The C. elegans even-skipped homologue,
vab-7, is essential for posterior epidermal and muscle patterning. As well as having posterior defects,
vab-7 mutants have uncoordinated forward movement. When moving backwards
vab-7 mutants can flex both dorsally and ventrally. However, in response to tail touch they curl ventrally and retain this posture for several seconds. Since the same muscles are used for forward and backward movement, the forward movement defect could be of neuronal origin. Using antibodies to VAB-7, we found that it is expressed in the DB motorneurones as well as 2 unidentified neurones in the head. The DB motorneurones innervate dorsal body muscles and are required for proper forward movement. Therefore, the forward movement defect of
vab-7 mutants might be due to lack of
vab-7 expression in the DB motorneurones. DA motorneurones (which are used for backward movement) and DB neurones have a similar structure: both send circumferential commissures to the dorsal nerve cord where they form neuromuscular junctions with dorsal body muscles. Some commissures run around the left hand side of the body and some on the right, depending on the neurone. Once the commissures reach the dorsal nerve cord they extend their axons along the dorsal nerve cord. DA axons turn anteriorly whereas DB axons turn posteriorly. Therefore, DA and DB motorneurones can be distinguished by means of their axonal polarity in the dorsal nerve cord. We examined the morphology of DA and DB motorneurones using an
unc-129::GFP integrated strain (expressed in DAs and DBs; thanks to J. Culotti). Examination of DA and DB motorneurones showed that all the DA and DB cell bodies were present and their commissures were on the correct sides of the body in
vab-7 mutants. However, the DB neurones were found to have anteriorly directed axons like DAs, instead of posteriorly directed ones. Furthermore, the dorsal nerve cords of
vab-7 mutants is defasciculated. Recently it has been shown that
unc-4::GFP is ectopically expressed in the DBs in
vab-7 mutants [1]. In wildtype
unc-4 is expressed in SABs, AVFs, DAs and VAs, and specifies proper presynaptic input to VAs [2]. To see whether the reversal of DB axonal polarity from posterior to anterior was due to ectopic expression of
unc-4 we examined the dorsal nerve cord of
unc-129::GFP;
unc-4 (
e120);
vab-7 (
e1562) animals. We found that in these animals the axonal polarity of DBs was restored to wildtype (posterior), but they still had a forward movement defect and dorsal cord defasciculations. Therefore,
vab-7 is probably performing other functions in the DBs in addition to repressing
unc-4. These results suggest that
vab-7 might determine the DB fate. To explore the functions of
vab-7 and
unc-4 in motorneurone fate further, we are currently testing whether ectopic expression of
vab-7 or
unc-4 are sufficient for determining axonal polarity. 1. Meir J.Y.J. et al., Early 1998 East Coast Worm Meeting 2. Milller D.M. et al., (1995). Development 121, 2877-2886.