SYMBIOTIC SPECIALIZATION AND THE POTENTIAL FOR GENOTYPIC COEXISTENCE IN A PLANT-BACTERIAL MUTUALISM

Genotypes of the annual legume Amphicarpaen bracteata vary in their de gree of specialization toward different nitrogen-fixing bacteria. Plan ts of lineages ''Ib'' and ''II'' are specialized for mutualism with a limited group of bacterial genotypes. Lineage ''Ia'' plants are symbio tic generalists, interacting with all bacteria associated with special ist (Ib and II) plants, and also with a distinct class of bacteria tha t only fix nitrogen with lineage Ia plants. The relative performance o f generalist and specialist plant lineages was measured in four symbio tic environments: (1) in the absence of nitrogen-fixing bacteria, (2) with bacteria with broad host ranges, (3) with bacteria specialized on lineage Ia hosts, and (4) with a mixture of the bacteria with broad a nd narrow host ranges. In the presence of bacteria with broad host ran ges, the relative performance of different plant lineages was inconsis tent among experimental replicates. However, lineage Ia plants had nea rly 3 times higher total biomass and 6 times higher seed biomass than lineage Ib or II plants when grown with bacteria specialized on Ia hos ts. When exposed to a mixture of bacteria with broad and narrow host r anges, generalist plants had 72% higher total biomass and >100% higher seed biomass than specialist plants. These results imply that in dive rse natural populations, where all plants have a choice of symbiotic p artners, mutualistic interactions are likely to foster competitive exc lusion rather than stable coexistence of different plant lineages.