Abstract
Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs. Cheater impact can be diminished if cooperators display a tag (‘greenbeard’) and recognise and preferentially direct cooperation towards other tag carriers. Despite its popular appeal, the feasibility of such greenbeards has been questioned because the complex patterns of partner-specific cooperative behaviours seen in nature require greenbeards to come in different colours. Here we show that a locus (‘Tgr’) of a social amoeba represents a polychromatic greenbeard. Patterns of natural Tgr locus sequence polymorphisms predict partner-specific patterns of cooperation by underlying variation in partner-specific protein–protein binding strength and recognition specificity. Finally, Tgr locus polymorphisms increase fitness because they help avoid potential costs of cooperating with incompatible partners. These results suggest that a polychromatic greenbeard can provide a key mechanism for the evolutionary maintenance of cooperation.
Highlights
Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs
The discrete greenbeard scenario provides a compelling illustration of the concept, natural populations typically contain individuals that vary in cooperative traits, often with complex patterns of partner-specific behaviours
Several thousand individual D. discoideum amoebae aggregate together to undergo multicellular development, which results in the formation of a multicellular fruiting body composed of a stalk and sporehead
Summary
Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs. Tgr locus polymorphisms increase fitness because they help avoid potential costs of cooperating with incompatible partners These results suggest that a polychromatic greenbeard can provide a key mechanism for the evolutionary maintenance of cooperation. Sequence variation and IdsD/IdsE protein binding in P. mirabilis[21,22,23] can explain partner-specific interactions during swarming, the extent of allelic variation and capacity for complex partner-specific interactions is unknown Both examples highlight the fact that cell adhesion proteins represent a standout candidate to encode a polychromatic greenbeard[31]. Elegant gene swapping experiments have shown that matching tgrB1 and tgrC1 alleles are required for co-aggregation[37,41,42] These studies raised the possibility that a polychromatic greenbeard system based on TgrB1- and TgrC1-mediated cell adhesion could underlie cooperative behaviour in natural populations of D. discoideum
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