Seasonal changes in juvenile hormone titers and rates of biosynthesis in honey bees

Honey bee colonies can respond to changing environmental conditions by showing plasticity in age related division of labor, and these responses are associated with changes in juvenile hormone. The shift from nest taks to foraging has been especially well characterized; foraging is associated with high juvenile hormone titers and high rates of juvenile hormone biosynthesis, and can be induced prematurely in young bees by juvenile hormone treatment or by a shortage of foragers. However, very few studies have been conducted that study plasticity in division of labor under naturally occurring changes in the environment. To gain further insight into how the environment and juvenile hormone influence foraging behavior, we measured juvenile hormone titers and rates of biosynthesis in workers during times of the year when colony activity in temperate climates is reduced: late fall, winter, and early spring. Juvenile hormone titers and rates of biosynthesis decreased in foragers in the fall as foraging diminished and bees became less active. This demonstration of a natural drop in juvenile hormone confirms and extends previous findings when bees were experimentally induced to revert from foraging to within-hive tasks. In addition, endocrine changes in foragers in the fall are part of a larger seasonally related phenomenon in which juvenile hormone levels in younger, pre-foraging bees also decline in the fall and then increase the following spring as colony activity increases. The seasonal decline in juvenile hormone in foragers was mimicked in summer by placing a honey bee colony in a cold room for 8 days. This suggests that seasonal changes in juvenile hormone are not related to photoperiod changes, but rather to changes in temperature and/or colony social structure that in turn influence endocrine and behavioral development. We also found that active foragers in the late winter and early spring had lower juvenile hormone levels than active foragers in late spring. In light of recent findings of a possible link between juvenile hormone and neuroanatomical plasticity in the bee brain, these results suggest that bees can forage with low juvenile hormone, after previous exposure to some threshold level of juvenile hormone leads to changes in brain structure.     

Effects of social environment and worker mandibular glands on endocrine-mediated behavioral development in honey bees

Previous studies suggest that older honey bee workers possess an inhibitory signal that regulates behavioral development in younger bees. To study how this inhibitor is transmitted, bees were reared for 7 days in double-screen cages, single-screen cages, or unrestricted in a typical colony (control bees). Double-screen cages prevented physical contact with colony members while single-screen cages allowed only antennation and food exchange. Bees reared in double-screen cages showed accelerated endocrine and behavioral development; they had significantly higher rates of juvenile hormone biosynthesis and juvenile hormone titers than did control bees and also were more likely to become precocious foragers. Relative to the other two groups, bees reared in single-screen cages showed intermediate juvenile hormone biosynthesis rates and titers, and intermediate rates of behavioral development. These results indicate that physical contact is required for total inhibition. We also began to test the hypothesis that worker mandibular glands are the sources of an inhibitory signal. Old bees with mandibular glands removed were significantly less inhibitory towards young bees than were sham-operated and unoperated bees. These results suggest that an inhibitor is produced by the worker mandibular glands. Accepted: 29 January 1998     

Sex-specific developmental profiles of juvenile hormone synthesis in honey bee larvae

Summary Juvenile hormone synthesis in drone larvae of the honey bee was measured by an in vitro radiochemical assay. The developmental profile of corpora allata activity in male larvae showed considerable differences from queen larvae, the presumptive reproductive females, and was comparable to workers, the sterile female morph. Drone and worker larvae, however, differed drastically in the regulation of juvenile hormone biosynthesis, as revealed by the addition of farnesoic acid to the culture medium. This precursor stimulated juvenile hormone synthesis of drone glands nearly eightfold, whereas in worker larvae it is known to lead to an accumulation of methyl farnesoate. The sex-specific differences in endocrine activity indicate a role for juvenile hormone in the expression of genetically determined sexually dimorphic characters during metamorphosis, a role not currently accounted for in models describing endocrine regulation of insect development. Correspondence to: K. Hartfelder     

Ultrastructure of corpora allata of known activity during the vitellogenic cycle in the cockroach Diploptera punctata

