Maximum titers of vitellogenin and total hemolymph protein occur during the canalized phase of grasshopper egg production

How Nutrients Mediate the Impacts of Global Change on Locust Outbreaks

Locusts are grasshoppers that can migrate en masse and devastate food security. Plant nutrient content is a key variable influencing population dynamics, but the relationship is not straightforward. For an herbivore, plant quality depends not only on the balance of nutrients and antinutrients in plant tissues, which is influenced by land use and climate change, but also on the nutritional state and demands of the herbivore, as well as its capacity to extract nutrients from host plants. In contrast to the concept of a positive relationship between nitrogen or protein concentration and herbivore performance, a five-decade review of lab and field studies indicates that equating plant N to plant quality is misleading because grasshoppers respond negatively or neutrally to increasing plant N just as often as they respond positively. For locusts specifically, low-N environments are actually beneficial because they supply high energy rates that support migration. Therefore, intensive land use, such as continuous grazing or cropping, and elevated ambient CO2 levels that decrease the protein:carbohydrate ratios of plants are predicted to broadly promote locust outbreaks.

The effects of dietary amino acid balance on post-embryonic development in a lubber grasshopper

Effects of dietary protein quality on insect development (not just growth) are unclear. Dietary amino acid blends matching yolk proteins support reproduction and juvenile development in Drosophila melanogaster. We matched amino acids to vitellogenin and tested development of juvenile male lubber grasshoppers, which do not produce vitellogenin. Last instars were fed classic dry diets with amino acids substituted for proteins. Matching amino acids to vitellogenin allowed molting to adulthood, while an unmatched isonitrogenous diet did not. Health on dry diets was poor, so we developed wet diets with agar, horse feed, and amino acids. Juveniles fed these diets matched to vitellogenin developed comparably to juveniles fed lettuce. However, wet diets with amino acids dissimilar to vitellogenin (low-quality) slowed development but maintained size at adulthood. We observed no compensatory feeding on low-quality diets. Theory suggests accumulation of proteins permits development. To detect a threshold, we started last juvenile instars on high-quality diets, then abruptly switched them to low-qualities diets. When switched to the poor-quality diet at 6d, grasshoppers molted at a similar age (∼17d) to grasshoppers continuously on the high-quality diet. Total hemolymph proteins levels were unaffected by the timing of diet switches. Last, methionine is essential but can be noxious at high levels. Diets with low-quality protein except for methionine slowed growth early but did not alter the time or size at molt. Overall, the feeding threshold is solely due to essential amino acids, and low-quality protein diets slowed development but did not affect adult size.

Protein storage and reproduction increase in grasshoppers on a diet matched to the amino acids of egg yolk protein

The diets of animals are essential to support development, and protein is key. Accumulation of stored nutrients can support developmental events such as molting and initiation of reproduction. Agricultural studies have addressed how dietary protein quality affects growth, but few studies have addressed the effects of dietary protein quality on developmental transitions. Studies on how dietary quality may affect protein storage and development are possible in arthropods, which store proteins in the hemolymph. We hypothesized that diets with a composition of amino acids that matches the precursor of egg yolk protein (vitellogenin, Vg) will be high quality and support both egg production and accumulation of storage proteins. Grasshoppers were fed one of two isonitrogenous solutions of amino acids daily: Vg-balanced (matched to Vg) or Unbalanced (same total moles of amino acids, but not matched to egg yolk). We measured reproduction and storage protein levels in serial hemolymph samples from individuals. The Vg-balanced group had greater reproduction and greater cumulative levels of storage proteins than did the Unbalanced group. This occurred even though amino acids fed to the Vg-balanced group were not a better match to storage protein than were the amino acids fed to the Unbalanced group. Further, oviposition timing was best explained by a combination of diet, age at the maximum level of storage protein hexamerin-270 and accumulation of hexamerin-90. Our study tightens the link between storage proteins and commitment to reproduction, and shows that dietary protein quality is vital for protein storage and reproduction.

