The C.elegans even-skipped homologue,
vab-7 , is required for posterior muscle and epidermal patterning.
vab-7 mutants are also forward Unc. VAB-7 is expressed in DB motorneurons, which innervate the dorsal body muscles and are required for proper forward movement. To examine the morphology of DB neurons, we used an
unc-129::gfp reporter gene, which labels DA and DB processes and cell bodies. Both DA and DB neurons send circumferential commissures to the dorsal nerve cord (DNC), but on reaching it, DAs send axons anteriorly and DBs posteriorly. We found that in
vab-7 mutants DB neurons have the same axonal polarity as the DAs (anteriorly directed) in the DNC, suggesting that they have DA characteristics. To investigate this further, we examined the expression of UNC-4, a homeodomain protein expressed in the DAs;
unc-4::gfp is ectopically expressed in the DBs in
vab-7 mutants.
unc-4 is responsible for the change in polarity of DBs in
vab-7 mutants, since their polarity is restored to wild type in
unc-4;
vab-7 double mutants. Furthermore expression of the DB marker,
acr-5::gfp (a nicotinic acetylcholine a-receptor subunit) is lost in
vab-7 mutants, also because of ectopic
unc-4 expression. These data indicate that
vab-7 is required for the DB fate. To investigate the sufficiency of
vab-7 for DB fate determination we ectopically expressed
vab-7 under the control of the
unc-3 promoter, which drives expression in the DAs and DBs and some postembryonic neurons (AS, VA, VB). This causes DAs to have DB characteristics: DA axonal polarity in the DNC is reversed,
unc-4::gfp expression is repressed, and
acr-5::gfp is ectopically expressed. Furthermore, ectopic expression of
vab-7 induces ectopic
unc-129::gfp expression in postembryonic neurons, suggesting a possible DB fate acquisition by these neurons. In support of this model, the postembryonic AS neurons, which have anteriorly directed axons in the DNC, have reversed (posterior) polarity when
vab-7 is ectopically expressed. Therefore,
vab-7 is sufficient to induce at least some aspect of DB fate in both embryonic and postembryonic neurons. Interestingly, even-skipped in Drosophila also controls the guidance of neurons that innervate dorsal muscles, so there is conservation in the function of these proteins.