Fig. 7.

The spindle checkpoint lies upstream of the programmed cell death pathway. (A) dpy-18 lis-1 and cep-1; dpy-18 lis-1 mutants exhibit no significant difference in the average number of corpses per gonad arm when stained with SYTO 12. On the other hand, they both contain a significantly greater average number of corpses per gonad arm than N2, dpy-18 and cep-1; dpy-18 (P<0.001, for all comparisons, Mann-Whitney test). The data presented are medians. (B) Treatment of N2 and unc-46 with lis-1 dsRNA produces a significant increase in the average number of corpses per gonad arm, compared to soaking buffer alone (P<0.05 and P<0.001 respectively, Mann-Whitney test). The unc-46 mdf-1 double mutant contains a similar average number of corpses per gonad arm as N2 and unc-46 in soaking buffer alone but does not show an increase in corpse number upon treatment with lis-1 dsRNA. The data are medians, while the averages ± the s.e.m. and are presented in Table 1. Because mdf-1 homozygotes display variable phenotypes (Kitagawa and Rose, 1999), we excluded animals exhibiting tumerous gonads (tum phenotype) or abnormal gonad development (gon phenotype) from our analysis. (C) The spindle checkpoint gene mdf-1 is required for lis-1(lf)-induced apoptosis, suggesting that the spindle checkpoint lies upstream of the programmed cell death pathway and in parallel to the DNA damage checkpoint. Genes linking the spindle checkpoint to the programmed cell death pathway remain unknown.