoptera: Trichogrammatidae)

Development and reproductive success of the solitary egg parasitoid Uscana lariophaga Steffan were examined after development in eggs of the bruchid storage pest Callosobruchus maculatus Fabricius reared at either low or high densities on cowpea seeds and laid at day 1 and 4 of maternal life. Both bruchid larval competition and maternal age negatively affected egg size, but the latter more than the former. Uscana lariophaga reared in small hosts developed slower, were smaller and produced fewer eggs compared to parasitoids reared in large hosts. Fecundity of the parasitoid was heavily influenced by host egg size. This was reflected in the values for the intrinsic rate of increase of U. lariophaga, which differed for wasps that developed in host eggs laid by bruchid females of different age. Wasps allocated marginally more female offspring to larger hosts.     

Acidic precipitation increases egg survival in Neodiprion sertifer

The effects of simulated acidic precipitation on the egg viability of the European pine sawfly, Neodiprion sertifer Geoffroy (Hymenoptera: Diprionidae) were studied by spraying egg clusters with a mixture of sulphuric and nitric acid (1:1, pH 2, 3, 4, 5, 6, distilled water, natural precipitation) for a month in April–May. Egg viability analyses showed that the proportion of hatched first instar larvae increased with increasing acidity. The better survival of sawfly eggs in pine needles in more acidic conditions may be partly due to changes in needle physiology affecting egg mortality. However, when the eggs were reared without needles in Petri-dishes, the proportion of successfully hatched larvae likewise increased with increasing acidity indicating direct acid-induced changes in the eggs themselves. Acidic precipitation as a stress factor of the host plant apparently changes population dynamics of pine sawflies and may be an important factor in triggering outbreaks.     

A lengthy solution to the optimal propagule size problem in the large-bodied South American freshwater turtle, <Emphasis Type="Italic">Podocnemis unifilis</Emphasis>

In oviparous vertebrates lacking parental care, resource allocation during reproduction is a major maternal effect that may enhance female fitness. In general, resource allocation strategies are expected to follow optimality models to solve the energy trade-offs between egg size and number. Such models predict that natural selection should optimize egg size while egg number is expected to vary with female size, thus maximizing offspring fitness and consequently, maternal fitness. Deviations from optimality predictions are commonly attributed to morphological constraints imposed by female size, such as reported for small-bodied turtle species. However, whether such anatomical constraints exist in smaller-bodied females within large-bodied clades remains unstudied. Here we tested whether resource allocation of the river turtle Podocnemis unifilis (a relatively smaller member of the large-bodied Podocnemididae) follows optimality theory, and found a pattern of egg elongation in smaller females that provides evidence of morphological constraints and of a reproductive trade-off with clutch size, whereas egg width supports the existence of an optimal egg size and no trade-off. Moreover, larger females laid larger clutches composed of rounder eggs, while smaller females laid fewer and relatively more elongated eggs. Elongated eggs from smaller females have larger volume relative to female size and to round eggs of equal width. We propose that elongated eggs represent a solution to a potential morphological constraint suffered by small females. Our results suggest that larger females may optimize fitness by increasing the number of eggs, while smaller females do so by producing larger eggs. Our data supports the notion that morphological constraints are likely more widespread than previously anticipated, such that they may not be exclusive of small-bodied lineages but may also exist in large-bodied lineages.     

Failure of white-tailed deer, Odocoileus virginianus L., to sustain a population of cattle ticks, Boophilus annulatus (Say), through successive generations