Silencing of ATP Synthase β Impairs Egg Development in the Leafhopper Scaphoideus titanus, Vector of the Phytoplasma Associated with Grapevine Flavescence Dorée

Scaphoideus titanus (Hemiptera: Cicadellidae) is the natural vector of Flavescence dorée phytoplasma, a quarantine pest of grapevine with severe impact on European viticulture. RNA interference (RNAi) machinery components are present in S. titanus transcriptome and injection of ATP synthase β dsRNAs into adults caused gene silencing, starting three days post injection (dpi) up to 20 dpi, leading to decrease cognate protein. Silencing of this gene in the closely related leafhopper Euscelidiusvariegatus previously showed female sterility and lack of mature eggs in ovaries. Here, alteration of developing egg morphology in S. titanus ovaries as well as overexpression of hexamerin transcript (amino acid storage protein) and cathepsin L protein (lysosome proteinase) were observed in dsATP-injected females. To evaluate RNAi-specificity, E.variegatus was used as dsRNA-receiving model-species. Different doses of two sets of dsRNA-constructs targeting distinct portions of ATP synthase β gene of both species induced silencing, lack of egg development, and female sterility in E. variegatus, indicating that off-target effects must be evaluated case by case. The effectiveness of RNAi in S. titanus provides a powerful tool for functional genomics of this non-model species and paves the way toward RNAi-based strategies to limit vector population, despite several technical and regulatory constraints that still need to be overcome to allow open field application.

Life-extending Dietary Restriction Reduces Oxidative Damage of Proteins in Grasshoppers but Does Not Alter Allocation of Ingested Nitrogen to Somatic Tissues

Dietary restriction (DR) extends life span and reduces reproduction in most animals. The disposable soma hypothesis suggests that this longevity is the result of reduced investment in reproduction and increased nutrient allocation to the soma, permitting an increase in cellular maintenance. To investigate the role of nutrient allocation upon life-extending DR, tissue-specific nitrogen allocation was tracked in grasshoppers (Romalea microptera) upon a full or restricted (60% of full) diet. In addition, carbonyl (oxidized protein) assays addressed tissue maintenance. To develop a labeled diet on which grasshoppers could thrive, hydroponically grown Romaine lettuce was enriched with 15N. This allowed quantification of nitrogen allocation upon a normal or restricted diet. There was a 50% decrease in reproductive investment upon DR. At the same time, relative allocation of 15N to the ovary did not change. Most important, relative allocation was similar between restricted and full diet grasshoppers for somatic tissues (ie, mandibular and femur muscle, dried hemolymph, gut, and fat body). Carbonyl assays of muscles, hemolymph, and gut revealed an overall reduction in protein oxidation upon DR. These data suggest that DR does not alter nutrient allocation but does reduce protein oxidation, an observation that is inconsistent with the basic predictions of the disposable soma hypothesis.

Vitellogenin-RNAi and ovariectomy each increase lifespan, increase protein storage, and decrease feeding, but are not additive in grasshoppers

Reduced reproduction has been shown to increase lifespan in many animals, yet the mechanisms behind this trade-off are unclear. We addressed this question by combining two distinct, direct means of life-extension via reduced reproduction, to test whether they were additive. In the lubber grasshopper, Romalea microptera, ovariectomized (OVX) individuals had a ~20% increase in lifespan and a doubling of storage relative to controls (Sham operated). Similarly, young female grasshoppers treated with RNAi against vitellogenin (the precursor to egg yolk protein) had increased fat body mass and halted ovarian growth. In this study, we compared VgRNAi to two control groups that do not reduce reproduction, namely buffer injection (Buffer) and injection with RNAi against a hexameric storage protein (Hex90RNAi). Each injection treatment was tested with and without ovariectomy. Hence, we tested feeding, storage, and lifespans in six groups: OVX and Buffer, OVX and Hex90RNAi, OVX and VgRNAi, Sham and Buffer, Sham and Hex90RNAi, and Sham and VgRNAi. Ovariectomized grasshoppers and VgRNAi grasshoppers each had similar reductions in feeding (~40%), increases in protein storage in the hemolymph (150-300%), and extensions in lifespan (13-21%). Ovariectomized grasshoppers had higher vitellogenin protein levels than did VgRNAi grasshoppers. Last but not least, when ovariectomy and VgRNAi were applied together, there was no greater effect on feeding, protein storage, or longevity. Hence, feeding regulation, and protein storage in insects, may be conserved components of life-extension via reduced reproduction.

