Multigenerational Regulation of the C. elegans Chromatin Landscape by Germline Small RNAs

Abstract

In evolutionary terms, propagation of a species is the only biological imperative. In sexually reproducing species, species survival requires the formation of specialized gametes that exist in a perpetual cycle of fertilization and establishment of totipotency, differentiation into the next generation of gametes, and then fertilization and the re-establishment of totipotency. This makes the germline an immortal cell lineage. To successfully repeat this cycle at every generation, the germline must maintain its replicative immortality. Equally important is the accurate transmission of genetic and epigenetic information to the next generation to facilitate the correct execution of the developmental program. Thus, pathways that protect genome fidelity and ensure appropriate gene expression at every generation are essential for germline immortality. In animals, small non-coding RNAs that are expressed in the germline and transmitted to progeny control gene expression to promote fertility. Germline-expressed small RNAs, including endogenous siRNAs (endo-siRNAs) and Piwi-interacting RNAs (piRNAs), drive the repression of deleterious transcripts such as transposons, repetitive elements, and pseudogenes. Small RNA pathways are highly conserved in metazoans and have been best described in the model organism Caenorhabditis elegans. Many features of C. elegans make it ideal for the study of small RNA biology and germline maintenance, including genetic tractability, rapid development, an invariant cell lineage, and propagation via self-fertilization. In C. elegans, endo-siRNAs are deposited from the maternal germline into the embryo, where they trigger the amplification of another generation of endo-siRNAs as well as heritable heterochromatin formation at target genes. Transgenerational inheritance of endo-siRNAs is critical for germline maintenance, as mutants that are heritable RNAi defective (Hrde) are germline mortal (progressively sterile). How endo-siRNAs control chromatin state and germline maintenance is unknown. I have identified morc-1, the sole C. elegans homolog of the highly conserved Microrchidia family of chromatin-binding proteins, as a crucial link between endo-siRNAs and multigenerational chromatin organization. I also identify and describe the first suppressor of the germline mortality phenotype of hrde pathway mutants. This work establishes a critical role for endo-siRNAs and MORC-1 in the control of transgenerational chromatin architecture to promote fertility.