Cattle ticks, Boophilus annulatus (Say), previously reared only on cattle, were placed on 3 white-tailed deer, Odocoileus virginianus L. Ticks were maintained through successive generations solely on the same deer as they aged (3, 6, and 9 mo of age) and received repeated challenges (0, 1, and 2 previous challenges). Cattle were infested simultaneously to assess tick viability and provide a comparison of tick numbers, female weight, egg mass weight, and egg hatch. The initial infestation (3,000 larvae/animal) produced a mean of 12.7 and 506.7 females from deer and cattle, respectively. Ticks recovered from deer weighed less, laid smaller egg masses, and had lower egg hatchability than cattle-reared ticks. A second infestation (3,000 larvae/animal) produced a 6.3-fold reduction in tick numbers on deer (means = 2.0 females/deer), whereas the number on cattle increased (means = 578.0 females/calf). Ticks reared on the deer were again smaller, laid fewer eggs, and had lower egg hatch, although differences were not significant. A third infestation of deer (1,900 larvae/deer) produced only 1 engorged female tick and no viable eggs, thus eliminating the population of deer-reared ticks within 3 generations. Results of the study suggest that a population of B. annulatus will not be sustained indefinitely through time solely on deer; thus, efforts to reduce deer populations severely as a means of eradicating ticks are unnecessary.     

Adaptive plasticity of egg size in response to competition in the cowpea weevil,Callosobruchus maculatus (Coleoptera: Bruchidae)

Life history theory predicts that larger propagules should be produced when the offspring are expected to experience intense competition. This study tested whether female cowpea weevils responded to high larval or adult density by producing larger eggs. In a splitbrood design I measured the effect of density experienced by females at their larval stage (1 vs. 4–6 larvae/cowpea) on the size of eggs produced just after emergence. The females were then kept either at low adult density (1 female+1 male per vial), or at high adult density (10 females+10 males) for 2 days, and tested for the effect of this adult density treatment on the size of eggs laid subsequently. I measured egg length and width, as well as the diameter of the entrance tunnel made by the larva, which can be regarded as a crude measure of larval size. Females that experienced high adult density subsequently laid slightly wider eggs than those kept at low density. This difference, albeit small (about 1–4% after correction for female weight and the effect of family, depending on the statistical model used), was statistically significant and robust to alterations of the statistical model. It may be a remnant of a larger plastic response of egg size to competition that has become eroded during many generations in high-density laboratory cultures. There was no difference in egg length or the diameter of the entrance tunnel. Eggs laid just after emergence by females reared at high larval density also tended to be wider than those produced by females that had no competitors. This effect was only marginally significant, however, and sensitive to the statistical model. Both egg length and width and the diameter of the entrance tunnel increased with female weight and decreased with female age. The tunnel diameter was positively correlated with both egg length and width, but the effect of width was larger.     

Effects of Single Versus Multiple Mates: Monogamy Results in Increased Fecundity for the Beetle <Emphasis Type="Italic">Phoracantha semipunctata</Emphasis>

Effects of polyandry versus monogamy were assessed for the beetle Phoracantha semipunctata (Coleoptera: Cerambycidae) by comparing measures of female reproductive success, including fecundity, egg viability, time until eclosion, and clutch size. The effect of intermittent presence or absence of a male was also evaluated. Polyandry was detrimental to female reproductive success in comparison to monogamy. Fecundity, egg viability, and clutch size were lower, and time to eclosion were increased for eggs from females with multiple mates compared with monogamous females. Intermittent presence of males had no effect on female reproductive success. Possible explanations for the decreased fecundity experienced by females with multiple mates include sperm competition intensity, costs of male harassment, and exceeding the optimal mating frequency. Females may reduce costs associated with polyandry by spending less time on host logs where mating occurs.     

Stressed mothers lay eggs with high corticosterone levels which produce low-quality offspring

Organisms frequently encounter stressful ecological conditions. In vertebrates, a major mechanism of physiological response to stress is mediated by the hypothalamic-pituitary-adrenal axis and results in increased secretion of glucocorticosteroids, which can have adverse consequences on diverse phenotypic traits affecting fitness. Maternal stress may thus have carry-over effects on progeny if it influences pre-natal offspring environment in terms of glucocorticosteroid concentration, although this hypothesis has never been tested in any species under field conditions. We manipulated stress experienced by female barn swallows Hirundo rustica, by exposing them to a predator during laying and measured egg corticosterone concentration. Stressed females laid eggs with greater corticosterone concentration than controls exposed to a herbivore. In another experiment, we injected physiological doses of corticosterone in the egg albumen and compared the phenotype of offspring originating from these eggs with their control siblings originating from either sham-inoculated or unmanipulated eggs and reared in the same nest. Eggs injected with corticosterone had lower hatchability and produced fledglings with smaller body size and slower plumage development than did control eggs. Nestling body size in our study population predicts long-term survival. Thus, maternal stress impaired offspring phenotype and viability by increasing transmission of glucocorticosteroids to the eggs. This study identifies a novel mechanism mediating early maternal effects whereby maternal stress affects offspring quality. These results are relevant to biological conservation because they disclose a mechanism that can link environmental conditions to population productivity and viability.     

