Somatic Programmed DNA Elimination is widespread in free-living Rhabditidae nematodes

All nuclei of an organism usually share an identical genome, faithfully transmitted through successive divisions. Yet, some species (e.g. in ciliates, nematodes, crustaceans, insects, vertebrates) deviate from this dogma, by systematically eliminating parts of their DNA in all their somatic nuclei, in a process called Programmed DNA Elimination (PDE). PDE leads to two coexisting genomes: a full germline genome, and a reduced somatic genome. PDE by chromosome fragmentation has been historically identified in one clade of parasitic nematodes, Ascarids, in 1887, and later also revealed in ciliate protists, in copepods and other obligate parasitic nematodes. However, and in contrast to extensive studies performed in ciliates, the molecular mechanisms, evolutionary origins, and functional significance of PDE in animals remain unclear. This lack of understanding partially stems from limitation in genetically tractable model species. Recently, PDE has been discovered in three non-parasitic, lab-tractable nematode species from the Rhabidtidae family. Identified using genomic approaches, Oscheius tipulae PDE targets 0.6% of its genome. We uncovered that two species in the Mesorhabditis genus eliminate ∼30% of their genome in the soma. In this study, we used cytological approaches to screen 25 other Rhabditidae species for PDE. We found evidence of PDE in 17 species. Our work reveals that PDE is present in 12 out of 17 tested genera, demonstrating its widespread presence in Rhabditidae nematodes, with the notable exception of C. elegans . Genetic tools have already been established for some of them. This work provides a collection of species that can be used to test many aspects of somatic Programmed DNA Elimination in animals.