Segment‐specific modifications of a neuropeptide phenotype in embryonic neurons of the moth, Manduca sexta

We have studied differences in the development of segmentally homologous neurons to identify factors that may regulate a neuropeptide phenotype. Bilaterally paired homologs of the peripheral neuron L1 were identified in the thoracic and abdominal segments in embryos of the moth Manduca: each bipolar neuron arises at a stereotyped location and, at 40% of embryogenesis, projects its major process within the transverse nerve of its own segment. Shortly after the initiation of axonogenesis (∼41%), L1 homologs in all but the prothoracic segment (T1) were labelled specifically by an antiserum to the molluscan neuropeptide Phe‐Met‐Arg‐Phe‐NH₂ (authentic‐ FMRFamide). Levels of peptide‐immunoreactivity (IR) were comparable in all such segmental homologs up to the ∼60% stage of embryogenesis, whereupon two distinct levels of peptide IR were displayed: homologs in the three most rostral segments (T2, T3, and A1; [abdominal segment 1]) showed high levels and were called Type I L1 neurons; homologs in the more caudal segments (A2‐A8) typically showed low levels of IR and were called Type II L1 neurons. This segment‐specific difference represented mature differentiated states and was retained in postembryonic stages. Intracellular dye fills of embryonic L1 neurons revealed that the morphogenesis of the Type I and II L1 neuron homologs was similar until ∼48% of embryogenesis; thereafter it differed in two salient ways: (1) the cell bodies of Type II L1 neurons migrated ∼150 m̈m laterally from their point of origin, and (2) the distal processes of the Type II L1 neurons contacted the heart, whereas those of Type I L1 neurons did not. Ultrastructural studies of both mature and developing L1 homologs showed that the FMRFamide‐like antigen(s) localized specifically to secretory granules. Further, whereas the secretory granules in segmental homologs appeared similar initially (i.e., at ∼50% of development), following the establishment of segment‐specific differences, secretory granules found in mature Type I and II L1 neurons were cell type‐specific.