The -3 3' untranslated region is essential for reproduction during temperature stress.

Organisms must sense temperature and modify their physiology to survive environmental stress. Elevated temperature reduces fertility in most sexually reproducing organisms. Maternally supplied mRNAs are required for embryogenesis. They encode proteins that govern early embryonic patterning. RNA-binding proteins (RBPs) are major effectors of maternal mRNA regulation. MEX-3 is a conserved RBP essential for anterior patterning of embryos. We previously demonstrated that the 3' untranslated region (3'UTR) represses MEX-3 abundance in the germline yet is mostly dispensable for fertility. Here, we show that the 3'UTR is essential during thermal stress. Deletion of the 3'UTR causes a highly penetrant temperature sensitive embryonic lethality phenotype distinct from a null. Loss of the 3'UTR decreases MEX-3 abundance specifically in maturing oocytes and early embryos during temperature stress. Dysregulation of reprograms the thermal stress response by reducing the expression of hundreds of heat shock genes. We propose that the primary role of the 3'UTR is to buffer MEX-3 expression during fluctuating temperature, ensuring the robustness of oocyte maturation and embryogenesis.