Alain Strambi

Hormonal and Genetic Control of Behavioral Integration in Honey Bee Colonies

The ability of insect colonies to adjust the division of labor among workers in response to changing environmental and colony conditions, coupled with research showing genetic effects on the division of labor in honey bee colonies, led to an investigation of the role of genetics and the environment in the integration of worker behavior. Measurements of juvenile hormone(JH) titers and allozyme analyses of worker honey bees suggest that two processes are involved in colony-level regulation of division of labor: (i) plasticity in age-dependent behavior is a consequence of modulation of JH titers by extrinsic factors, and (ii) stimuli that can affect JH titers and age-dependent behavior do elicit variable responses among genetically distinct subpopulations of workers within a colony. These results provide a new perspective on the developmental plasticity of insect colonies and support the emerging view that colony genetic structure affects behavioral organization.

Caste and metamorphosis : hemolymph titers of juvenile hormone and ecdysteroids in last instar honeybee larvae

Juvenile hormone (JH) and ecdysteroid titers are critical factors for caste development and metamorphosis in the last larval instar of the honeybee, Apis mellifera. Two highly sensitive radioimmunoassays were used for the determination of these hormones in the hemolymph. For juvenile hormone, which is of prime importance for the control of caste development in honeybees, our data show a caste-specific peak in queen larvae of the early fifth instar. A second peak appears in prepupae of both castes which probably is responsible for the regulation of the pupal moult. A single peak of ecdysteroids was observed in prepupae of both castes. In queens, however, the titer increases distinctly earlier than in worker larvae. The ecdysteroid composition of this prepupal peak was determined by high-performance liquid chromatography separation followed by radioimmunoassay. Makisterone A proved to be the main ecdysteroid compound, but 20-hydroxyecdysone was also found in significant amounts.

Hormonal regulation of behavioural development in the honey bee is based on changes in the rate of juvenile hormone biosynthesis

Abstract In the adult worker honey bee (Apis mellifera L.), increases in the haemolymph titre of juvenile hormone underlie behavioural development, from nest duties to foraging. However, the physiological basis of juvenile hormone titre regulation was unknown. Using a radiochemical assay for juvenile hormone biosynthesis in vitro, we demonstrate that differences in juvenile hormone titres among bees performing different age-dependent tasks are a consequence of changes in rates of hormone synthesis by the corpora allata. Rates of juvenile hormone biosynthesis were low in newly emerged bees, 7–9-day-old nurse bees, and 14–15-day-old bees collected from the nest periphery, and high in foragers. Rates of biosynthesis were highly correlated with haemolymph titres of juvenile hormone measured in the same individuals. Corpora allata contained mostly methyl farnesoate, the immediate precursor of juvenile hormone, and released principally juvenile hormone III into the incubation medium, indicating no appreciable hormone storage. We also report similarities and differences in parameters of juvenile hormone biosynthesis between nurse bees and foragers that were found during the course of a detailed characterization of the radiochemical assay for adult worker honey bees. These results, coupled with the fact that it is possible to measure rates of juvenile hormone biosynthesis from individual bees, indicate that the radiochemical assay will be useful in further studies of hormonal regulation of bee behaviour.

Neurogenesis in adult insect mushroom bodies.

The occurrence of neurogenesis in mushroom bodies of adult insects belonging to several orthopteroid and coleopteran families is described. Using injections of 5‐bromo, T2′‐deoxy we showed that neuroblasts, which are progenitors of Kenyon cells during preimaginal instars continue to divide in adult Acheta domesticus. Their progeny constitute a central column in mushroom body cortices of 3‐week‐old females. Other Gryllidae, Gryllus bimaculatus and Gryllomorpha dalmatina, show the same pattern of neuroblast activity and migration of their progeny. Immunocytochemical staining of glial cells failed to reveal any immunoreactivity, either in proliferating regions or in the resulting cells.

Comparison of juvenile hormone and ecdysteroid haemolymph titres in adult worker and queen honey bees (Apis mellifera)

Abstract Measurements of both juvenile hormone and ecdysteroid haemolymph titres were made from the same individuals to explore the possibility that there is an interaction between these hormones in the regulation of adult honey bee behaviour and physiology. Queens, egg-laying workers, and workers engaged in brood care (nurses) had low titres of juvenile hormone whereas foragers had significantly higher titres, as in previous studies. In contrast, ecdysteroid titres were undetectably low in both nurses and foragers, higher in laying workers, and higher still in laying queens. Measurements of juvenile hormone titres are consistent with previous findings demonstrating that this hormone regulates worker age polyethism but does not play a typical role in reproduction. Comparison of juvenile hormone and edcysteroid titres suggests that ecdysteroids are not involved in the regulation of age polyethism but may play a role in the regulation of reproduction in honey bees.

