Sexual differentiation in the CNS of the moth, Manduca sexta. I. Sex and segment-specificity in production, differentiation, and survival of the imaginal midline neurons.

We analyzed the development of several sets of postembryonic sex-specific motoneurons in Manduca sexta which belong to a group of homologous lineages of neurons called the imaginal midline neurons (IMNs). Adult female oviduct motoneurons and male sperm duct motoneurons are IMNs that show similar anatomical features and differentiate during metamorphosis, despite appearing in different segments: A7 for oviduct neurons, A9 for sperm duct neurons. These cells are born at the same time and, initially, similar sets are found in A7 and A9 ganglia of larvae of both sexes. The dimorphic adult pattern is generated by sex-specific production and cell death. A7 IMNs differentiate in both sexes through early pupal stages, whereupon they disappear in the male and become the oviduct motoneurons in the female. A9 IMNs are overproduced in the male, and subsequent cell death reduces male cell number and eliminates the small complement of female cells; the surviving male cells develop into the sperm duct motoneurons. Similar IMN arrays are generated in nongenital ganglia, but show non-sex-specific fates. This suggests that both the sex of these cells and their segment of residence play major roles in their subsequent differentiation.