In C. elegans hermaphrodites, the gonad acquires a U-shape by the directed migration of distal tip cells (DTCs). They start migration at late L2 stage from the ventral mid body of the animal, and by late L4 stage they finish migration. The first phase of DTC migration is longitudinal migration along the ventral muscle bands. The second phase is a circumferential migration, where DTCs migrate from ventral to dorsal along the epidermis, then finally they migrate retrogradely along the dorsal muscle band back to mid-body. We have characterized two phenotypic classes of
mig-6 mutants. In class 1
mig-6 mutant, originally described by Hedgecock et al. (1), the first longitudinal migration is defective but ventral to dorsal migration is normal, resulting in short and immature gonad, which is sterile. By a large scale F1 screen for DTC migration mutants, we have obtained a new class of
mig-6 mutant (class 2). The heterozygotes of class 2 display ventral reflection of gonad, which indicates that the circumferential migration of DTCs is defective but longitudinal migration is normal, similar to the defects seen in
unc-5, -6, -40 , and - 130 (2, 3). Interestingly, homozygotes of this class 2 show leaky embryonic/early larval lethality, with escapers (average 17) growing to adulthood and their gonads displaying supernumerary turns, indicating disruption of both longitudinal and circumferential guidance of DTC migration. This lethality, but not migration defects, is rescued by maternal effect, since most of homozygote animals from heterozygote parents grow to adults with a complete penetrance of supernumerary turn of DTCs. It seems
mig-6 affects all of the three phases of DTC migration, but separately. Both classes of
mig-6 are rescued by a genomic fragment containing a single locus. There are two alternative transcripts, designated
mig-6a and
mig-6b. They have the same exon structure down to exon 10 and have different 3' extensions. The proteins are putative secreted proteins with a signal peptide. The N-terminal half of the proteins has a cluster of thrombospondin type I domains and N-linked carbohydrate attachment sites. The C-terminal half of the proteins has a repeat of Kunitz-type serine protease inhibitor (KU) domains. Both forms of the proteins share the same structural features until the 6th KU domain. The large one has 5 extra KU domains and a immunoglobulin C2 type domain in the end. By designing dsRNA based on transcript specific 3' sequence, each transcript was disrupted independently or together. Quite surprisingly, it was shown that RNAi of
mig-6b phenocopied the class 1 mutant, and RNAi of
mig-6a alone or together with
mig-6b phenocopied class 2 mutant recessive phenotype. All four class 1 alleles are nonsense mutations that affect just the
mig-6b transcript. At least one of the class 2 mutations is a missense codon affecting both transcripts.
mig-6 promoter:: gfp reporter showed
mig-6 expressions in various tissues including DTCs, body wall muscles, the head mesodermal cell, coelomocytes, and some neuronal cells. Preliminary results suggest that expression of the two transcripts is under distinct regulation. Hence the expression patterns of the two transcripts, in addition to structural difference may define the class 1 and class 2 phenotypes. (1) Hedgecock et al . (1987) Development 100 , 365-382 (2) Hedgecock et al . (1990) Neuron 2 , 61-85 (3) Nash et al. (2000) Genes & Development 14 , 2486-2500