Development and morphology of class II Kenyon cells in the mushroom bodies of the honey bee, Apis mellifera.

Class II Kenyon cells, defined by their early birthdate and unique dendritic arborizations, have been observed in the mushroom bodies of evolutionarily divergent insects. In the fruit fly Drosophila melanogaster, Class II (also called clawed) Kenyon cells are well known for their extensive reorganization that occurs during metamorphosis. The present account reports for the first time the occurrence of mushroom body reorganization during metamorphosis in holometabolous insect species outside of the Diptera. In the honey bee, Apis mellifera, Class II Kenyon cells show signs of degeneration and undergo a subtle reshaping of their axons during metamorphosis. Unlike in Drosophila, reorganization of Class II Kenyon cells in the honey bee does not involve the loss of axon branches. In contrast, the mushroom bodies of closely related hymenopteran species, the polistine wasps, undergo a much more dramatic restructuring near the end of metamorphosis. Immunohistochemistry, dextran fills, and Golgi impregnations illuminate the heterogeneous nature of Class II Kenyon cells in the developing and adult honey bee brain, with subpopulations differing in the location of dendritic arbors within the calyx, and branching pattern in the lobes. Furthermore, polyclonal antibodies against the catalytic subunit of Drosophila protein kinase A (anti-DC0) label an unusual and previously undescribed trajectory for these neurons. The observed variations in morphology indicate that subpopulations of Class II Kenyon cells in the honey bee can likely be further defined by significant differences in their specific connections and functions within the mushroom bodies.