The spawning activity of cod, Gadus morhua L.

Aspects of the reproduction of reared cod, Gadus morhua L., with special emphasis on the females, were studied under laboratory conditions. The fecundity and condition factor were 2–5 and 1–5 times, respectively, that of wild cod. A total of 18 spawning females were kept in separate tanks/ chambers, each with one or two males. Seven of the 18 females were classified as stressed, based upon behaviour, irregular spawning intervals and low fertilization rates of the eggs. The reared cod were found to spawn 17–19 batches. The number of eggs liberated in each batch normally followed a smooth, dome-shaped curve with time. The fertilization rate was normally 100%. Egg size decreased from first to last batch and the egg dry weight decreased by about 20–30%. The reared cod showed the same egg diameter to dry weight relation as wild cod. Egg diameter of first batch and maternal fish length were significantly positively correlated. The mean spawning interval for the female and the mean water temperature during its spawning were negatively correlated. The reared cod spawned in both the night and the day for about 50–60 days.     

Hypoxia and life‐history traits in a eurytopic African cichlid

This study quantified variation in key life‐history traits of the widespread African mouth‐brooding cichlid Pseudocrenilabrus multicolor victoriae. Egg size, number, batch reproductive effort, size at maturity and brooding efficiency were compared among field populations across a wide range of dissolved oxygen (DO) concentrations from extreme hypoxia to normoxia. In the laboratory, a similar suite of characters was quantified in F1 of low‐ and high‐DO origin reared under low or high DO. In general, females from low‐DO habitats and females reared under low DO were characterized by a smaller size at maturity and no difference in batch reproductive effort when compared with females from high‐DO habitats or females reared under normoxia. A trade‐off between egg size and number was evident in the field and in the laboratory‐rearing experiment, but the direction of the trade‐off differed. Egg size was negatively correlated with egg number across field populations; females collected from low‐DO sites generally had more, smaller eggs relative to females from high‐DO sites. In the laboratory‐rearing experiment, F1 females of high‐DO origin produced larger, fewer eggs than F1 females of low‐DO origin, lending support to the field results and suggesting a heritable component to these traits. There was also an element of developmental plasticity, F1 females raised under low DO produced larger, fewer eggs compared with F1 females raised under high DO (regardless of population) suggesting that DO may interact with other variables to determine egg size in the field.     

Reproductive strategy of the ghost shrimp Callichirus major (Crustacea: Axiidea: Callianassidae) from the southwestern Atlantic: sexual maturity of females,fecundity, egg features,and reproductive output

Reproduction and reproductive investment of females is an intriguing feature in axiidean shrimps. They have a cryptic behavior and great ecological importance in sediment turnover, and recycling of organic material and nutrient. Herein, we describe different aspects of the reproductive biology (size at sexual maturity of females, fecundity, egg characteristics, and reproductive output [RO]) of the ghost shrimp Callichirus major from the southeastern coast of Brazil. Females reached sexual maturity at sizes ≥11.85?mm dorsal oval length. Fecundity increased significantly with female size and varied between 1455 and 9931 eggs (average 4564 eggs). Individual egg volume almost doubled during embryogenesis, and average egg water content increased during the incubation period from 75.0 to 93.3%. The egg mass comprised on average 13.2% of the dry body weight of females. This relatively high RO value is probably compensatory to the relatively low number of ovigerous females in axiidean populations. The few published data on RO values suggest that female investment in reproduction of axiidean shrimps is somewhat higher than in other decapods. The high investment in egg production reflects most likely an effort to maximize the viability of the progeny. Studies with additional congeneric species will clarify whether there is a tendency of axiidean shrimps to have RO values at the upper end of the range reported for decapods.     

