Contributions of embryonic and postembryonic neurogenesis to the development of the antennal lobe in the sphinx moth Manduca sexta.

Abstract

This thesis investigates the roles of embryonic and postembryonic neurogenesis in the transformation of the larval antennal center (LAC) into the adult antennal lobe (AL) in the brain of the sphinx moth Manduca sexta. By means of birthdating methods, postembryonic neurogenesis in the CNS was first observed in the brain of the mid-first larval instar, and then proceeded in an anterior-to-posterior pattern. Five stereotypic neuroblast nests (three lateral, two medial) were identified that appeared to be associated with the LAC. Cell death was observed shortly after onset of postembryonic neurogenesis from the mid-first larval instar until wandering stage W2. Fates of the five LAC-associated lineages were traced histologically with respect to the anterior, medial, and lateral cell groups (AC, MC, LC) of the AL. At pupal stage P3, the three lateral nests coalesce and give rise to the postembryonic component of the LC, which is clearly distinguishable by pupal stage P4. Nests D and E remain separate and give rise to the postembryonic components of the AC and MC, respectively. A pronounced sexual dimorphism in the number of neurons in the MC was observed as early as pupal stage P5. The increased number of neurons in lineage E of males is evident as early as larval stage V-O, suggesting that regulation of postembryonic lineage size may contribute to the dimorphism of adults. The cellular distributions of two neuropeptide phenotypes (allatotropin-like-immunoreactivity, ATIR; and FMRFa-like-immunoreactivity, FIR) were examined immunocytochemically in larval, pupal and adult olfactory regions. Both phenotypes were found in a small number of cells lateral to the LAC, but significantly greater immunostaining was observed in the LC of the AL. For both ATIR and FIR, double-label immunocytochemistry (using BrdU) demonstrated that a subpopulation of the cells observed in the adult are born postembryonically. The larval and adult positions of ATIR and FIR neurons, of an identified serotonin-immunoreactive neuron, and of the five neuroblast nests, were compared. In every case, the lateral position of cells with respect to the LAC neuropil, regardless of embryonic or postembryonic origin, predicts a positional fate in the LC of the AL.