Self-recognition is abundant in the natural world where it regulates diverse behaviours of myriad social interactions, including mating behaviours and predator-prey dynamics. Perhaps the most striking example of kin recognition is found in organisms with the capability to harm or even kill their relatives, whereby it is fundamental to prevent cannibalism of kin. Despite the prevalence of self-recognition behaviours, many examples have only been described in non-model species currently lacking the necessary molecular, neurological, and evolutionary tools required to fully investigate these systems and the associated mechanisms. Furthermore, self-recognition behaviours often act at the interface between neurobiology and immunology, making exploration of the behavioural processes additionally complex.

To overcome these challenges, we explore the self-recognition system evident in the omnivorous roundworm, Pristionchus pacificus. This nematode has evolved teeth-like denticles and is capable of both feeding on bacteria and killing other nematode larvae, including those of its more famous cousin Caenorhabditis elegans. However, while P. pacificus kills other nematode species and strains, remarkably, it does not kill its own progeny; we therefore investigate this self-recognition system which protects the offspring from the predatory parent. Firstly, we are identifying the signals transmitted by P. pacificus to indicate self and prevent attack by their relatives. Secondly, we are elucidating the receptors and circuits behind the killing decision and which distinguish between foreign and self-progeny. Lastly, we are understanding the evolution of these processes by utilising a worldwide representation of P. pacificus from a vast strains library, many of which display strong killing interactions with each other.

Self-recognition is observed across the tree of life, in organisms as diverse as (top left and clockwise) bacteria, tiger salamanders, social amoebae, fire ants, tunicates, and spade foot toad tadpoles.

(1) Molecular mechanisms behind the self-recognition signal

Pristionchus nematodes have evolved a sophisticated mechanisms to avoid killing their own progeny. We have previously isolated the first component of the self-recognition signalling system which functions through the sensing of a small peptide, likely found on the nematode surface coat. While this small peptide is a major component of the self-signalling mechanism, it is not the only component. Therefore we are currently attempting to identify other elements involved in this process to begin to understand how Pristionchus larvae ensure they are not cannibalised by their parents.

Expression pattern of the self-recognition component self-1 with robust expression observed throughout all the hypodermal cells. (Image from Lightfoot and Wilecki, 2019, Science).

(2) Receptors and circuitry required to process self-recognition

In addition to understanding how the nematodes signal self, it is also essential to understand how these signals are recieved and processed. As the self-recognition behaviour requires direct contact between the nose of the predator and the nematode larvae, the head sensory circuit of these nematodes are prime candidates for mediating these interactions. Therefore, using a combination of genetic screens and neuronal imaging we are currently investigating the receptors and circuits to elucidated the mechanism behind prey sensing.

SEM image looking into the mouth of a predatory nematode. (Image from Lightfoot and Wilecki, 2019, Science).

(3) Understanding the ecology and evolution of predation and self-recognition

All behaviours are evolved traits generated via natural selection with predation and self-recognition no exception. Fortunately, with access to a large collection of P. pacificus natural isolates, from around the world along with deep and repeated sampling from Réunion Island, we are elucidating how the genes and behaviours have evolved, the mechanism of generating diversity and crucially, their role within a natural population. Our preliminary observations of killing and recognition interactions between thse P. pacificus natural populations demonstrate the existence of a complex assortment of behaviours. These studies will help to answer key questions related to predation, competition and kin selection as well as furthering our understanding of the selection pressures and environment which has led to the evolution of the predatory and self-recognition traits.

P. pacificus nematodes are found all around the world and show varying degrees of relatedness between strains. Many diverse strains also kill one another. Image is of Réunion Island where repeated deep sampling of P. pacificus facilitates the study of ecologically relevant interactions.