Vitellogenin RNAi halts ovarian growth and diverts reproductive proteins and lipids in young grasshoppers

Reduced reproduction extends lifespan of females in many animals. To test the effects of reproduction on storage of macronutrients, we block reproductive output in the lubber grasshopper by injecting RNAi against the precursor to egg-yolk protein, vitellogenin, in early adulthood. Controls were injected with either buffer or RNAi against the major storage protein in the hemolymph, hexamerin-90. Vitellogenin RNAi greatly reduced both levels of mRNA for vitellogenin and ovarian growth, in comparison to both controls. Fat body mass was increased upon vitellogenin RNAi, but concentrations of the three hexameric storage proteins from the hemolymph were not. Surprisingly, hemolymph vitellogenin levels were increased upon vitellogenin RNAi. Total reproductive protein (hemolymph vitellogenin plus ovarian vitellin) was unchanged by vitellogenin RNAi, as reproductive protein was diverted to the hemolymph. Similarly, the increased lipid storage upon vitellogenin RNAi was largely attributable to the reduction in lipid in the ovary, due to decreased ovarian growth. A BLAST search revealed that the 515 bp sequence of vitellogenin used for RNAi had three 11 bp regions identical to the vitellogenin receptor of the cockroach Leucophaea maderae. This suggests that our treatment, in addition to reducing levels of vitellogenin transcript, may have also blocked transport of vitellogenin from the hemolymph to the ovary. This would be consistent with halted ovarian growth simultaneous with high levels of vitellogenin in the hemolymph. Nonetheless, the accumulation of vitellogenin, instead of hexameric storage proteins, is inconsistent with a simple model of the trade-off between reproduction and storage. This was observed in young females; future studies will address whether investment of proteins may shift to the soma as individuals age. Overall, our results suggest that blockage of reproduction in young grasshoppers redirects lipids to storage and reproductive proteins to the hemolymph.

Life-extending ovariectomy in grasshoppers increases somatic storage, but dietary restriction with an equivalent feeding rate does not

Reduced diet or reduced reproduction each extends lifespan in many animals. It is often thought that reduced reproduction and reduced diet may act through the same mechanisms. In grasshoppers, ovariectomy extends lifespan and reduces feeding to a level similar to that used for life extension by dietary restriction, further suggesting mechanistic overlap. Here, we measure the feeding rate of ovariectomized grasshoppers and, by manipulating feeding levels, create a sham-operated & dietary restricted group with matched daily feeding. Both groups show ~25% increased survivorship near the median age of mortality for fully fed and reproductive controls. Ovariectomy results in a doubling of fat body mass and hemolymph volume in comparison to both a feeding-matched dietary restriction group and a sham-operated & fully fed control, which do not differ from each other. Total anti-oxidant activity in the hemolymph and the skeletal muscle was unchanged upon ovariectomy or dietary restriction, so it does not appear to be a major factor in lifespan extension. Next, we measured mitochondrial counts using qPCR to determine mitochondrial cytochrome-b concentrations relative to nuclear (genomic) beta-actin. Mitochondrial counts in the ovariectomized group were lower than sham-operated and fully fed controls but not than the dietary restriction group. Last, in the fat body, transcript levels of hexamerin-90 (a hemolymph storage protein) were affected by neither ovariectomy nor dietary restriction. Hence, ovariectomy resulted in large magnitude increases in organismal storage. The matched-fed dietary restricted group differed from the ovariectomized group only in organismal storage, and not in any of the cellular parameters measured here. This study suggests that longevity via ovariectomy has distinct physiological mechanisms from longevity via dietary restriction in grasshoppers that are independent of daily feeding rate, particularly for protein and fat storage.

