The nematode Oscheius tipulae as a genetic model for programmed DNA elimination

Abstract

Programmed DNA elimination (PDE) is a notable exception to the paradigm of genome integrity. In metazoa, PDE often occurs coincident with germline to somatic cell differentiation. During PDE, portions of genomic DNA are lost, resulting in reduced somatic genomes. Prior studies have described the sequences lost, as well as chromosome behavior, during metazoan PDE. However, a system for studying the mechanisms and consequences of PDE in metazoa is lacking. Here, we present a functional and genetic model for PDE in the free-living Rhabditidae nematode Oscheius tipulae, a family that also includes Caenorhabditis elegans. O. tipulae was recently suggested to eliminate DNA. Using staged embryos and DNA FISH, we showed that O. tipulae PDE occurs during embryogenesis at the 8-16 cell stages. We identified a conserved motif, named Sequence For Elimination (SFE), for all 12 break sites on the six chromosomes at the junctions of retained and eliminated DNA. SFE mutants exhibited a "fail-to-eliminate" phenotype only at the modified sites. END-seq revealed that breaks can occur at multiple positions within the SFE, with extensive end resection followed by telomere addition to both retained and eliminated ends. We identified many functional SFEs at the chromosome ends through END-seq in the wild-type embryos, genome sequencing of SFE mutants, and comparative genomics of 23 wild isolates. We suggest that these alternative SFEs provide flexibility in the sequences eliminated and a fail-safe mechanism for PDE. These studies establish O. tipulae as a new, attractive model for studying the mechanisms and consequences of PDE in a metazoan.

Keywords: CRISPR; DNA double-strand break; END-seq; Oscheius tipulae; genetic model; nematode; programmed DNA elimination; sequence for elimination; telomere addition; wild isolates.