At the last C. elegans meeting, we reported the isolation of the mutation nl 792, which causes certain asymmetric cell divisions to reverse polarity. We have continued our analysis of the role of the gene defined by this mutation,
lin44 1. Genetic analysis. We have isolated a new
lin-44 allele,
n2111, in a non-complementation screen. Both nl 792 and
n2111 behave similarly in homozygotes, deficiency heterozygotes, and nl 792/n2111 transheterozygotes. The two alleles differ in that nl 792 is suppressed by a single copy of the amber suppressor
sup-7, while
n2111 is not suppressed. These alleles could define the null phenotype of
lin44. Cell-interactions. Mutations in
lin44 reverse the polarity of cell divisions in the B, F, U, and T lineages. These blast cells lie in the posterior of the male and are related primarily by position rather than by lineal history, suggesting that cell-interactions might be involved in defining the polarities of asymmetric cell divisions in these lineages. Chisholm and Hodgkin (Genes Dev. 3: 1413, 1989) have shown that when the B cell is kined with a laser microbearn in wild- type males, the polarity of the F and U lineages are reversed as they are in
lin44 mutant males. Thus, the B cell influences the polarities of the F and U cells. What cell or cells influence the polarities of B and T? To answer this question, we have killed the mother of the B cell in both wild-type and
lin44 embryos and looked at the polarity of T. We have not observed an effect on the T cell lineage in either case, and conclude that B does not influence the polarity of T. In an analogous experiment, we have killed the T cells before B is born in the embryo and looked for an effect on the polarity of the B cell division. We have not observed an effect in wild-type animals. Results with
lin44 animals are not yet conclusive, although it appears T does not influence the polarity of B. Molecular analysis. To learn how
lin44 controls the polarity of asymmetric cell division at the molecular level, we are cloning the locus. We have mapped the endpoints of the small deficiency hDp (which spans
lin44 ) to the
unc-73 contig by hybridization of labeled cosmids to wildtype and deficiency genomic DNA. Microinjection of cosmids in this interval does not rescue
lin44. However, part of the interval is represented only by YAC clones, and we suspect that
lin44 lies in this 'YAC gap.' We have injected the 350 kb YAC clone Y48F5 into
lin44 animals and obtained two rescued lines. We are currently attempting to nanow down the size of the rescuing DNA.