The excretory canals reflect the migration of their growth cones during development: first the cell sends out two branches dorsally, which, when reaching the lateral midline, split to form an anterior and a posterior branch, which first pass the region of the gonad primordium and grow then further into the tail. Previous genetic analysis of single and multiple mutants showed that
unc-53 ,
unc-5 and
unc-71 represent genes of different pathways that interact in the dorsoventral growth cone steering of the excretory canals. Mutations in the three genes strongly enhance each other: whereas in
unc-53(
n152) less than 5% of the animals show ectopic ventral canals and in
unc-71(
e541) and
unc-5(
e53) there are 22% and 35% of them, respectively, we find up to 94% of ventral canals in the triple mutant strain. Based on this enhancement we designed forward genetic screens in the
unc-53 background to find mutants identifying new genes in the other two pathways. As an alternative approach to identify components of these genetic pathways we performed a Yeast-Two-Hybrid screen for UNC-5 interactors and functionally analyzed the genetic interactions of the positives with
unc-53 and
unc-71 . We expect an enhancement comparable to that by
unc-5 (see above). We found 10 positives, all of which were unknown to be involved in the pathway previously. Here we focus primarily on
apr-1 and on
unc-14 , together with its direct interactor
unc-51 (Ogura et al., 1997). The phenotypes of the doubles of
unc-14 and
unc-51 mutants with
unc-53 and
unc-71 confirmed the positions of the former in the netrin pathway downstream of
unc-5 . During the migration of a growth cone, the extracellular environment is explored by filopodia extended in three dimensions. The filopodium encountering the most favorable environment is then stabilized, a process regulated by microtubules (Bentley and O'Connor, 1994). APR-1 and UNC-53 are the only known proteins to be located at the microtubule plus ends, this could be the point where the directional information from different pathways converges to the final decision the cell takes to stabilize a growth cone in a specific direction. Two of the mutants isolated in the genetic screen are new alleles of
unc-71 , thus validating the screen. One other mutant,
bg7 , strongly enhances the ventral excretory canal phenotype of
unc-53 and moreover on itself it shows not only ectopic ventral canals but also ectopic dorsal canals. This mutant points towards another mechanism controlling dorsalward migration. Our genetic analysis reveals that the apparently simple dorsalward migration of the excretory canals is in fact a complex decision involving an extended genetic network, composed by partially redundant genes and pathways. The identification of UNC-5 interactors on the other hand gives a first insight in the genes acting in the pathway downstream of this netrin receptor . Ogura,K., Shirakawa, M., Barnes, T.M. , Hekimi, S., Ohshima, Y., Genes Devel., 11(1997), 1801-1811 Bentley, D. and O'Connor, T.P., Curr. Opin. Neurob. 4(1994), 43-48 This work was supported partially by European TMR programme NdegFMRX-CT98-0217