Ovariectomy in grasshoppers increases somatic storage, but proportional allocation of ingested nutrients to somatic tissues is unchanged

Reduced reproduction increases storage and extends lifespan in several animal species. The disposable soma hypothesis suggests this life extension occurs by shifting allocation of ingested nutrients from reproduction to the soma. A great deal of circumstantial evidence supports this hypothesis, but no direct tracking of nutrients has been performed in animals that are long-lived because of direct reduction in reproduction. Here, we use the stable isotopes to track carbon and nitrogen from ingestion to somatic organs in long-lived, ovariectomized grasshoppers. Three estimates of somatic storage (viz., quantity of hemolymph storage proteins, amount of femur muscle carbohydrates, and size of the fat body) all doubled upon ovariectomy. In stark contrast, ovariectomy did not increase the proportion of these tissues that were made from recently ingested foods. In other words, the physiology underlying relative allocation to these somatic tissues was not affected by ovariectomy. Thus, at the level of whole tissue storage, these results are consistent with a trade-off between reproduction and longevity. In contrast, our stable isotope data are inconsistent with the prediction that enhanced storage in ovariectomized females results from a physiological shift in allocation of ingested nutrients.

Life-extending dietary restriction and ovariectomy result in similar feeding rates but different physiologic responses in grasshoppers

Dietary restriction (DR) and reduced reproduction each extend life span in many species. Females undergoing DR typically experience a reduction in their fecundity, which raises the question of whether the two treatments are actually extending life span in overlapping ways. Life span in lubber grasshoppers has been shown to be increased by DR, and separately by ovariectomy (OVX). Here, we test the combination of these on life span. If life extension by the two treatments are additive, it would suggest that they likely act through separate pathways. The experimental groups were: fully reproductive and fully fed (ShamFD); ovariectomized and fully fed (OVXFD); fully reproductive and restricted diet (ShamDR); and ovariectomized and restricted diet (OVXDR). The median life spans of these groups were: ShamFD=245 d, OVXFD=285 d, ShamDR=286 d, and OVXDR=322 d. Feeding rate for the OVXFD group was 64% of ad libitum, similar to the 70% of ad libitum that was used for ShamDR. We also measured hemolymph parameters of physiology in these same individuals. Hemolymph levels of vitellogenin (the egg yolk-precursor protein) were increased 5-fold by OVX, but were not affected by DR. In addition, hemolymph total anti-oxidant activity (per μg protein) was significantly reduced by OVX, but was not affected by DR. We show that OVX and DR produce different physiological responses in grasshoppers, despite life extensions and feeding levels that were not significantly different. These data suggest that OVX and DR might extend life span via distinct pathways.

Does it pay to delay? Flesh flies show adaptive plasticity in reproductive timing

Life-history plasticity is widespread among organisms. However, an important question is whether it is adaptive. Most models for plasticity in life-history timing predict that animals, once they have reached the minimal nutritional threshold under poor conditions, will accelerate development or time to reproduction. Adaptive delays in reproduction are not common, especially in short-lived species. Examples of adaptive reproductive delays exist in mammalian populations experiencing strong interspecific (e.g., predation) and intraspecific (e.g., infanticide) competition. But are there other environmental factors that may trigger an adaptive delay in reproductive timing? We show that the short-lived flesh fly Sarcophaga crassipalpis will delay reproduction under nutrient-poor conditions, even though it has already met the minimal nutritional threshold for reproduction. We test whether this delay strategy is an adaptive response allowing the scavenger time to locate more resources by experimentally providing supplemental protein pulses (early, mid and late) throughout the reproductive delay period. Flies receiving additional protein produced more and larger eggs, demonstrating a benefit of the delay. In addition, by tracking the allocation of carbon from the pulses using stable isotopes, we show that flies receiving earlier pulses incorporated more carbon into eggs and somatic tissue than those given a later pulse. These results indicate that the reproductive delay in S. crassipalpis is consistent with adaptive post-threshold plasticity, a nutritionally linked reproductive strategy that has not been reported previously in an invertebrate species.