Summary Ultrastructure was correlated with rates of juvenile hormone synthesis in corpora allata from females of the viviparous cockroach Diploptera punctata at seven daily intervals during the first vitellogenic cycle. Synthetic activity of the glands was determined by in vitro radiochemical assay before the glands were fixed for electron microscopic analysis. The cycle in rates of juvenile hormone synthesis progressed from about 20 pmol h^-1 per gland pair (oocytes 0.60 mm long) to a maximum mean rate of 140 pmol h^-1 per pair (oocytes 1.40–1.47 mm long) and declined to about 20 pmol h^-1 per pair at ovulation (oocytes about 1.65 mm long). Conspicuous ultrastructural changes occurred with changing synthetic rates. In glands with increasing rates of synthesis, mitochondria showed less electron-dense matrix, greater diameter and more irregular shape. Smooth endoplasmic reticulum changed from easily seen to obscure tubules, networks, and vesicles. Rough endoplasmic reticulum appeared in longer, more curved segments. Newly formed autophagic vacuoles appeared in all glands of highest activity rates. In glands with decreasing rates of synthesis, the mitochondrial matrix became denser, width smaller, and shapes less irregular. Smooth endoplasmic reticulum again appeared tubular and distinct. Golgi complexes were more conspicuous. Rough endoplasmic reticulum in whorls and large numbers of autophagic vacuoles continued to be present.This work was supported by USPH Grant AI 15230. We thank Kuen-Kuen Chan for skillful and thoughtful technical assistance     

Ultrastructural dynamics of the corpus allatum of Choleva angustata Fab. (Coleoptera,Catopidae)

Summary The ultrastructure of the corpora allata (CA) during postembryonic stages of Choleva angustata Fab. shows cyclic changes, in particular regarding the endoplasmic reticulum and the mitochondria. During the last larval instar, at the short transitory period which follows the cessation of alimentation, we observed a highly unusual appearance of the CA, characterized by intensive vacuolization. After this transitory period, the smooth endoplasmic reticulum (ser) disappears until the imago emerges. These observations along with our experimental data permit a physiological interpretation.We thank Departments of Electron Microscopy and Photography of the Institute of Neurophysiology and Psychophysiology, C.N.R.S., Marseille, France     

A stage-specific ovarian factor with stable stimulation of juvenile hormone synthesis in corpora allata of the cockroach Diploptera punctata

This is a study of a feedback loop from a stimulated organ to glands that produce the stimulatory hormone in the cockroach Diploptera punctata. In this insect as in many others, juvenile hormone (JH) produced by corpora allata (CA) stimulates vitellogenesis. In our previous studies, transplantations of ovaries at certain stages of development into ovariectomized mated females stimulated JH synthesis within 24h. An in vitro study by other investigators showed that all stages of ovaries release a stimulatory factor into culture medium that was not retained on a solid-phase extraction column but occurred in the aqueous flow-through. The present study is a comparison of the effect of medium conditioned with ovaries from days 1-4 and 8 of the first reproductive cycle, to the effect of the flow-through of that medium on members of a pair of CA from day 3 females. Results provide evidence for an ovarian factor that stimulates JH synthesis by CA in vitro after removal from the conditioning medium (i.e., stable stimulation). Only medium conditioned with ovaries from days 2 or 3 females significantly stimulated CA more than flow-through. Stimulation was dose dependent, sensitive to trypsin, and survived freezing. These results indicate that CA can be directly and stably stimulated by a stage-specific peptidergic ovarian factor.     

