Specialized cellular identity requires unique protein expression control. Such expression control in gametes and reproductive neurons ensures fertility. By studying cell- and tissue-level genetic code regulation, we discovered multiple links to reproductive biology. Specifically, we found that in both flies (Drosophila) and humans, the germ cells of the testis but not the ovary upregulate numerous mRNAs that are enriched in rarely used codons. We then revealed that rare codons restrict the ribosomal protein RpL10Aa to the fly testis to ensure fertility. Few other fly cell types express a rare codon reporter outside of the testis, with an additional example being most types of neurons of the brain. From genetic screening, we identified the conserved mRNA binding proteins Not3 and Orb2 as critical regulators of rare codon-dependent expression in both germ cells of the testis and in neurons throughout the brain. Notably, Orb2 has two main functions- ensuring spermatogenesis and control of mating behavior in reproductive neurons of the brain. Thus, our data are revealing important rare codon regulation of reproduction in both male gametes and neurons (including those neurons that control reproduction). In this proposal, we capitalize on our model and robust preliminary data to find new rare codon-dependent regulation in the testis and brain, and to define its function in fertility and reproductive behavior. The significance of our proposed work lies in the conservation of our findings in human testis biology and the impact on reproductive biology. Further significance is in the conservation of the new regulators of rare codon expression to be studied (Not 3 and Orb2). Our studies are innovative because we will study testis/neuron biology and reproduction in a new way, from the perspective of tissue-specific interpretation of the redundant code. In Aim1, we will identify the role of testis- and neuron-specific tRNA levels and mRNA stability on rare codon expression. To answer this question, we will determine the impact of rare codon tRNA levels, rates of mRNA decay, and the CCR4-Not de- adenylation complex on rare codon expression. We will compare each of these mRNA translation inputs in the testis vs. ovary and neurons vs. neural progenitors. In Aim 2, we will define the mechanism of Orb2 rare codon regulation in the testis and neurons. To do so, in the testis and neurons we will identify critical Orb2 domains required for rare codon expression, determine the role of rare codons in mRNA/Orb2 interactions, and identify rare codon-enriched Orb2 targets involved in fertility and reproductive behavior. In Aim 3, we will define the function of specific rare codon-derived proteins in the testis and neurons. To do so, we will further our mechanistic study of the testis-specific RpL10Aa in gametogenesis and fertility and use candidate genetics and ribosome profiling to find new examples where rare codons direct expression and/or function, focusing on testis-...