Klaus Hartfelder

2 Social Insect Polymorphism: Hormonal Regulation of Plasticity in Development and Reproduction in the Honeybee

Publisher Summary This chapter discusses the hormonal regulation of plasticity in development and reproduction in the honeybee. The insight that hormones may be the cornerstones for understanding the developmental regulation, not only in preimaginal caste differentiation but also in adult life, has turned honeybee into a model for studies on caste in social insects, the reasons being its relatively large size and its ready availability together with a wealth of knowledge concerning honeybee biology and management. More recent views on caste differentiation seek to understand the developmental regulation in honeybee in the context of metamorphosis, particularly focusing on the role of the morphogenetic hormones, juvenile hormone, and ecdysone. These hormones exert additional and apparently novel functions in the adult life of honeybee. Reproduction and division of labor are also discussed in the chapter in the context of endocrine regulation. Long-term integration of the developmental processes and reproduction cycles with environmental conditions poses a key problem for any organism. In general, the required interaction of external stimuli with the genotype of an organism is mediated and orchestrated by hormones. During critical periods of caste differentiation, research on the regulatory pathways in the endocrine system of the honeybee larvae has shown that the corpora allata exhibit a caste-specific program of activity during the last larval instar and that this program can set the activity pattern of the prothoracic gland.

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.

Juvenile hormone effect on DNA synthesis and apoptosis in caste-specific differentiation of the larval honey bee (Apis mellifera L.) ovary.

Caste-specific differentiation of the honey bee ovary commences in the last larval instar. In this process, formation of germ cell clusters by synchronous and incomplete mitoses occurs in the queen ovary, whereas in the worker ovary programmed cell death is the dominant feature. BrdU and TUNEL labeling were used to study dynamics of cell proliferation and apoptosis-dependent DNA degradation in ovaries of naturally developing queens and workers, as well as in juvenile hormone-treated worker larvae. Cell proliferation in ovaries of last-instar queen larvae generally exceeded that in workers, except for the late feeding phase. This inversion in cell proliferation patterns coincided with the onset of apoptosis in worker ovaries, as evidenced by TUNEL labeling. Juvenile hormone application to early-fifth-instar worker larvae had two noticeable effects. First, it diminished the number of S-phase nuclei in ovaries of late feeding-phase workers, bringing them to queen-like levels. Second, it prevented the induction of apoptotic DNA degradation. Caste-specific regulation of cell division in connection with programmed cell death can thus be attributed to the previously described differences in juvenile hormone titer in queen and worker larvae, adding a new facet to this hormones multiple functions.

Corpora allata activity, a prime regulating element for caste-specific juvenile hormone titre in honey bee larvae (Apis mellifera carnica)

Abstract Corpora allata activity of queen and worker larvae of the honey bee in late larval development was studied in vitro by a radiochemical assay. Prospective queens showed a high peak of corpora allata activity in the fourth and early fifth larval stadium. This peak coincides with a queen-specific maximum in juvenile hormone titre, demonstrating that modulation of juvenile hormone release is of prime importance in the regulation of the caste-specific juvenile hormone titre. In both castes, hormone release is strictly correlated with juvenile hormone synthesis. The conversion of the precursor methyl farnesoate to juvenile hormone may be regulated caste-specifically, since only in queens but not in workers a linear correlation between intraglandular contents of juvenile hormone and methyl farnesoate could be found.

Differentially expressed regulatory genes in honey bee caste development.

Abstract. In the honey bee, an eminently fertile queen with up to 200 ovarioles per ovary monopolizes colony level reproduction. In contrast, worker bees have only few ovarioles and are essentially sterile. This phenotype divergence is a result of caste-specifically modulated juvenile hormone and ecdysteroid titers in larval development. In this study we employed a differential-display reverse transcription (DDRT)-PCR protocol to detect ecdysteroid-regulated gene expression during a critical phase of caste development. We identified a Ftz-F1 homolog and a Cut-like transcript. Ftz-F1 could be a putative element of the metamorphic ecdysone response cascade of bees, whereas Cut-like proteins are described as transcription factors involved in maintaining cellular differentiation states. The downregulation of both factors can be interpreted as steps in the metamorphic degradation of ovarioles in worker-bee ovaries.

Not Only Single Mating in Stingless Bees

Abstract. Queens of the large, pantropical and fully eusocial taxon Meliponinae (stingless bees) are generally considered to be singly mated. We indirectly estimated queen mating frequency in two meliponids, Melipona beecheii and Scaptotrigona postica, by examining genotypes of workers at microsatellite DNA loci. Microsatellites were highly variable, providing suitable markers with which to assign patrilinial origin of workers within colonies headed by single queens. Queen mating frequency varied between 1 and 3 (M. beecheii) and 1 and 6 (S. postica), representing the first clear documentation of polyandry in the Meliponinae. Effective paternity frequency, me, was lower, although above 2 for S. postica. Stingless bees may provide suitable subjects for the testing of recent inclusive fitness arguments describing intracolony kin conflict in social Hymenoptera.

