The hermaphrodite distal tip cells (DTCs). undergo a 3-phase pattern of migration sequentially moving along the AP, DV and AP axes. The second migration phase, from the ventral to the dorsal side, requires the UNC-6/Netrin guidance cue and its receptors UNC-5 and UNC-40 (1). UNC-5, which is up-regulated at the L3/L4 transition, is both necessary and sufficient to cause ventral to dorsal migration. Premature
emb-9 promoter-driven UNC-5 in the DTCs causes a precocious ventral to dorsal migration (2). This
emb-9::
unc-5 induced precocious migration was partially dependent upon the endogenous
unc-5 suggesting that
unc-5, albeit undetected, is probably expressed and tightly regulated during L3. In addition, it was also suggested that some putative unknown negative regulation is relieved at the time of the turn (2). In order to identify putative
unc-5 regulators and to get a better insight into the mechanisms involved in the transition from one migratory phase to another, I had conducted modifier screens for both suppression and enhancement of this UNC-5 induced precocious migration phenotype. Since the precocious migration is occurring under unfavorable conditions, it is reasonable to assume that it would be very sensitive to any changes with respect to the ability of the DTC to detach from the ventral side and reorient dorsally. This might allow the identification of molecules that participate in this process, but may act redundantly or have no marked phenotype when mutated on their own. As a starting strain for the screen I generated a strain that carries
emb-9::
unc-5 along with
emb-9::gfp (evIs129). This strain has a more complex phenotype. The DTCs undergo precocious migrations at a higher penetrance (~90%) and in addition, once the DTCs reach the dorsal side, instead of reorienting towards the midbody, they migrate towards either the head or the tail. This sometimes involves extra turns of the DTC. This third phase migratory defect suggests that similar to the situation on the ventral side, in which
unc-5 must be tightly regulated for the ventral to dorsal migration to occur, the reduction in
unc-5 levels which occur as the DTC approach the dorsal muscle band (2) might be necessary for the normal execution of the third phase of migration. I have conducted a clonal screen looking for mutations that would suppress
emb-9::
unc-5 induced precocious migration, or enhance it in a balanced evIs129 heterozygote strain. 6 suppressors and 5 enhancers were isolated and will be described. Analysis of these mutants will, we believe, shed light on the mechanisms governing the directed cell migration of the DTC's. 1. Hedgecock et al. (1990). Neuron 4, 61-85. 2. Su et al. (2000). Development 127,585-594.