Environmental Cues, Endocrine Factors, and Reproductive Diapause in Male Insects

Environmental cues, mostly photoperiod and temperature, mediated by effects on the neuroendocrine system, control reproductive diapause in female insects. Arrest of oocyte development characterizes female reproductive diapause, which has two major adaptive functions: It improves chances of survival during unfavorable season(s), and/or it confines oviposition to that period of the year that is optimal for survival of the eggs and progeny. Although reproductive diapause is less well studied in male insects, there may be no sex-dependent differences in regard to the first of these functions. The second one, however, is not valid for the male; instead, selection pressure directs the male's reproductive strategy toward maximum chances of fertilization of the female's eggs with minimum waste of energy. Therefore, in species with female reproductive diapause, the males may or may not exhibit diapause, but if they do, their diapause must be adapted to that existing in conspecific females. Male reproductive diapause is defined as a reversible state of inability of the male to inseminate receptive females. In relation to reproductive diapause, there are several patterns of coadaptations between male reproductive strategy and timing of female receptivity, (a) In some insects, the females are receptive in the early part of their diapause; mating occurs during this period and there is no diapause in the male. The male dies shortly after copulation and the female stores the sperms to fertilize the eggs that develop after termination of the female's diapause, (b) In some species, as in the grasshopper Anacridium aegyptium, females are receptive during diapause; though oocyte development is arrested, copulation occurs and the stored sperms fertilize the eggs when the female's diapause ends. Males were claimed to have no diapause, but recent studies have revealed the presence of a reproductive diapause in a proportion of the males. This and other cases show that female receptivity during reproductive diapause may or may not be accompanied by male reproductive diapause. If there is a reproductive diapause in the male, it is controlled by the same endocrine mechanism, the corpora allata (CA), as in the females, (c) In many species females are refractory during their diapause. In these cases, males exhibit reproductive diapause, which may be light, as in the beetle Oulema melanopus, or well established, as in certain grasshoppers, butterflies, and beetles. In the latter cases, male diapause is controlled by similar environmental cues (photoperiod, temperature) and by the same intrinsic mechanism (neuroendocrine system, especially CA) as female diapause. Nevertheless, male diapause is less intense; the environmental cues leading to its termination are less complex and/or less extreme, so male diapause terminates before that of the females. Presumably, male diapause is under two antagonistic selection pressures: A male should not waste energy by courting dia-pausing refractory females, but he should be ready to copulate as soon as the females become receptive, otherwise he may lose in the competition between males for females. Some further strategies, which do not seem to fit the above patterns, are also outlined.     

Environmental Cues,Endocrine Factors,and Reproductive Diapause in Male Insects

Environmental cues, mostly photoperiod and temperature, mediated by effects on the neuroendocrine system, control reproductive diapause in female insects. Arrest of oocyte development characterizes female reproductive diapause, which has two major adaptive functions: It improves chances of survival during unfavorable season(s), and/or it confines oviposition to that period of the year that is optimal for survival of the eggs and progeny. Although reproductive diapause is less well studied in male insects, there may be no sex-dependent differences in regard to the first of these functions. The second one, however, is not valid for the male; instead, selection pressure directs the male's reproductive strategy toward maximum chances of fertilization of the female's eggs with minimum waste of energy. Therefore, in species with female reproductive diapause, the males may or may not exhibit diapause, but if they do, their diapause must be adapted to that existing in conspecific females. Male reproductive diapause is defined as a reversible state of inability of the male to inseminate receptive females. In relation to reproductive diapause, there are several patterns of coadaptations between male reproductive strategy and timing of female receptivity, (a) In some insects, the females are receptive in the early part of their diapause; mating occurs during this period and there is no diapause in the male. The male dies shortly after copulation and the female stores the sperms to fertilize the eggs that develop after termination of the female's diapause, (b) In some species, as in the grasshopper Anacridium aegyptium, females are receptive during diapause; though oocyte development is arrested, copulation occurs and the stored sperms fertilize the eggs when the female's diapause ends. Males were claimed to have no diapause, but recent studies have revealed the presence of a reproductive diapause in a proportion of the males. This and other cases show that female receptivity during reproductive diapause may or may not be accompanied by male reproductive diapause. If there is a reproductive diapause in the male, it is controlled by the same endocrine mechanism, the corpora allata (CA), as in the females, (c) In many species females are refractory during their diapause. In these cases, males exhibit reproductive diapause, which may be light, as in the beetle Oulema melanopus, or well established, as in certain grasshoppers, butterflies, and beetles. In the latter cases, male diapause is controlled by similar environmental cues (photoperiod, temperature) and by the same intrinsic mechanism (neuroendocrine system, especially CA) as female diapause. Nevertheless, male diapause is less intense; the environmental cues leading to its termination are less complex and/or less extreme, so male diapause terminates before that of the females. Presumably, male diapause is under two antagonistic selection pressures: A male should not waste energy by courting dia-pausing refractory females, but he should be ready to copulate as soon as the females become receptive, otherwise he may lose in the competition between males for females. Some further strategies, which do not seem to fit the above patterns, are also outlined.     

