To identify new genes that modulate the rate of reproductive aging, we performed a forward genetic screen for mutant worms with an extended mated reproductive span, leading to the identification of the
am117 mutation (Hughes et al., 2011). We used positional cloning approaches to identify the
am117 molecular lesion as a premature stop codon in the
phm-2 gene, which was previously identified genetically by not molecularly.
phm-2 encodes a protein with homology to human scaffold attachment factor B, a protein proposed to bind DNA and influence transcription in vertebrates.
phm-2(lf) animals also displayed substantially extended lifespan and increased stress resistance, indicating the affected gene modulates somatic and reproductive aging. The mutant worms display an abnormal pharynx grinder and a scrawny morphology, suggesting that the mutation might lead to caloric restriction. Consistent with this hypothesis, we showed that genetic interactions of
phm-2(lf) behaved as predicted with
eat-2,
rsks-1 and
pha-4. Multiple alleles of the
phm-2 gene were discovered based on a defect in pharyngeal pumping; our molecular analysis of these alleles showed they affect the same open reading frame as
am117. These results identify the
phm-2 locus, a new allele of
phm-2, and novel
phm-2 phenotypes. We observed that
phm-2 mutant animals are frequently outside the bacterial lawn, a phenotype called food avoidance. We hypothesized that the pharyngeal grinder defect results in bacterial colonization of the gut, which was confirmed. Furthermore, this bacterial colonization resulted in the activation of the innate immune response based on transcriptional changes and the nuclear localization of HLH-30. Bacterial colonization caused food avoidance and caloric restriction, since the
phm-2 mutant phenotypes (extended lifespan, reproductive span and scrawny body morphology) can all be suppressed by culturing the mutant animals on UV-killed E. coli or non-pathogenic bacteria such as Comamonas DA1877 and B. subtilis. To elucidate the basis of the food avoidance behavior, we demonstrated that
phm-2 animals required serotonergic signaling to avoid food. We conclude that the pharynx grinder defect leads to bacterial colonization, which activates innate immunity pathways and leads to food avoidance, which leads to caloric restriction. The combination of these factors results in delayed somatic and reproductive aging. The analysis of
phm-2 represents a new mechanism of longevity extension in C. elegans - bacterial colonization and behavioral food avoidance - and we discuss implications for the interpretation of other genetic models of caloric restriction in worms. Caloric restriction is the original lifespan extending intervention; Different CR regimes are reported to have different consequences in worms, and our results may clarify these puzzles.