ABSTRACT
Octopod (Octo) is a mutation of the moth Manduca sexta, which results in the homeotic transformation of the ventral surface of the first (Al) and less often the second (A2) abdominal segments in the anterior direction. The extent of the transformation ranges from a slight deformation of the ventral cuticle, up to the formation of miniature thoracic legs on Al. The extent of the transformation is always less within A2 as compared to Al. A genetic analysis revealed that Octo is an autosomal mutation which shows incomplete dominance. The effect of this mutation on the central nervous system (CNS) was assessed by examining the distribution and fate of the postembryonic neuroblasts in the segmental ganglia of Octo larvae. In each of the thoracic ganglia of wildtype larvae, there is a set of 45–47 neuroblasts; a reduced but homologous array of 24 and 10 neuroblasts are found in Al and A2, respectively. Ganglion Al of Octo larvae had 1 to 6 supernumerary neuroblasts, and 20% of the A2 ganglia showed a single ectopic neuroblast. The supernumerary neuroblasts corresponded to identifiable neuroblasts normally found in more anterior ganglia. The Octo mutation also influenced the mitotic activity of stem cells normally present in Al. In this case, the neuroblasts generated a lineage of cells that were typical of a thoracic location rather than Al. These data demonstrate that homeotic mutations can influence the fate of identifiable pattern elements within the CNS of an insect.
