Components of the spindle assembly checkpoint regulate the anaphase-promoting complex during meiosis in Caenorhabditis elegans

Abstract

Temperature-sensitive mutations in subunits of the Caenorhabditis elegans anaphase-promoting complex (APC) arrest at metaphase of meiosis I at the restrictive temperature. Embryos depleted of the APC co-activator FZY-1 by RNAi also arrest at this stage. To identify regulators and potential substrates of the APC, we performed a genetic suppressor screen with a weak allele of the APC subunit MAT-3/CDC23/APC8, whose defects are specific to meiosis. Twenty-seven suppressors that resulted in embryonic viability and larval development at the restrictive temperature were isolated. We have identified the molecular lesions in 18 of these suppressors, which correspond to five genes. In addition to a single intragenic suppressor, we found mutations in the APC co-activator fzy-1 and in three spindle assembly checkpoint genes, mdf-1, mdf-2, and mdf-3/san-1, orthologs of Mad1, Mad2, and Mad3, respectively. Reduction-of-function alleles of mdf-2 and mdf-3 suppress APC mutants and exhibit pleiotropic phenotypes in an otherwise wild-type background. Analysis of a single separation-of-function allele of mdf-1 suggests that MDF-1 has a dual role during development. These studies provide evidence that components of the spindle assembly checkpoint may regulate the metaphase-to-anaphase transition in the absence of spindle damage during C. elegans meiosis.

Figure 1.—

Most suppressors are dominant suppressors of mat-3(or180). mat-3(or180); suppressor males were mated into the mat-3(or180) dpy-1 strain. Hermaphrodite non-Dpy F₁ cross progeny were shifted to 24° at the L4 stage and embryonic viability of the F₂ generation was monitored and compared to viability of the homozygous mat-3(or180); suppressor strain. ▪, homozygous; □, heterozygous.

Figure 2.—

Suppressors of mat-3(or180) map to a minimum of nine genes, five of which have been molecularly identified. Thick horizontal lines represent linkage groups I–V (LG X is not shown). Thick vertical lines indicate molecularly identified suppressors. The number of alleles recovered for a given gene is indicated in brackets following the gene name. Markers shown below the line represent morphological markers used for mapping. Thin vertical lines indicate the location of uncloned suppressors based on genetic mapping and phenotypic characterization. Bar, 5 cM.

Figure 3.—

mdf-3/san-1(av20) and mdf-3/san-1(av31) are loss-of-function mutations. Genetic experiments with three mdf-3/san-1 alleles, mdf-3/san-1(ok1580), mdf-3/san-1(av20), and mdf-3/san-1(av31) indicate that mat-3(or180); mdf-3/san-1 suppression at 24° is recessive. Embryonic viability for all three mdf-3/san-1 strains was determined. For these experiments, an mdf-3/san-1(ok1580) deletion marked with unc-13 was used.

Figure 4.—

mdf-1(av19) is a separation-of-function allele. (A) mdf-1(av19) contains a point mutation in a conserved alanine at residue 508, near the defined Mad2-binding region (underlined). The red residues indicate conserved amino acids. The green residue indicates the mutation in the mdf-1(av19) allele. The alignment is adapted from Sironi et al. (2002). C.e., C. elegans; D.m., Drosophila melanogaster; S.c., S. cerevisiae. (B) Genetic experiments with mdf-1 strains including the deletion strain mdf-1(gk2) and the mutant allele mdf-1(av19) to assess embryonic viability at 24°. All strains are homozygous for mat-3(or180). (C) RNAi has weak effects on the wild-type N2 strain at 24°, but greatly reduces hatching of mat-3(or180); mdf-1(av19). Diamonds indicate conditions where the majority of embryos were arrested at a multicellular stage. OP50 is the normal food source for C. elegans.

Figure 5.—

fzy-1(av15) has a reduced brood at 24°. unc-4, fzy-1(av4) unc-4, and fzy-1(av15) unc-4 hermaphrodites were grown at 24° and assessed for brood size. Asterisk indicates a P-value of <0.05 by Student's t-test. Error bars indicate SEM.

Figure 6.—

Brood size and F₁ phenotypes of mdf-2 mutants. (A) unc-17, mdf-2(av14) unc-17, mdf-2(av16) unc-17, and mdf-2(av18) unc-17 hermaphrodites were grown at 24° and assessed for brood size. All three strains have a significantly smaller brood size than unc-17 (P < 0.05). Error bars indicate SEM. (B) F₁ progeny were phenotypically classified. Pie charts depict the distribution of phenotypes in each of the four indicated genotypes.

Figure 7.—

Brood size and F₁ phenotypes of mdf-3/san-1 mutants. (A) unc-13, mdf-3/san-1(ok1580) unc-13, mdf-3/san-1(av20) unc-13, and mdf-3/san-1(av31) unc-13 hermaphrodites were grown at 24° and assessed for brood size. Asterisks denote a P-value of <0.05. Error bars reflect SEM. (B) F₁ progeny from unmarked control and mutant strains were phenotypically classified. Pie charts depict the distribution of phenotypes in each of the four genotypes: wild type, mdf-3/san-1(ok1580), mdf-3/san-1(av20), and mdf-3/san-1(av31). Egg-laying defective (Egl) refers to animals that retain embryos in their uterus.