Liu, QinwenThe modification of transcriptional regulation is a well-documented evolutionary mechanism in both plants and animals, but post-transcriptional controls have received less attention. The derived hermaphrodite of <italic>C. elegans<italic> has regulated spermatogenesis in an otherwise female body. PUF family RNA-binding proteins FBF-1 and FBF-2 limit XX spermatogenesis by repressing the male-promoting proteins FEM-3 and GLD-1. For my dissertation research, I examine the function of PUF homologs from other <italic>Caenorhabditis<italic> species, with emphasis on <italic>C. briggsae<italic>, which evolved selfing convergently. <italic>C. briggsae<italic> lacks a <italic>bona fide fbf-1/2<italic> ortholog, but two members of the related PUF-2 subfamily, <italic>Cbr-puf-2<italic> and <italic>Cbr-puf-1.2<italic>, do have a redundant germline sex determination role. Surprisingly, this is to promote, rather than limit, hermaphrodite spermatogenesis. I provide genetic, molecular, and biochemical evidence that <italic>Cbr-puf-2<italic> and <italic>Cbr-puf-1.2<italic> repress <italic>Cbr-gld-1<italic> by a conserved mechanism. However, <italic>Cbr-gld-1<italic> acts to limit, rather than promote, XX spermatogenesis. As with <italic>gld-1<italic>, no sex determination function for <italic>fbf<italic> or <italic>puf-2<italic> orthologs is observed in gonochoristic <italic>Caenorhabditis<italic>. These results indicate that PUF family genes were coopted for sex determination in each hermaphrodite via their long-standing association with <italic>gld-1<italic>, and that their precise sex-determining roles depend on the species-specific context in which they act. Finally, I document non-redundant roles for <italic>Cbr-puf-2<italic> in several aspects of somatic development. I show <italic>Cbr-puf-2<italic> is required for reliable embryonic development, and that it is essential for vulval development and normal progression from early larval stage. I provide evidence suggesting that this latter role is related to pharyngeal muscle physiology. Thus, recently duplicated PUF paralogs, while redundant for some roles, can also rapidly acquire distinct non-redundant functions. This is consistent with theoretical models for the preservation of gene duplicates.DissertationEvolution & developmentGeneticsMolecular biologyCaenorhabditisEvolutionGerm cellsHermaphroditismPUF proteinsTranslation