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Abstract

Self-recognition is observed abundantly throughout the natural world regulating diverse biological processes. Although ubiquitous, often little is known about the associated molecular mechanisms and despite the prevalence of nematodes in nearly every ecological niche and the pre-eminence of Caenorhabditis elegans as a model organism, evidence of self-recognition has thus far never been described in nematodes. Here we investigate the predatory nematode Pristionchus pacificus and through interactions with its prey, reveal a self- recognition mechanism acting on the nematode surface, capable of distinguishing self-progeny from even closely related strains. We identified a key component of the self-recognition machinery located within a region of minimal recombination activity and as such, developed a novel method of inducing informative recombination events via CRISPR/Cas9. Using this system, we have successfully identified the small peptide SELF-1 as a major component of self-recognition, which is expressed in all hypodermal cells at all stages of the life-cycle. SELF-1 is composed of an invariant domain including a signal peptide and a hyper-variable C- terminus. Comparative analysis of this region reveals extreme variability in sequence, length and also self-1 copy number. Disruptions to the hyper-variable region including even a single amino acid substitution were sufficient to eliminate protection from predation. Thus, the self-recognition system identified in P. pacificus enables this nematode to avoid cannibalism, while promoting the killing of competing nematodes.