Small RNAs and Argonautes Provide a Paternal Epigenetic Memory of Germline Gene Expression to Promote Thermotolerant Male Fertility: A Dissertation

Abstract

During each life cycle, gametes must preserve and pass on both genetic and epigenetic information, making the germline both immortal and totipotent. In the male germline the dramatic morphological transformation of a germ cell through meiosis, into a sperm competent for fertilization, while retaining this information is an incredible example of cellular differentiation. This process of spermatogenesis is inherently thermosensitive in numerous metazoa ranging from worms to man. Here, I describe the role of two redundant AGO-class paralogs, ALG-3/4, and their small RNA cofactors, in promoting thermotolerant male fertility in Caenorhabditis elegans. alg-3/4 double mutants exhibit temperature dependent sterility resulting from defective spermiogenesis, the postmeiotic differentiation of haploid spermatids into spermatozoa competent for fertilization. The essential Argonaute CSR-1 functions with ALG-3/4 to positively regulate target genes required for spermiogenesis by promoting transcription via a small RNA positive feedback loop. Our findings suggest that ALG-3/4 functions during spermatogenesis to amplify a small-RNA signal loaded into CSR-1 to maintain transcriptionally active chromatin at genes required for spermiogenesis and to provide an epigenetic memory of male-specific gene expression. CSR-1, which is abundant in mature sperm, appears to transmit this memory to offspring. Surprisingly, in addition to small RNAs targeting male-specific genes, we show that males also harbor an extensive repertoire of CSR-1 small RNAs targeting oogenesis-specific mRNAs. The ALG-3/4 small RNA pathway also initiates silencing small RNA signals loaded into WAGO vii Argonautes, which function to posttranscripitonally silence their target mRNAs. Silencing WAGO/small RNA-complexes are present in sperm and presumably transmitted to offspring upon fertilization. Together these findings suggest that C. elegans sperm transmit not only the genome but also epigenetic activating and silencing signals in the form of Argonaute/small-RNA complexes, constituting a memory of gene expression in preceding generations.