Incomplete division of labor: error-prone multitaskers coexist with specialists

Division of labor (DoL) occurs when individual members of a group specialize by performing particular tasks toward some common goal. Under complete DoL, every individual acts as a specialist and so performs only one kind of task. But under incomplete DoL, some individuals may act as generalists and so have the capacity to perform more than one kind of task. This persistence of generalists in the presence of specialists presents a theoretical challenge, particularly if generalists must pay an extra cost, an inefficiency penalty, for their capacity to perform more than one type of task. Prior work focused on how such costs tend to drive evolution toward complete DoL, with only specialists typically remaining at equilibrium [Wahl, L.M., 2002a. Evolving the division of labor: generalists, specialists and task allocation. J. Theoret. Biol. 219, 371-388; Wahl, L.M., 2002b. The division of labor: genotypic versus phenotypic specialization. Am. Nat. 160, 135-145]. Relaxing this key assumption, we show that generalists, despite paying some extra costs, can coexist with specialists. Relaxing another assumption, we also show that this coexistence can hold even when generalists often perform the wrong task. How can stable multitasking emerge despite this flawed decision-making? From the perspective that cognitive errors matter only when they translate into fitness decrements, we observe that error-prone generalists may persist most commonly in situations in which their mistakes do little to jeopardize group success. Our findings show that incomplete DoL can emerge even when generalists often err and must pay extra costs for their multitasking capacity.     

Dufour’s gland secretion,sterility and foraging behavior: Correlated behavior traits in bumblebee workers

Bombus terrestris colonies go through two major phases: the “pre-competition phase” in which the queen is the sole reproducer and aggression is rare, and the “competition phase” in which workers aggressively compete over reproduction. Conflicts over reproduction are partially regulated by a group of octyl esters that are produced in Dufour’s gland of reproductively subordinate workers and protect them from being aggressed. However, workers possess octyl esters even before overt aggression occurs, raising the question of why produce the ester-signal before it is functionally necessary?In most insect societies, foragers show reduced aggression and low dominance rank. We hypothesize that ester production in B. terrestris is not only correlated with sterility but also with foraging, signaling cooperative behavior by subordinate workers. Such a signal helps to maintain social organization, reduce the cost of fights between reproductives and helpers, and increase colony productivity, enabling subordinates to gain greater inclusive fitness. We demonstrate that foragers produce larger amounts of esters compared to non-foragers, and that their amounts positively correlate with foraging efforts. We further suggest that task performance, potential fecundity, and aggression are interlinked, and that worker–worker interactions are involved in regulating foraging behavior.B. terrestris, being an intermediate phase between primitive and derived eusocial insects, provides an excellent model for understanding the evolution of early phases of eusociality. Our results, combined with those in primitively eusocial wasps, suggest that at early stages of social evolution, reproduction was regulated by a “primordial division of labor”, that comprised foragers and reproducers, which further evolved to a more complex division of labor, a hallmark of eusociality.     

Influence of temperature and carbon dioxide concentration on juvenile hormone titre and dependent parameters of adult worker honey bees (Apis mellifera L.)