Influence of insect hormones on the establishment of dominance hierarchies among foundresses of the paper wasp, Polistes gallicus

SummaryLaboratory studies on overwintered foundresses of Polistes gallicus show that differences in the endocrine activity are mainly responsible for achieving the dominance rank. Females that became dominant had either larger corpora allata or more developed ovaries than subordinate females. Body size did not contribute to dominance rank. Since a correlation exists between the length of terminal oocytes and the ecdysteroid titre in haemolymph as well as between the volume of corpora allata and the synthesis of juvenile hormone, dominant behaviour is thought to depend upon an elevated hormone titre in haemolymph. Injections of juvenile hormone (JHI) and 20-hydroxyecdysone, separately and simultaneously, significantly increased the probability that the treated female would be the dominant female of a test pair. After a hierarchy has been established, endocrine activity in subordinate foundresses is inhibited by the dominant foundress that then monopolizes reproduction.

The common properties of neurogenesis in the adult brain: from invertebrates to vertebrates

Until recently, it was believed that adult brains were unable to generate any new neurons. However, it is now commonly known that stem cells remain in the adult central nervous system and that adult vertebrates as well as adult invertebrates are currently adding new neurons in some specialized structures of their central nervous system. In vertebrates, the subventricular zone and the dentate gyrus of the hippocampus are the sites of neuronal precursor proliferation. In some insects, persistent neurogenesis occurs in the mushroom bodies, which are brain structures involved in learning and memory and considered as functional analogues of the hippocampus. In both vertebrates and invertebrates, secondary neurogenesis (including neuroblast proliferation and neuron differentiation) appears to be regulated by hormones, transmitters, growth factors and environmental cues. The functional implications of adult neurogenesis have not yet been clearly demonstrated and comparative study of the various model systems could contribute to better understand this phenomenon. Here, we review and discuss the common characteristics of adult neurogenesis in the various animal models studied so far.

Reproduction in worker honey bees is associated with low juvenile hormone titers and rates of biosynthesis

Three experiments were performed to determine the role of juvenile hormone (JH) in worker reproduction in queenless colonies of honey bees. In Experiment 1, egg-laying workers had low hemolymph titers of JH, as did bees engaged in brood care, while foragers had significantly higher titers. Experiment 2 confirmed these findings by demonstrating that laying workers have significantly lower rates of JH biosynthesis than foragers do. In Experiment 3, ovary development was inhibited slightly by application of the JH analog methoprene to 1-day-old bees, but was not affected by application to older bees, at least some already displaying egg-laying behavior. These results, which are consistent with earlier findings for queen honey bees, are contrary to a common model of insect reproduction, in which elevated JH titers trigger ovary development, which then leads to oviposition. Previous experiments have demonstrated that JH regulates nonreproductive behavior in workers that is associated with colony division of labor; perhaps this function is incompatible with a traditional role for JH in reproduction.

Role of ovaries and ecdysteroids in dominance hierarchy establishment among foundresses of the primitively social wasp, Polistes gallicus

SummaryThe contribution of ovaries and ovarian ecdysteroids to dominance and nest initiation was tested in foundresses of Polistes gallicus at the end of hibernation. Removal of the ovaries resulted in a significant decrease of the ecdysteroid titre in haemolymph. Dominance in ovariectomized foundresses was found to be closely correlated to the size of corpora allata. The operated wasps started nest building, but they did not show any egg-laying behaviour. We conclude that both juvenile hormone and ovarian ecdysteroids contribute to degree of dominance, and that factors other than ovary development may stimulate building.

Juvenile hormone and ecdysteroid titres during critical periods of wing morph determination in Gryllus rubens

Abstract Juvenile hormone and ecdysteroid titre developmental profiles were compared between Gryllus rubens that were genetically-determined to develop into long-winged vs short-winged adults. Hormone titres were measured during the penultimate and last stadia, two periods during which wing length development is sensitive to exogenous juvenile hormone. No large differences in juvenile hormone titres were observed between nascent wing morphs during any stage in development. Slightly elevated titres were observed in long-wing vs short-wing-destined crickets during the middle third of the penultimate stadium. However, titre differences were opposite those inferred from earlier experiments involving topical application of juvenoids. An earlier decline in the juvenile hormone titre was also suggested in presumptive long-winged crickets during the last stadium. The earlier decline in macropters is consistent with previous juvenile hormone topical application experiments as well as with morph specific differences in juvenile hormone esterase activity. Peak ecdysteroid titres were higher in presumptive macropterous vs brachypterous males, while an elevated ecdysteroid titre occurred for a longer duration in macropterous females during the penultimate stadium. In both sexes, ecdysteroid titres were higher in presumptive long-winged vs short-winged morphs throughout the last stadium; peak titres were significantly higher in macropterous vs brachypterous females but not males. Both the longer duration of elevated ecdysteroid titres and the higher hormone titres in presumptive long-winged crickets are consistent with a regulatory role of ecdysteroids in wing morph determination.