Oviposition Behaviour and Reproductive Performance of Trichogramma australicum Girault (Hymenoptera: Trichogrammatidae) Reared in Artificial Diet

Biotypes of Trichogramma australicum were reared on artificial diet "A", Helicoverpa armigera (Hübner) (natural host) eggs and Sitotroga cerealella (Olivier) (factitious host) eggs. We evaluated the quality of these biotypes by comparing their oviposition behaviour and reproductive performance on H. armigera eggs. Female T. australicum reared in vitro accepted H. armigera eggs for oviposition and displayed similar behavioural components of oviposition to females reared on natural hosts. However, handling time during oviposition in H. armigera eggs by females from in vitro rearing was significantly longer than handling time of females reared on the natural host because the females needed a significantly longer time for host feeding. In vitro reared females produced significantly more progeny and parasitised more H. armigera eggs than the females reared on S. cerealella. Improvements to the artificial diet for T. australicum may reduce handling time of the in vitro reared females and increase the female population. These improvements may include changes in nutrient content and size of the artificial eggs.     

Artificial miniaturization causes eggs laid by crowd-reared (gregarious) desert locusts to produce green (solitarious) offspring in the desert locust, Schistocerca gregaria

The mechanism underlying the phase-dependent polyphenism in hatchling body coloration was studied by testing for a possible causal relationship with egg size in the desert locust, Schistocerca gregaria. Crowd-reared (gregarious) females typically produce large, black offspring, whereas females reared in isolation (solitarious) deposit small, green offspring. We first tested for possible genetic differences in the role of egg foam by washing or separating eggs from two strains of locust. No solitarizing effect was found in either of the strains tested, supporting a previous finding, using another laboratory strain, to show that the hatchling body coloration and size are pre-determined in the ovary of the mother and no egg foam factor is involved in the control of the hatchling body coloration. Topical application of fenoxycarb, a juvenile hormone analog (JHA), and implantation of extra corpora allata (CA), taken from Locusta migratoria, caused gregarious female adults of S. gregaria to produce small eggs. Some eggs laid by CA-implanted females produced green hatchlings. All large eggs chosen among those deposited by gregarious females produced black hatchlings. When eggs were either kept on dry filter paper at nearly saturated relative humidity during embryogenesis or pricked with a needle so that some egg yolk was squeezed out, some produced small, green hatchlings. These results suggested that the amount of egg yolk or the availability of yolk material may determine the body coloration of hatchlings.     

Relatively large males lower reproductive success in female zebrafish

Females can adjust their reproductive effort in relation to their partner’s perceived fitness value. In zebrafish (Danio rerio), large males are typically preferred mating partners. However, females have been observed to reduce their reproductive output with exceptionally large males but it remains unknown whether it is due to sexual harassment or aggressive behavior to establish and maintain dominance. Here, we study the association between relative male size, sexual harassment and dominance behavior, female stress status (stress behaviors and whole-body cortisol concentration), and reproductive success during a 4 day spawning trial. We found female cortisol to correlate negatively with female body size and positively with female dominance behavior. However, male and female behavior as well as female cortisol level were not related to relative male size. Females mating with relatively large males produced more and most of their eggs during the first spawning day, while females with smaller males produced few eggs during the first day but then increased egg production. Despite females produced more eggs when mating with relatively larger males, their eggs had substantially lower fertilization rates compared to females mating with relatively smaller males. Hence, overall, the reproductive fitness was lowest when females mated with a relatively large male. These findings could help to explain the maintenance of male size variation under natural conditions.     