It is known that juvenile hormone plays an important role in the regulation of labour division and of the different life spans, and that the microclimate of the bee hive is characterized by its high CO₂ concentration and its varying temperature depending on the presence of brood.We have investigated the influence of microclimates characteristic of breeding and broodless areas on the juvenile hormone titre in the haemolymph and whole body extracts, on the corpora allata in vitro activity, on the degradation of juvenile hormone and on the dry weight of the hypopharyngeal glands using bees of known ages. A microclimate of 35°C and 1.5% CO₂, as observed in the breeding area, induces a rapid and pronounced increase in the juvenile hormone titre. On the other hand, this titre remains low in bees kept at 27°C and 1.5% CO₂, a microclimate associated with broodless combs. Rates of juvenile hormone synthesis by corpora allata in vitro were found to be extremely low, even in the presence of farnesenic acid, and not related to the juvenile hormone titre. In vitro incubation of juvenile hormone in haemolymph revealed no degradation while injected juvenile hormone was found to be degraded and taken up by the gut at rates only weakly correlated with the juvenile hormone titre.We propose a hypothetical model for the regulation of the juvenile hormone titre as well as the course of labour division by the varying microclimates observed in the bee hive.     

Short neuropeptide F (sNPF) is a stage-specific suppressor for juvenile hormone biosynthesis by corpora allata,and a critical factor for the initiation of insect metamorphosis

Molting and metamorphosis are essential events for arthropod development, and juvenile hormone (JH) and its precursors play critical roles for these events. We examined the regulation of JH biosynthesis by the corpora allata (CA) in Bombyx mori, and found that intact brain-corpora cardiaca (CC)–CA complexes produced a smaller amount of JH than that in CC–CA complexes and CA alone throughout the 4th and 5th (last) instar stadium. The smaller amount of synthesis was due to allatostatin-C (AST-C) produced by the brain. The CC synthesized short neuropeptide F (sNPF) that also suppressed the JH synthesis, but only in day 3 4th stadium and after the last larval ecdysis. For the suppression, both peptides prevented the expression of some of the distinct JH biosynthetic enzymes in the mevalonate pathway. Allatotropin (AT) stimulated sNPF expression in the CC of day 1 5th instar stadium, not of day 3 4th; therefore the stage-specific inhibition of JH synthesis by sNPF was partly due to the stimulative action of AT on the sNPF expression besides the stage-specific expression of the sNPF receptors in the CA, the level of which was high in day 2 4th and day 0 5th instar larvae. The cessation of JH biosynthesis in the last instar larvae is a key event to initiate pupal metamorphosis, and both sNPF and AST-C are key factors in shutting down JH synthesis, along with the decline of ecdysone titer and dopamine.     

Ultrastructural changes induced by precocene II and 3,4-dihydroprecocene II in the corpora allata of Blattella germanica

Summary Ultrastructural studies on corpora allata (CA) from different stages during the first gonadotropic cycle of the cockroach Blattella germanica have shown well defined changes which have a correspondence with oocyte length, CA volume and juvenile hormone (JH) biosynthesis. The most significant variations concern the mitochondria and the endoplasmic reticulum. Topically applied precocene II (P II) at a dose of 200 g induced a transient arrest of CA function, although cytotoxic effects were occasionally observed. When CA were maintained in vitro with 10^-3 M of P II, a relationship between the time of treatment (3, 6 or 9 h) and the intensity of the effects was apparent. The 9-h treatment led to an irreversible inhibition of JH production which parallels the severe damages observed in the CA (membrane lysis, nuclear pyknosis, vacuolization). Equivalent studies performed with the chroman derivative 3,4-dihydroprecocene II (DHP II) showed that it is less active than P II. Only treatments as severe as 12 h of incubation with a 10^-3 M concentration elicited cytotoxic effects which could be due to radical species involved in the in situ oxidative bioactivation of DHP II. Thus, this compound could be regarded as a new type of pro-allatocidin.     

