| One of the characteristic features of metamorphosis in many holometabolous insects is the coalescence of specific thoracic and abdominal ganglia that occurs during pupal-adult development. Although the close association of behaviorally-relevant circuits and postembryonically-derived neurons within these adult-specific compound ganglia are necessary for adult behavior (i.e., flight), little is known about the underlying cellular mechanisms that facilitate migration and fusion.; Here, I demonstrate that cell adhesion molecules play important roles in nerve cord reorganization in the moth Manduca sexta. Specifically, I examined the temporal and spatial expression pattern of M. sexta -specific isoforms of the homophilic cell adhesion molecule fasciclin II (MFasII). Using antibodies against the transmembrane isoform of MFasII (TM-MFasII, known to label neurons) and the glycosyl phosphatidylinositol-linked isoform of MFasII (GPI-MFasII, which can label glial cells), I present the first study of MFasII expression and its endocrine regulation during ganglionic migration and fusion. I show that TM-MFasII expression is differentially regulated, both temporally and spatially, on migrating vs. non-migrating ganglia, and is linked tightly to the same changes in the steroid hormone, 20-hydroxyecdysone (20E), which have been shown previously to orchestrate ganglionic migration and fusion (Amos et al., 1996).; Curiously, the few neurons that showed TM-MFasII immunostaining independent of the ecdysteroid titers also were immunoreactive to an antiserum against eclosion hormone (EH). It is unclear what role, if any, the EH-containing ventromedial (VM) neurons play during nerve cord reorganization; however, because the VM cells span the entire length of nerve cord, their chronic TM-MFasII (steroid-independent) expression is notable. In addition, I provide preliminary evidence that the small stellate glial cells within the neuropile-somata interface are recognized by the lectin wheat germ agglutinin (WGA), which binds to specific carbohydrate sequences. These 'sticky' moieties were concentrated on the leading edges of neuropile within migrating ganglia. WGA labeling was also closely associated with elongating axons that were TM-MFasII immunoreactive. The WGA-stained stellate glial cells may provide the motive force, during nerve cord reorganization, necessary to move ganglia in concert with the interstitial growth of the hundreds of elongating axons that express TM-MFasII at this time. |
