PARENT SESSION
Poster Session #52: Evolutionary Ecology.
Thursday, August 8. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC
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Evidence for partner-choice in the legume-rhizobia system.
Taylor, Lee1, Cushing, Lara*,1, Povich, Joshua1, Simms, Ellen1, 1 University of Califonia at Berkeley, Berkeley, CA
ABSTRACT- The association between leguminous plants and nitrogen-fixing bacteria is generally assumed to be mutualistic, but existence of ineffective rhizobia indicates that cheating occurs. Although evolutionary models predict that mutualisms should not persist if cheating is unconstrained, this association is widespread and ancient. Theory suggests that partner choice in a market of potential partners can constrain cheating if (1) a range of partners is available, (2) there is a mechanism by which choice can be exercised, and (3) the cost of evaluating partners is less than the benefit derived from choosing a good partner. In a regional survey of nodules collected from Lupinus spp. in Northern California, 16s-23s RFLP analysis showed that 36% of 70 sampled plants had been infected by more than one strain of Bradyrhizobium. This estimate is likely to be low because only one locus was analyzed and fewer than four nodules were examined per plant. This result indicates that these lupines are exposed to a market of potential partners. We also present data from greenhouse experiments examining the effectiveness of these strains, and propose that differential resource allocation to nodules after infection is a mechanism by which lupines choose among partners. For nodule-specific allocation to constrain cheating, few nodules should be multiply infected. In our survey, only 1.3% of nodules housed multiple strains. Our results thus meet initial requirements to indicate partner-choice as a mechanism for maintaining the mutualism.
KEY WORDS: partner-choice, symbiosis, nitrogen-fixation, rhizobia