Multicellular organisms transmit genetic information through an immortal germ line which must undergo self-renewal at each generation. Failure of germ cells to maintain telomeres, small RNA expression, or repair DNA leads to a progressive loss of fertility as germ cells become unable to proliferate or enter diapause from accumulation of stress, a phenomenon known as germline mortality. We have identified a novel class of mutants that display transgenerational sterility which is unrelated to accumulation of stress. Ablation of the ZFAND3/5/6 homologs F22D6.2 and F56F3.4, which we propose to name
mstr-1 and
mstr-2 (multigenerational sterility, temperature regulated) leads to a progressive transformation of the nematode male germline to female over multiple generations thus causing sterility. Unlike canonical mortal germline mutants, sterile
mstr-1;
mstr-2 mutants are near-fully fertile when outcrossed to wild-type males. In these mutants, the commitment of germ cells to spermatogenesis can be rescued by novel mutations in the proteasome or the terminal sex regulating transcription factor GLI/tra-1. We then show that MSTR-1 coimmunoprecipitates with the mRNA of QKI/GLD-1, a key sex determinant upstream of
tra-1, and spatiotemporally restricts its expression from proximal germ cells fated for spermatogenesis. In
mstr-1;
mstr-2 mutants, each generation of maintenance at 25oC shows accumulation of ectopic GLD-1 expression in spermatogenic cells, which is rescued by the proteasomal suppressors. Perplexlingly,
gld-1 mRNA expression remains constant over multiple generations while its protein product increases. Therefore, we propose a model where MSTR-1 binds to mRNA and targets nascent proteins for proteasomal degradation to ensure proper spatial and temporal-generational expression of cell fate determinants. We are currently investigating the mechanistic basis of this novel post-transcriptional transgenerational regulation of gene expression. Overall, our work shows that in sexually reproducing organisms, germline immortality not only requires the clearance of stress but also renewal of germ cell commitment to spermatogenesis every generation.