The canal-associated neurons (CANs) provide a simple model for studying cell migration in C. elegans. These two bilaterally symmetric cells migrate posteriorly from the head to the middle of the nematode during embryogenesis. Two phenotypes are associated with CAN defects: ablation of both CANs results in a clear, larval lethal phenotype (Clr), and mutations which disrupt CAN cell migration cause a withered tail phenotype (Wit). To find mutations that disrupt CAN development or migration, two screens were conducted. In the first screen, mutants with CAN defects were identified by looking for Clr or Wit animals. Two Clr mutants that appeared to lack CANs were identified, and these mutations,
gm58 and
gm71, failed to complement each other. In a second screen, mutants with CAN defects were identified by screening directly for misplaced or missing CAN's using a
ceh-23::
unc-76::GFP reporter (provided by J. Zallen and C. Bargmann) that expresses GFP in CAN cells. Three Clr mutants (
gm100,
gm131, and
gm133), which seemed to lack CANs, and two Wit mutants (
gm120 and
gm127), which had misplaced CANs, were found. These five mutations also failed to complement
gm58. These seven mutants represent the first mutations reported in
ceh-10.
ceh-10, originally identified as a C. elegans homeobox containing gene, encodes a paired-like homeoprotein expressed in several neurons, including the CANs (P. Svendsen and J. McGhee (1995), Dev. 121:1253-62). All seven mutations mapped to a region on chromosome III near
ceh-10. Furthermore, an extrachromosomal array carrying wild-type
ceh-10 rescued one of the Clr alleles,
gm58. Thus,
gm58 appears to be a mutation in
ceh-10. Because the remaining six alleles all failed to complement
gm58, they too are mutations in
ceh-10. The Clr alleles may be null mutations: they have the strongest possible phenotype and arose at a frequency expected for null mutations (5/7000 haploid genomes). Unfortunately, deficiencies that delete
ceh-10 are not yet available. The CAN migration defect of
gm120 becomes more severe in trans to Clr alleles, consistent with
gm120 being hypomorphic. All seven putative
ceh-10 mutations are being sequenced to verify that they are in
ceh-10. Sequencing the mutant genes also will demonstrate whether any of the Clr alleles are null and will provide insight into residues required for
ceh-10 function. So far, we have found
gm58 to have a conserved alanine changed to a threonine in the CVC domain, a conserved 60 amino acid region shared by
ceh-10, Vsx-1, and Chx10 downstream from the homeodomain. The CANs migrate an average of 50% of their normal distance in
ceh-10 Wit mutants. We cannot detect the CANs in Clr
ceh-10 mutants; if present, they fail to express the
ceh-23::
unc-76::GFP transgene. Similarly, the AIYL/R cells, which also express
ceh-10 and
ceh-23::
unc-76::GFP in wild-type animals, fail to express
ceh-23::
unc-76::GFP in Clr mutants. To determine whether these cells are present but just fail to express
ceh-23,
ceh-10(
gm58) mutants bearing a
ceh-10::lacZ reporter (provided by Svendsen and McGhee) were stained for beta-galactosidase expression. lacZ expression was not detected in any cells suggesting that
ceh-10 may regulate its own expression. In summary, we have identified seven mutations which cause developmental defects in several neurons, including the CANs. Genetic experiments have shown that these mutations are in
ceh-10, which encodes a putative transcription factor . Presumably,
ceh-10 regulates other genes involved in CAN cell migration and differentiation.