Primer effect of queen pheromone on juvenile hormone biosynthesis in adult worker honey bees

Juvenile hormone synthesis in adult worker honey bees was measured by an in vitro corpora allata bioassay. Adult queenless workers exhibit higher rates of juvenile hormone biosynthesis than queenright workers. Hormone synthesis is not correlated with the volume of the glands. Extract of queen mandibular glands, applied to a dummy, reduces juvenile hormone biosynthesis in caged queenless workers to the level of queenright workers. The same result was obtained with synthetic (E)-9-oxo-2-decenoic acid, the principal component of the queen mandibular gland secretion. This pheromonal primer effect may function as a key regulating element in maintaining eusocial colony homeostasis. The presence of brood does not affect the hormone production of the corpora allata.Abbreviations BSA bovine serum albumin - CA Corpora allata - JH juvenile hormone - 9-ODA (E)-9-oxo-2-decnoic acid     

Larval juvenile hormone treatment affects pre-adult development,but not adult age at onset of foraging in worker honey bees (Apis mellifera)

Previous research has shown that juvenile hormone (JH) titers increase as adult worker honey bees age and treatments with JH, JH analogs and JH mimics induce precocious foraging. Larvae from genotypes exhibiting faster adult behavioral development had significantly higher levels of juvenile hormone during the 2nd and 3rd larval instar. It is known that highly increased JH during this period causes the totipotent female larvae to differentiate into a queen. We treated third instar larvae with JH to test the hypothesis that this time period may be a developmental critical period for organizational effects of JH on brain and behavior also in the worker caste, such that JH treatment at a lower level than required to produce queens will speed adult behavioral development in workers. Larval JH treatment did not influence adult worker behavioral development. However, it made pre-adult development more queen-like in two ways: treated larvae were capped sooner by adult bees, and emerged from pupation earlier. These results suggest that some aspects of honey bee behavioral development may be relatively insensitive to pre-adult perturbation. These results also suggest JH titer may be connected to cues perceived by the adult bees indicating larval readiness for pupation resulting in adult bee cell capping behavior.     

Changes in weight of the pharyngeal gland and haemolymph titres of juvenile hormone,protein and vitellogenin in worker honey bees

Four physiological parameters (haemolymph-juvenile hormone titre, protein concentration, vitellogenin concentration, and pharyngeal gland dry weight) were examined in the following categories of queenright adult worker bees: summer bees 1–40 days old, winter bees 80–130 days old, 12–100-day old bees at the beginning of winter, 100–195-day old bees at the end of winter, and 1–100-day old bees experimentally induced to live longer in summer.In contrast to the continuously increasing titre of juvenile hormone in ageing summer bees, winter bees kept a constant low level. In bees at the beginning of winter, the hormone titre never reached high values. However, at the end of winter it rose from a low to a high level, comparable with the high titre of 24–40-day old summer bees. In experimentally induced longlived bees in summer, the juvenile hormone titre did not increase as in normal summer bees but remained low as in bees at the beginning of winter. Among the known natural juvenile hormones, only juvenile hormone III was present in the haemolymph of winter bees.The results support the hypothesis of polyphenism being regulated by the titre of juvenile hormone in the haemolymph.     

Identification and action of juvenile hormone III from sexually mature alate females of the red imported fire ant,Solenopsis invicta

Analysis of extracts of hemolymph obtained from sexually mature alate females of Solenopsis invicta from monogyne colonies resulted in identification of juvenile hormone III (JH III). The average amount of JH III was 0.32±0.04 pmol/μmolof hemolymph. Topical application of 0.038 pmol of JH III was sufficient to stimulate alates to shed their wings in the presence of the queen. The time in which alates were induced to dealate decreased linearly with increasing concentrations of JH III from 0.038 to 3.8 pmol. However, higher JH III concentrations deviated from linearity and did not reach dealation times comparable with those that occur after mating flights. Thus, it appears that the mechanism of dealation that occurs when female alates are out of the influence of their queen is different from the one associated with mating flights. Application of 0.42 μmol of precocene II inhibited dealation of alates in queenless colonies. However, this inhibition was reversed after applying 38 pmol JH III to precocene-treated alates. The sizes of corpora allata (CA) from sexuals treated with JH III did not differ from those of controls. However, the sizes of CA were reduced in alates treated with precocene II. The results indicated that JH was important to dealation.