Juvenile-hormone-dependent interaction of actin and spectrin is crucial for polymorphic differentiation of the larval honey bee ovary.

Abstract. Programmed cell death in the worker ovary of Apis mellifera reduces the number of ovarioles during metamorphosis from 150–200 primordia to less than 10. In contrast, practically all ovarioles in the ovary of queens survive to the adult stage. The correct formation and persistence of polyfusomes has been suggested as a critical factor for ovariole survival. We have analyzed the developmental dynamics of F-actin and α-spectrin in fusomes of queen and worker larvae, and in juvenile-hormone-treated worker larvae. Small fusomes containing actin and spectrin can be detected in the ovaries of fourth instar larvae in both castes. After molting to the fifth instar, the actin-spectrin association persists in the enlarged fusomes of queen ovarioles. In workers, actin dissociates from the fusomal and cortical α-spectrin. Coinciding with the appearance of apoptosis markers, large agglomerates of actin are detectable in worker ovarioles. Treatment of fourth-instar worker larvae with juvenile hormone rescues ovarioles from apoptosis and maintains the actin-spectrin association. Juvenile-hormone-dependent actin-spectrin interaction is thus one of the earliest steps in the differentiation of a polymorphic ovary. Plasticity in ovariole numbers as a result of hormone-dependent fusome formation may be a more widespread phenomenon in insects, extending beyond caste polymorphism in highly eusocial Hymenoptera.

Germ cell cluster formation and cell death are alternatives in caste-specific differentiation of the larval honey bee ovary

Summary Caste-specific differentiation of the female honey bee gonad takes place in the fifth larval instar. In queen larvae most ovarioles exhibit almost simultaneous formation of numerous germ cell clusters within the first 20 h after the last larval molt. Ultrastructurally distinctive fusomal cytoplasm connects these cystocytes. Germ cell differentiation is accompanied by morphological changes in somatic components of the ovarioles, the follicle and the terminal filament cells. Subsequently, queen ovarioles elongate and differentiate basal stalks that coalesce in a basal calyx. A second round of mitotic activity was found to occur in the late prepupal and early pupal queen ovary. This round may elevate germ cell numbers composing each cluster to levels observed in follicles of adult honey bee queens. In contrast, germ cell cluster formation does not occur in most of the 120–160 ovarioles of the larval worker ovary, but instead many cells in such ovarioles show signs of impending degeneration, such as l...

Caste determination is a sequential process: effect of larval age at grafting on ovariole number, hind leg size and cephalic volatiles in the honey bee (Apis mellifera carnica)

SUMMARYArtificial queen rearing with worker larvae grafted at different developmental stages resulted in gradual effects on ovary size (number of ovarioles per ovary), as well as hind leg and wax gland structures in adults. A significant decrease in ovariole number was observed when third instar larvae were grafted. Basitarsus shape was affected when fourth instar larvae were grafted. Queen—worker intermediates developed when early-fifth instar worker larvae were transferred. As newly emerged adults, spectra of cephalic volatiles of queens and workers are still very similar, and do not yet exhibit the caste-specific elements of the mandibular glands. At one day after emergence, most of the dominant compounds in these spectra are represented at higher levels in workers.

The composition of larval food in stingless bees: Evaluating nutritional balance by Chemosystematic methods

SummaryNutritional balance of larval food supposedly plays an important role in honey bee caste formation. Whether this is similar in stingless bees was an open question. We analysed the major water-soluble constituents in the larval food of 7 species of meliponids. The data were used to evaluate the hypothesis that a balanced composition of larval food is shaped by selection. Chemosystematic trees were calculated and compared to a published phylogenetic tree based on morphological characters. A considerable degree of congruence between the chemosystematic trees for the general composition of larval food and the phylogenetic tree allowed us to discuss the composition of larval food in highly eusocial bees in relation to functional aspects of larval nutrition.ZusammenfassungDie ausgewogene Zusammensetzung des Larvenfutters wird allgemein als ein wichtiger Faktor in der Kastenbildung der Honigbiene angesehen. Ob dies für Stachellose Bienen in ähnlicher Weise gilt, war eine offene Frage. Wir analysierten die wasserlöslichen Hauptkomponenten im Larvenfutter von 7 Meliponinen-Arten. Die Ergebnisse wurden herangezogen, um die Hypothese zu überprüfen, daß eine ausgewogene Zusammensetzung des Larvenfutters der Selektion unterliegt. Dazu wurden chemosystematische Bäume berechnet und mit einem bereits publizierten, strukturell-morphologisch begründeten Stammbaum verglichen. Ein beträchtlicher Grad an Kongruenz zwischen den chemosystematischen Bäumen für die allgemeine Zusammensetzung des Larvenfutters und dem Stammbaum erlaubte es, die Zusammensetzung des Larvenfutters hoch-eusozialer Bienen in Bezug auf die funktionalen Aspekte der Larvenernährung zu diskutieren.