A peptide against cannibalism

A worm whose favorite dish is – of all things – worm larvae must be very careful not to eat their own offspring by accident. Scientists from the Max Planck Institute for Developmental Biology in Tübingen recently discovered how these tiny worms only one millimeter away can distinguish their own offspring from that of other worms and avoid cannibalism. They found that Pristionchus recognizes his offspring by means of a complex mechanism. Worms carry a small, highly variable protein on their surfaces, which seems to be detected by the worm's nose. The variable part of the protein probably functions as a code of self-recognition and the change of a single amino acid leads to cannibalism.

"Self-recognition is found in all animal and plant kingdoms," says Ralf Sommer, a leading scientist at the Max Planck Group in Tübingen, who established Pristionchus as the model organism for comparative studies of the famous nematode Caenorhabditis elegans. "This includes being at the heart of biological processes ranging from the aggregation of unicellular organisms to social or aggressive behaviors in various animals, and it is also a key component of the human immune system responsible for protecting humans." against pathogens -recognition in general, this type of self-recognition by the organism has never been described in nematodes, "explains Sommer.

To study how accurately worms can distinguish theirs from those that are not, researchers conducted a series of experiments to evaluate the preferences of their prey. In multiple separate experiments, adult worms of different Pristionchus species were offered their own larvae, C. elegans larvae, larvae of another Pristionchus species or larvae of a related line of their species as prey. .

Signals on the body surface

In all experiments, worms avoided attacking the larvae of their own lineage; but they did not hesitate to feed on larvae of C. elegans, descendants of closely related species, or even descendants of lineages within their own species. Even predators that have been put into a mixed culture with their own larvae and those of another species of Pristionchus have exclusively chosen "non-self" offspring as prey. "This discovery excludes that their own larvae are recognized by pheromone-like substances that are emitted into the environment in the form of self-recognition remains intact in the mixed culture," says James Lightfoot, first author of the study. The observation that feeding of adult animals always begins with contact with the nose before biting is further evidence that the decisive signals are located on the surface of the body.

Thanks to targeted mutations, Sommer and his team have been able to identify a gene that plays a vital role in the distinction between "self" and "non-self" and which has been rightly named by scientists. self-1 code for a small protein of 63 amino acids – the building blocks of proteins. Large parts of the protein are highly conserved, which means that they possess the same amino acid sequence, even in distant species.

Region of the variable molecule

A small region at the end of the protein is however highly variable – with clear differences even between closely related P. pacificus lineages. In mutation experiments, the exchange of a single amino acid in this region has proven sufficient to prevent self-recognition and disable protection against cannibal attacks. "So, SELF-1 seems to represent a code of self-recognition and is held on the surface of the worm throughout its life," says Sommer.

Other experiments, however, have shown that possession of a species-specific SELF-1 protein is not the only signal upon which the decision to attack or not depends. C. elegans larvae were also attacked by P. pacificus after being genetically engineered with P. pacificus SELF-1. "Obviously, SELF-1 is part of a more complex mechanism, which governs the distinction between" self "and" not-self, "Sommer said. "P. pacificus therefore provides an ideal model system for studying the mechanism underlying self-recognition as well as its evolution and the strategies used by this worm in competitive situations."