Abstract
Cooperation can be favoured through the green-beard mechanism, where a set of linked genes encodes both a cooperative trait and a phenotypic marker (green beard), which allows carriers of the trait to selectively direct cooperative acts to other carriers. In theory, the green-beard mechanism should favour cooperation even when interacting partners are totally unrelated at the genome level. Here, we explore such an extreme green-beard scenario between two unrelated bacterial species-Pseudomonas aeruginosa and Burkholderia cenocepacia, which share a cooperative locus encoding the public good pyochelin (an iron-scavenging siderophore) and its cognate receptor (green beard) required for iron-pyochelin uptake. We show that pyochelin, when provided in cell-free supernatants, can be mutually exchanged between species and provide fitness benefits under iron limitation. However, in co-culture we observed that these cooperative benefits vanished and communities were dominated by P. aeruginosa, regardless of strain background and species starting frequencies. Our results further suggest that P. aeruginosa engages in interference competition to suppress B. cenocepacia, indicating that inter-species conflict arising from dissimilarities at the genome level overrule the aligned cooperative interests at the pyochelin locus. Thus, green-beard cooperation is subdued by competition, indicating that interspecific siderophore cooperation is difficult to evolve and to be maintained.