Allocation of nutrients to somatic tissues in young ovariectomized grasshoppers

The disposable soma hypothesis predicts that when reproduction is reduced, life span is increased because more nutrients are invested in the soma, increasing somatic repair. Rigorously testing the hypothesis requires tracking nutrients from ingestion to allocation to the soma or to reproduction. Fruit flies on life-extending dietary restriction increase allocation to the soma "relative" to reproduction, suggesting that allocation of nutrients can be associated with extension of life span. Here, we use stable isotopes to track ingested nutrients in ovariectomized grasshoppers during the first oviposition cycle. Previous work has shown that ovariectomy extends life span, but investment of protein in reproduction is not reduced until after the first clutch of eggs is laid. Because ovariectomy does not affect investment in reproduction at this age, the disposable soma hypothesis would predict that ovariectomy should also not affect investment in somatic tissues. We developed grasshopper diets with distinct signatures of ¹³C and ¹⁵N, but that produced equivalent reproductive outputs. These diets are, therefore, appropriate for the reciprocal switches in diet needed for tracking ingested nutrients. Incorporation of stable isotopes into eggs showed that grasshoppers are income breeders, especially for carbon. Allocation to the fat body of nitrogen ingested as adults was slightly increased by ovariectomy; this was our only result that was not consistent with the disposable soma hypothesis. In contrast, ovariectomy did not affect allocation of nitrogen to femoral muscles. Further, allocation of carbon to the fat body or femoral muscles did not appear to be affected by ovariectomy. Total anti-oxidant activities in the hemolymph and femoral muscles were not affected by ovariectomy. These experiments showed that allocation of nutrients was altered little by ovariectomy in young grasshoppers. Additional studies on older individuals are needed to further test the disposable soma hypothesis.

Life history plasticity after attaining a dietary threshold for reproduction is associated with protein storage in flesh flies

Body condition affects the timing and magnitude of life history transitions. Therefore, identifying proximate mechanisms involved in assessing condition is critical to understanding how these mechanisms affect the expression of life history plasticity. Nutrient storage is an important body condition parameter, likely playing roles in both attaining minimum body-condition thresholds for life history transitions and expression of life history traits.We manipulated protein availability for females of the flesh fly Sarcophaga crassipalpis to determine whether reproductive timing and output would remain plastic or become fixed. Liver was provided for 0, 2, 4, or 6 days of adult pre-reproductive development. Significantly, liver was removed after the feeding threshold had been attained and females had committed to producing a clutch.We also identified the major storage proteins and monitored their abundances, because protein stores may serve as an index of body condition and therefore may play an important role in life history transitions and plasticity.Flesh flies showed clear post-threshold plasticity in reproductive timing. Females fed protein for 2 days took ~30% longer to provision their clutch than those fed for 4 or 6 days. Observations of oogenesis showed the 2-day group expressed a different developmental program including slower egg provisioning.Protein availability also affected reproductive output. Females fed protein for 2 days produced ~20% fewer eggs than females fed 4 or 6 days. Six-day treated females provisioned larger eggs than 4-day treated females, followed by 2-day treated females with the smallest eggs.Two storage proteins were identified, LSP-1 and LSP-2. LSP-2 accumulation differed across feeding treatments. The 2- and 4-day treatment groups accumulated LSP-2 stores but depleted them during provisioning of the first clutch, whereas the 6-day group accumulated the greatest quantity of LSP-2 and had substantial LSP-2 stores remaining at the end of the clutch. This pattern of accumulation and depletion suggests that LSP-2 could play roles in both provisioning the current clutch and future clutches, making it a good candidate molecule for affecting reproductive timing and allotment. LSP-1 was not associated with post-threshold plasticity; it was carried over from larval feeding into adulthood and depleted uniformly across all feeding groups.

Protein accumulation underlying lifespan extension via ovariectomy in grasshoppers is consistent with the disposable soma hypothesis but is not due to dietary restriction

Reduced reproduction extends lifespan in many experimental animals, but the mechanism by which this occurs is unclear. The disposable soma hypothesis suggests that when reproduction is reduced, more nutrients are allocated to the soma and lifespan is extended. Alternatively, the reproductive tissues or the process of reproduction may have a direct (i.e., non-nutritional) negative effect on lifespan. We used ovariectomized grasshoppers to examine the effects of reduced reproduction throughout the lifespan at the physiological level. We focused on protein, the limiting nutrient for egg production. Ovariectomized females lived significantly longer than sham females. Because both groups ingested similar amounts, the effect was independent of dietary restriction. Despite this, ovariectomized females gained less body mass than sham females. Ovariectomized grasshoppers produced the egg yolk-precursor protein vitellogenin. At the time sham females laid their first clutch, cumulative reproductive protein was similar in ovariectomized and sham females. By advanced ages, however, ovariectomized females had produced about five-fold less cumulative reproductive protein than sham females. In contrast, old ovariectomized females had at least two-fold more hemolymph storage protein. These results are consistent with ovariectomy extending lifespan in part via enhanced protein allocation to storage at the expense of reproduction.

