Most transposons are silenced by a class of small RNAs called piRNAs that interact with the conserved Argonaute protein Piwi that is present in germ cells. We discovered that dysfunction of the C. elegans Piwi ortholog PRG-1 displays normal fertility for several generations, but then fertility declines, ultimately resulting in complete sterility after growth for ~20 generations. This so-called Mortal Germline phenotype indicates that Piwi promotes germ cell immortality. We found that the sterility of Piwi/prg-1 mutants can be reversed, indicating that Piwi sterility is a form of reproductive arrest. Therefore, the sterility of Piwi mutants may be analogous to sterility that occurs in response to severe environmental stresses like starvation. We therefore hypothesize that Piwi/prg-1 mutant germ cells transmit an epigenetic factor, likely due to progressive de-silencing of one or more transposons, which accumulates to trigger a stress response that induces reproductive arrest. We are interested in understanding what stress is transmitted by prg-1 mutant germ cells and how the organism responds to this stress. We are interested in the potential relevance of heritable stress to development, aging and disease.