GERM-CELL CLUSTER FORMATION AND CELL-DEATH ARE ALTERNATIVES IN CASTE-SPECIFIC DIFFERENTIATION OF THE LARVAL HONEY-BEE OVARY

Caste-specific differentiation of the female honey bee gonad takes pla ce in the fifth larval instar. In queen larvae most ovarioles exhibit almost simultaneous formation of numerous germ cell clusters within th e first 20 h after the last larval molt. Ultrastructurally distinctive fusomal cytoplasm connects these cystocytes. Germ cell differentiatio n 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 coalesc e in a basal calyx. A second round of mitotic activity was found to oc cur in the late prepupal and early pupal queen ovary. This round may e levate germ cell numbers composing each cluster to levels observed in follicles of adult honey bee queens. In contrast, germ cell cluster fo rmation 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 i mpending degeneration, such as large autophagic bodies. DNA extracted from worker ovaries did not reveal nucleosomal laddering, and ultrastr ucturally, chromatin in germ cell nuclei appeared intact. In the 4-7 s urviving ovarioles of the small worker ovary, germ cell clusters were found with ultrastructural characteristics identical to those in queen ovarioles. The temporal window during which divergence in development al pathways of the larval ovaries initiates shortly after the last lar val molt coincides with caste-specific differences in juvenile hormone titer which have long been considered critical to caste-specific morp hogenesis.