A cumulative feeding threshold required for vitellogenesis can be obviated with juvenile hormone treatment in lubber grasshoppers

Developmental thresholds can ensure that an adequate condition has been attained to proceed through major transitions (e.g. initiation of reproduction, metamorphosis). Nutrition is critical to attaining most thresholds, because it is needed for both growth and storage. Attaining a threshold typically stimulates the release of hormones that commit the animal to the developmental transition, yet the relationships between the nutrition needed for developmental thresholds and these endocrine signals are poorly understood. Lubber grasshoppers require a cumulative feeding threshold to initiate vitellogenesis and potentially commit to oviposition. We tested the relative roles of the nutritional threshold and the major gonadotropin (juvenile hormone; JH) in initiating vitellogenesis and committing to oviposition. The source of JH was removed from all females, and then JH analog was applied after different amounts of feeding. Threshold feeding was not required to initiate vitellogenesis, suggesting that sub-threshold grasshoppers are competent to respond to JH. Further, sub-threshold grasshoppers went on to oviposit earlier than supra-threshold grasshoppers treated with JH at the same time. Hence, threshold feeding is required only to cause the production and release of JH. At the same time, we also found that individuals that were restored with JH late in life tended to favor current reproduction, at the expense of future reproduction. Both time to oviposition and vitellogenin profiles were consistent with this developmental allocation. Taken together, our results suggest that lubber grasshoppers adjust reproductive tactics primarily in response to nutrition (which only serves to release JH) and secondarily in response to age.

Calorie restriction and late-onset calorie restriction extend lifespan but do not alter protein storage in female grasshoppers

Calorie restriction (CR) and late-onset CR enhance longevity in many organisms. Resource allocation theory suggests that longevity is enhanced by increasing somatic storage, at the expense of current reproduction. Phytophagous insects accumulate amino acids as hemolymph storage proteins for major developmental events. We hypothesized that protein storage is involved in life extension from CR. In a longitudinal experiment, we tested whether CR altered protein storage in female grasshoppers. Individuals on CR (60 or 70% of ad libitum) or late-onset CR had at least 60% greater longevity than ad libitum individuals. Age at first oviposition, dry mass of the first clutch, or lifetime fecundity were not affected by CR, but CR did increase the number of clutches produced. Most important, females on life-extending CR and late-onset CR did not differ in the concentration of hemolymph storage of proteins in comparison to ad libitum females. Protein storage changed with time in all groups, demonstrating sufficient sensitivity in our methods. Previous experiments have shown that severe CR ( approximately 30% of ad libitum) can reduce hemolymph storage. Therefore, the reduction in intake needed to extend lifespan is not sufficient to reduce protein storage in the hemolymph. These results do not support the hypothesis that protein storage is involved in life extension from CR.

Larval nutrition affects lipid storage and growth, but not protein or carbohydrate storage in newly eclosed adults of the grasshopper Schistocerca americana

Nitrogen availability from dietary protein can have profound effects on the physiology and evolutionary ecology of insect herbivores. While many studies consider the effects of nutrition on consumption and gross body composition of protein and other important nutrients, few consider partitioning to storage for future use. I used chemically defined artificial diets to quantitatively manipulate the amount of dietary carbohydrates and proteins available to growing larvae of the grasshopper Schistocerca americana to determine how larval nutrient availability affects growth and all three classes of stored nutrients (proteins, lipids, and carbohydrates) carried over from larval feeding into adulthood. Larvae on poor diets increased consumption, but could not compensate for diet quality, eclosing small and containing no significant nutrient stores at adulthood. Individuals fed intermediate to high nutrient content diets as larvae were significantly larger and contained a significantly greater proportion of lipid stores at adult eclosion, but not protein or carbohydrate stores than individuals fed low nutrient content diets. This suggests that larvally derived lipid stores may be more important to adult fitness than carbohydrate or protein stores. This result is contrary to previous studies performed on the role of larval nutrition and allocation to protein stores, and this difference is likely due to variation in the relative availability of protein in adult diets across species.

