Multicoloured greenbeards, bacteriocin diversity and the rock-paper-scissors game

Greenbeard genes identify copies of themselves in other individuals and
cause their bearer to behave nepotistically towards those individuals. Bacterial
toxins (bacteriocins) exemplify the greenbeard effect because producer
strains carry closely linked genes for immunity, such that toxicity is limited
to nonproducer strains. Bacteriocin producers can be maintained in a
dynamic polymorphism, known as rock-paper-scissors (RPS) dynamics, with
immune and susceptible strains. However, it is unclear whether and how
such dynamics will be maintained in the presence of multiple toxin types
(multiple beard ‘colours’). Here, we analyse strain dynamics using models of
recurrent patch colonization and population growth. We find that (i) polymorphism
is promoted by a small number of founding lineages per patch,
strong local resource competition and the occurrence of mutations; (ii) polymorphism can be static or dynamic, depending on the intensity of local
interactions and the costs of toxins and immunity; (iii) the occurrence of
multiple toxins can promote RPS dynamics; and (iv) strain diversity can be
maintained even when toxins differ in toxicity or lineages can exhibit
multitoxicity/multi-immunity. Overall, the factors that maintain simple RPS
dynamics can also promote the coexistence of multiple toxin types (multiple
beard colours), thus helping to explain the remarkable levels of bacteriocin
diversity in nature. More generally, we contrast these results with the
maintenance of marker diversity in genetic kin recognition.