NB 1-2 DETAILS
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NB 1-2 delaminates as an S2 NB.
There is no lineage information from other insects for this NB.
NB 1-2 is intriguing for its seemingly dual anterior and posterior characteristics. It expresses both engrailed (en) (posterior marker) and mirror-lacZ (mrr-lacZ) (marking the anterior border of the segment, McNeill et al, 1997). In addition to these markers, NB 1-2 expresses Klumpfuss (Klu), (castor) cas, and (seven-up-lacZ) svp-LacZ by S5 (Yang, et al, 1997; Cui and Doe, 1992; Broadus et al, 1995). Despite being an intermediate column NB, intermediate neuroblast defective (ind) is not expressed in NB 1-2; by the time NB 1-2 delaminates, ind expression has been inactivated in all but NBs 6-2 and 7-2 (Weiss et al, 1998).
Bossing et al, 1996 describe the clone as consisting of 16-24 interneurons, including the TB neuron which has a neurite trajectory that is unique and separate from the rest of the clone. In addition, they found 1 or 2 cells of the clone to be en+.
A. Motoneurons:
The larval body wall muscle pattern in the thorax differs from the abdominal one by the presence of mouthpart-associated muscles and by alterations in the lateral and ventral muscles (Hooper, 1986). The muscles associated with mouthparts are numbered 31-36 (nomenclature by Crossley, 1978) and are found primarily in segments T1-T3, with muscle 31 present only in A1 (see Fig 1-2). Muscle 33 in T3 was found to express Semaphorin II (previously known as Fasc IV, Kolodkin et al, 1993; Matthes et al, 1995). Kolodkin et al, (1993) used DiI to back fill NMJs from muscle 7,6,13 and 12 in A1-A7 and 31 in A1, and showed that a single ipsilateral motoneuron known as DC1 innervated this muscle in A1 via SNb.
We observe that the 1-2 clone generates all of the DC motoneurons.
In T3, NB 1-2 forms a small single motoneuron at the posterior edge of the clone; this MN innervates muscle 31 in A1 via SNb and is therefore DC1. (This is not diagrammed in Fig 1-2, but is visible in panel C.) It extends through the CNS via the posterior tract before exiting to muscle 31. At stage 15 its cell body is just posterior to the previously identified TB neuron and is only 5.8 microns in diameter (n=1) (see Fig 1-2A); by late stage 16, the TB neuron has migrated medially, away from the other cells of the clone and towards the midline, whereas the DC1 neuron remains at the medial and posterior extent of the clone, but enlarges dramatically. It eventually becomes a large ovoid motoneuron, approximately 6 by 8.2 microns in size (n=2), and is dorsally positioned in the CNS.
In T2, the same NB generates a more robust cluster of MNs (containing at least 2 motoneurons and possibly as many as 4) that innervate at least muscle 33 and possibly all of the mouthpart-associated musculature in T3 (see Figure 1-2, B and diagram). These MNs are large (8 microns in diameter, n=2) and laterally located in the clone; they are in the bottom of the CNS, i.e., in the ventral 1/3 of the ganglion. They extend through the CNS via the anterior tract, but we were unable to clearly identify the nerve pathway used to exit the CNS. We believe these axons to enter SNb. They begin to extend axons by stage 15, but don’t appear to form mature muscle connections until early in stage 17 (data not shown).
We never generated a NB 1-2 clone in T1. NB 1-2 does not produce motoneurons in abdominal segments.
B. Interneurons:
At least three of the neurons derived from this clone are intersegmental, one of these being the Torsten Bossing (TB) neuron previously described (Bossing, et al, 1996). It is a large round cell, averaging 7.0 microns in diameter (n=17). It separates itself from the remaining cells of the cluster and moves medially at the end of stage 16. It extends an axon early (prior to stage 14), anteriorly in its ipsilateral longitudinal connective to the anterior commissure of its own segment; here it crosses the midline (at stage 14) and on reaching the contralateral longitudinal connective extends anteriorly again, to the next segment’s anterior commissure. At stage 17, we see a large dendrite extending posteriorly from the TB neuron; it is situated in the medial part of the ipsilateral longitudinal connective. The second and third intersegmental interneurons in the clone do not separate themselves from the remaining cells of the cluster as does the TB neuron, but remain at its anterior and medial edge. These too are large cells, averaging 6 microns in diameter (n=8). Their axons appear to make contact with the TB neuron and then repel from it, remaining in the ipsilateral longitudinal connective (see arrow, Fig 1-2, G) and extending almost as far anteriorly as does the TB neuron. A fourth intersegmental interneuron is observed posterior to A3 (data not shown).
In addition to these intersegmental interneurons, there are approximately 10-15 local interneurons derived from this clone; they fall into two distinct size classes. Approximately 5 cells/clone were on average 3.8 microns in diameter (n=14 clones); approximately 7 cells/clone were on average 5.1 microns in diameter (n=13 clones). They extend neurites in the posterior commissure of their own segments and in the anterior commissure of the next adjacent posterior segment. They bifurcate at the contralateral longitudinal connective and make complex arborizations in the neuropil.
References:
Bossing, T., Udolph, G., Doe, C. Q., and Technau, G. M. (1996). "The Embryonic CNS lineages of Drosophila melanogaster I. Neuroblast lineages derived from the ventral half of the neurectoderm." Dev Biol 179: 41-64.
Broadus, J., Skeath, J.B., Spana, E. P., Bossing, T., Technau, G.M., and Doe, C.Q. (1995). New neuroblast markers and the origin of the aCC/pCC neurons in the Drosophila central nervous system. Mech Dev 53: 393-402.
Crossley, A. C. (1978). "The morphology and development of the Drosophila muscular system." in The Genetics and Biology of Drosophila, vol 2b, (ed. M. Ashburner and T.R.F. Wright) pp 499-560, Academic Press, New York.
Cui, X., and Doe, C.Q. (1992). ming is expressed in neuroblast sublineages and regulates gene expression in the Drosophila central nervous system. Development 116(4): 943-52.
Hooper, J. (1986). "Homeotic gene function in the muscles of Drosophila larvae". EMBO J 5(9): 2321-2329.
Kolodkin, A. L., Matthes, D. J., and Goodman, C. S. (1993). "The Semaphorin genes encode a family of transmembrane and secreted growth cone guidance molecules." Cell 75: 1389-99.
Matthes, D. J., Sink, H., Kolodkin, A. L., and Goodman, C. S. (1995). "Semaphorin II can function as a selective inhibitor of specific synaptic arborization." Cell 81: 631-9.
McNeill, H., Yang, C.H., Brodsky, M., Ungos, J., and Simon, M.A. (1997). Mirror encodes a novel PBX-class ofhomeoprotein that functions in the definition of the dorsal-ventral border in the Drosophila eye. Genes Dev 11(8): 1073-82.
Yang, X., Bahri, S., Klein, T., and Chia, W. (1997). Klumpfuss, a putative Drosophila zinc finger transcription factor, acts to differentiate between the identities of two secondary precursor cells within one neuroblast lineage. Genes Dev 11(11):1396-1408.