Plasticity of grasshopper vitellogenin production in response to diet is primarily a result of changes in fat body mass

Life history plasticity is the developmental production of different phenotypes by similar genotypes in response to different environments. Plasticity is common in early post-embryonic or adult development. Later in the developmental stage, the transition from developmentally plastic to canalized (i.e., inflexible) phases is often associated with the attainment of a threshold level of storage. Thresholds are often described simply as total body mass or cumulative consumption of food. The physiological characteristics of thresholds, such as the contributions of the growth of particular organs or the production rate of proteins, are largely unstudied. To address the physiology underlying a threshold-induced developmental transition, total vitellogenin production in response to diet quality in the lubber grasshopper was studied. For individuals that differed in age or dietary protein, somatic mass, ovarian mass, fat body mass, mass-specific vitellogenin production, vitellogenin titer, and storage protein titer were measured. Age and diet strongly affected these parameters, with ovarian mass and fat body mass contributing most to the differences. During mid vitellogenesis, females were highly plastic in response to changing food quality. Only during late vitellogenesis were females unresponsive to changes in food quality. Fat body mass was a more important component of plasticity than was mass-specific vitellogenin production. Because these two variables together make up total vitellogenin production, the greater contribution of fat body mass than mass-specific vitellogenin production suggests that growth factors may be more important than tissue stimulators in producing developmental changes in total vitellogenin production. To our knowledge, this is the first study to demonstrate that mass gain of an organ is more important to developmental plasticity than is the output of that same organ.

Laying-sequence-specific variation in yolk oestrogen levels, and relationship to plasma oestrogen in female zebra finches (Taeniopygia guttata)

We investigated the relationship between plasma and yolk oestrogens in laying female zebra finches (Taeniopygia guttata) by manipulating plasma oestradiol (E2) levels, via injection of oestradiol-17beta, in a sequence-specific manner to maintain chronically high plasma levels for later-developing eggs (contrasting with the endogenous pattern of decreasing plasma E2 concentrations during laying). We report systematic variation in yolk oestrogen concentrations, in relation to laying sequence, similar to that widely reported for androgenic steroids. In sham-manipulated females, yolk E2 concentrations decreased with laying sequence. However, in E2-treated females plasma E2 levels were higher during the period of rapid yolk development of later-laid eggs, compared with control females. As a consequence, we reversed the laying-sequence-specific pattern of yolk E2: in E2-treated females, yolk E2 concentrations increased with laying-sequence. In general therefore, yolk E2 levels were a direct reflection of plasma E2 levels. However, in control females there was some inter-individual variability in the endogenous pattern of plasma E2 levels through the laying cycle which could generate variation in sequence-specific patterns of yolk hormone levels even if these primarily reflect circulating steroid levels.

Starvation affects vitellogenin production but not vitellogenin mRNA levels in the lubber grasshopper, Romalea microptera

The interaction of juvenile hormone (JH) and nutrition was studied during the oviposition cycle of the Eastern Lubber grasshopper (Romalea microptera). Starvation of females early or in the middle of the cycle inhibited oocyte growth. Starvation for 4 days also reduced hemolymph levels of JH III and vitellogenesis (Vg) to 25% and 15%, respectively, of the levels in fed animals. Likewise, Vg production by fat body fragments incubated in vitro was reduced to 2% of the levels in fed animals and total protein synthesis was reduced to 25%, suggesting that starvation had a stronger effect on Vg synthesis than on protein synthesis. These effects were reversed when starved animals were fed again. However, fat body levels of Vg-mRNA were similar in fed and starved animals, indicating that starvation did not affect transcript levels. We tested whether the decline in JH levels mediated the other starvation effects by infusing animals with JH III or vehicle for 2 days at the onset of starvation. Infusion of JH elevated JH and Vg-mRNA levels 670% and 103%, respectively, above the levels in vehicle-infused animals. However, Vg production and hemolymph levels of Vg were similar to the levels in vehicle-infused animals. These data suggest that JH alone is insufficient to stimulate Vg production.

