Colette 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.

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.

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.

Development of cricket mushroom bodies.

Mushroom bodies are recognized as a multimodal integrator for sensorial stimuli. The present study analyzes cricket mushroom body development from embryogenesis to adulthood. In the house cricket, Kenyon cells were born from a group of neuroblasts located at the apex of mushroom bodies. Our results demonstrate the sequential generation of Kenyon cells: The more external they are, the earlier they were produced. BrdU treatment on day 8 (57% stage) of embryonic life results, at the adult stage, in the labelling of the large Kenyon cells at the periphery of the mushroom body cortex. These cells have specific projections into the posterior calyx, the gamma lobe, and an enlargement at the inner part of the vertical lobe; they represent a part of mushroom bodies of strictly embryonic origin. The small Kenyon cells were formed from day 9 (65% stage) of the embryonic stage onward, and new interneurons are produced throughout the entire life of the insect. They send their projections into the anterior calyx and into the vertical and medial lobes. Mushroom body development of Acheta should be considered as a primitive template, and cross‐taxonomic comparisons of the mushroom body development underscore the precocious origin of the gamma lobe. As a result of continuous neurogenesis, cricket mushroom bodies undergo remodeling throughout life, laying the foundation for future studies of the functional role of this developmental plasticity. J. Comp. Neurol. 452:215–227, 2002.

The application and evaluation of a radioimmunoassay for measuring juvenile hormone titres in Colorado beetle haemolymph

Abstract This paper demonstrates that the radioimmunoassay for juvenile hormone developed by Strambi et al. (1981) ( Eur. J. Biochem. 118, 401–406) is suitable for the quantification of JH-titres in insect haemolymph. The results obtained with this assay agree well with those from physico-chemical analyses. With the aid of this RIA we have measured the JH-titre in the haemolymph of the Colorado potato beetle, Leptinotarsa decemlineata , under various experimental conditions. The high JH-titre in long-day and low titre in short-day animals is in agreement with earlier findings. The JH-titre was similar in males and females, and was unaffected by mating in females. After injection of 50 μg JH-III dissolved in 1 μl olive oil per animal extremely high titres were found for at least 6 days in short-day animals.