Fig. 6. The action of mau-2 is independent of that of slt-1 to guide AVM ventrally. (A) AVM was visualized with mec-4::gfp. Anterior is towards the right, ventral at the bottom. (a) A wild-type AVM axon in which the axon first projects ventrally and then turns anteriorly. The axon of AVM fails to migrate ventrally (b-d): the axon of AVM projects anteriorly in mau-2 and slt-1 single mutants (b), or posteriorly in mau-2 single mutant (c), and dorsally in mau-2;slt-1 double mutants and in mau-2(qm160) misexpressing SLT-1 under the myo-3 promoter (d,e). (e) The axon of AVM projects dorsally (past the dorsoventral position of the neurone ALM) and, although not in the plane of focus here, then turns anteriorly, migrating alone for a distance and finally joining the axon of ALM. A short anterior branch is also present. Arrows and arrowheads in a-e indicate AVM and ALM, respectively. (B) Phenotype of the axon of AVM in mau-2;slt-1 double mutants and in mau-2(qm160) animals misexpressing SLT-1 (kyIs218 and kyIs209 are integrated myo-3::slt-1 transgenes). Light grey indicates that the AVM axon projected anteriorly; dark grey, posteriorly; black, dorsally. The mau-2;slt-1 double mutants exhibit an enhancement of the ventral guidance defect compared with the single mutants, and display a novel phenotype (dorsal projection; n=170-300). Error bars indicate standard error of the proportion. (C) The function of mau-2 appears to be acting in parallel to those of slt-1 and unc-6 to guide the axon of AVM. The AVM neurone is shown in the right-hand panel. Its cell body rests at the boundary between the ventral muscle quadrant and the lateral hypodermis. The axon of AVM normally projects ventrally (towards the bottom), along the ventral muscle quadrant, and then turns anteriorly (towards the right). The broken arrows indicate that the axon of AVM can project abnormally (anteriorly, posteriorly or dorsally) in some mutant situations. Loss of function of unc-6, slt-1 or mau-2 results in abnormal anteriorly projecting AVM axons; loss of function of unc-6 or mau-2 can also result in posteriorly oriented axons. When the activity of both mau-2 and slt-1 are altered, the axon of AVM can project dorsally. It appears that three mechanisms (unc-6-mediated attraction, slt-1-mediated repulsion and that involving mau-2) are partially redundant to guide AVM ventrally. Scale bars: 5 µm.