Interpopulation variation in developmental titers of vitellogenin, but not storage proteins, in lubber grasshoppers

We examined simultaneous plastic and latitudinal interpopulation variation in the time course of hemolymph protein titers during egg production in the lubber grasshopper. Our goal was to gain insight into possible evolutionary changes in the physiology underlying reproductive plasticity. We used lubbers from three locations in the United States (Florida [FL], Louisiana [LA], and Georgia [GA]), each offered three daily food rations. Previous genetic analysis indicated that grasshoppers from FL (the low-latitude population) and GA (the high-latitude population) were phylogenetically closer to each other than to LA grasshoppers (the intermediate-latitude population). The ages at maximum titers of vitellogenin (Vg(max)) and three storage proteins that were referred to as major hemolymph proteins (MHP(max)) were used as indices of the progress of oocyte development. Age at Vg(max) was affected significantly both by diet and by population. Perhaps most importantly, age at Vg(max) was less for GA grasshoppers than for FL and LA grasshoppers; this pattern differs from the phylogenetic relationships of the populations. Age at MHP(max) was significantly affected only by diet and not by population. Hence, the regulation of these proteins may differ across populations. Finally, we found no evidence that plasticity of reproductive investment in response to food availability differs across populations (as indicated by nonsignificant interactions of population and feeding environment).

Presence of a single abundant storage hexamerin in both larvae and adults of the grasshopper, Schistocerca americana

We identified a single hexameric storage protein in the grasshopper, Schistocerca americana, and monitored its abundance through the last larval instar and up until reproductive competence in adults of both sexes. This storage hexamerin, termed Schistocerca americana Persistent Storage Protein (saPSP) was the most abundant soluble protein in both larvae and adults. In both sexes, saPSP abundance started out low at the onset of the last larval instar and accumulated during feeding, peaking just prior to molting. Adults of both sexes contained significant amounts of saPSP after eclosion. In adult males, saPSP content dropped continuously after eclosion and was lowest once individuals reached reproductive maturity. In contrast, adult females depleted saPSP reserves during the first days of adulthood, but subsequently accumulated significant saPSP stores. In adult females, saPSP stores peaked just prior to the completion of egg provisioning. Given the overall patterns of abundance, saPSP has functions in both larvae and adults. In addition, the observed pattern of storage hexamerin accumulation differs from patterns of accumulation in the other known grasshoppers, Locusta migratoria and Romalea microptera, suggesting that significant functional diversity has evolved in storage hexamerins among the grasshoppers.

Hemolymph ecdysteroids do not affect vitellogenesis in the lubber grasshopper

The role of hemolymph ecdysteroids in the reproduction of non-dipteran insects is unclear. We examine the role(s) of hemolymph ecdysteroids during egg production in the lubber grasshopper, Romalea microptera. In all individuals, hemolymph ecdysteroids rose to a sharp peak with similar maxima and then fell to undetectable levels. The time from the adult molt to the maximum ecdysteroid titer (E(max) titer) varied in response to food availability, whereas the time from E(max) titer to oviposition was unrelated to food availability. Because both the timing of egg production and the timing of E(max) responded similarly to environmental changes, ecdysteroids may be involved in egg production. We hypothesized that this role is the stimulation of vitellogenesis. Ovariectomized females had vitellogenin but no ecdysteroids, so ecdysteroids are not necessary for vitellogenin production. In addition, treatment of females with ecdysteroids altered neither Vg titers nor ovarian growth. Ovarian ecdysteriods increased at the same age in development as hemolymph ecdysteroids. In contrast to hemolymph ecdysteroids, ovarian ecdysteroids persisted until oviposition. Despite this, [(3)H]ecdysone injected into the hemolymph was detected later only at very low levels in the ovary, suggesting that hemolymph ecdysteroids are not sequestered by the ovary. In summary, our studies indicate that hemolymph ecdysteroids in adult females of the lubber grasshopper are associated with the timing of egg production, but they neither regulate vitellogenesis nor act as a source of ecdysteroids for the ovary.