Reproduction is essential to species survival. We systematically investigated the essential role of superoxide dismutases (SODs) for the efficient reproduction in C. elegans. SODs are a group of enzymes that specifically scavenge the hyperactive free radical superoxide. Five C. elegans genes encode SOD:
sod-1 (constitutive) and
sod-5 (inducible) encode the cytoplasmic SODs;
sod-2 (constitutive) and
sod-3 (inducible) encode the mitochondrial SODs; and
sod-4 encodes the extracellular SOD. Deletion of the mitochondrial sod gene in either fly or mouse causes neonatal lethality, but C. elegans has a regular lifespan even in the absence of all five sod genes [1]. However, in the quintuple sod knockout, the progeny number drops to ~40 per-generation, as compared to ~300 in wild type. Progeny numbers are ~120 in the
sod-1,5 (cytoplasmic null) mutant or ~90 in
sod-2,3 (mitochondrial null) mutants, but there is no change in reproduction in the
sod-4 (extracellular) mutant. The number of C. elegans self progeny is dictated by the number of functional sperm, so we first examined the sperm number with whole worm DAPI-staining in sod mutants. It turns out that the sod quintuple mutant can generate ~150 sperm per-generation, which is much larger than its progeny number. After further investigation, we find that the in vitro sperm activation rate of the sod quintuple mutant is about 30% lower than wild type. Thus, the fertility defect in the sod quintuple mutant may be partially due to the failure of sperm activation. We also measured the rate of oocyte production (progeny/day) by counting the progeny of sod mutants mated with wild type males. We found the mated
sod-1,2,3,4,5 produced ~80 oocytes on the peak day, compared to ~170 of the wild type. Consistent with defective or slowed oocyte production, DAPI staining of young adult worms showed that the sod quintuple mutant had a low number of nuclei in the gonad. We also found there were no unfertilized eggs and no significant increase of dead embryos from the mated
sod-1,2,3,4,5 hermaphrodites. Thus, the quality of oocytes appears close to normal in the absence of all superoxide management, but the quantity of oocytes is largely decreased in sod quintuple mutant. We found the absence of either mitochondrial SOD (
sod-2,3) or cytosolic SOD (
sod-1,5) also largely reduced the maximal daily oocyte production to ~80. Thus, either a cytosolic or mitochondrial superoxide increase can impact germ-line cells, with a slightly less severe outcome compared to the impact of missing all SODs. References: 1. Van Raamsdonk, J.M. and S. Hekimi, Superoxide dismutase is dispensable for normal animal lifespan. Proc Natl Acad Sci U S A, 2012. 109(15): p. 5785-90.