Facultative blood-feeding in the crabhole mosquito, Deinocerites cancer

1. Most of the female crabhole mosquitoes Deinocerites cancer Theobald entering bait traps had undeveloped ovaries. 2. Nearly half of the D. cancer females that were captured in bait traps, but denied blood, were gravid following a 1 week holding period; they developed relatively small egg clutches autogenously (mean = 7.4 eggs per gravid female). 3. Although both parous and nulliparous D. cancer females were captured exiting from crabholes, only parous females were attracted to and collected in bait traps. 4. Much larger egg clutches (mean = 41.8 eggs per female) were produced anautogenously by mosquitoes that blood-fed on chickens. 5. All D. cancer females from field collected pupae developed eggs autogenously when allowed to mate and sugar feed in the laboratory. These mosquitoes produced an initial egg clutch with a mean size equivalent to that found in blood-fed, parous mosquitoes from bait traps. 6. In the field, mating and sugar feeding rather than blood-feeding seem to stimulate production of the initial egg clutch. 7. D. cancer blood-feeding appears to be delayed until after the first egg clutch has been laid; some parous crabhole mosquitoes are facultative blood feeders and can produce an egg clutch either with or without a blood meal.     

Cryptic maternal effects in Hippodamia convergens vary with maternal age and body size

Maternal effects can mold progeny phenotypes in various ways and may constitute ecological adaptations. By examining the effect of oviposition sequence on progeny produced by different size classes of female ladybird beetles (produced by controlling larval access to food), we show that maternal signals can change through adult life and alter the developmental programs of progeny, ostensibly to synchronize their life histories with predictable resource dynamics, thus maximizing maternal fitness. We also show that female body size, as determined by larval food supply, interacts with female age to influence progeny fitness. When fed ad libitum as adults, small females reared with limited food access laid fewer, smaller eggs than large females reared with ad libitum food access. Maternal body size interacted with oviposition sequence to influence progeny development, but the latter had greater impact. Eggs laid later by medium and large females hatched faster than those laid earlier, larvae fed longer in the fourth instar, their pupation period was shorter, total developmental time was reduced, and adults emerged with greater mass, most notably daughters. Oviposition sequence effects on progeny from small mothers were non‐significant for total developmental time and progeny mass. Only large mothers increased egg size over time and egg mass was not consistently correlated with developmental parameters, indicating that progeny phenotype was impacted by other, more cryptic, maternal signals. Such signals appear costly, as food limitation during development constrained not only fecundity and egg size but also maternal ability to manipulate progeny phenotype. The production of faster‐developing offspring that mature to larger sizes late in the oviposition cycle may be adaptive for exploitation of ephemeral aphid outbreaks with predictable dynamics of prey abundance and competition.     

Egg size manipulation in the migrant skipper, Parnara guttata guttata (Lepidoptera: Hesperiidae), in response to different host plants

Female adults of the migrant skipper, Parnara guttata guttata, lay different-sized eggs on different host plants in different generations. Moreover, P. g. guttata maintains large egg size variation either in the population or in the individual. Why such phenotypic plasticity in egg size is maintained has not been clarified. In the present study we performed a series of experiments to verify whether or not females of P. g. guttata discriminate between the different host plants, i.e., rice plant Oriza sativa in the first and second generations and cogon grass Imperata cylindrica in the third (overwintering) generation and manipulate egg size accordingly on these host plants. When a caged female was allowed to lay eggs alternatively on soft-leafed rice plant and tough-leafed cogon grass, the size of the eggs laid on cogon grass was significantly larger than that on rice plant. When a caged female was allowed to lay eggs on the two host plants that were supplied on alternate days, the size of eggs laid on cogon grass was also significantly larger than that on rice plant. A preliminary experiment using crude extract from the plants suggested that females did not manipulate egg size in response to chemical stimulants alone. The skipper female was able to lay different-sized eggs instantaneously after assessing the host plant. However, the reaction norm to different host plants differed among females. Eggs of various sizes were matured in each ovariole, and egg size variation at the lowest part of the ovariole ranged from the size of the larger eggs laid on cogon grass to that of the smaller eggs laid on rice plant, although how the female chose the appropriate-sized egg at each occasion is unknown.