COS 128-3 - Has domestication affected the ability of peas to sanction poor-quality rhizobia?

Abstract: Mutualisms provide access to resources, services, or habitats that would otherwise be limiting. However, the benefits of such mutualisms can be undermined by partners that fail to reciprocate. A mutualism that is taken advantage of by a poor-quality or “cheating” partner would be evolutionarily unstable. Exploitation could be particularly costly if the context of the mutualism changes, either by de-valuing the resource a partner can gain, or by limiting the resource used to “pay” a partner. The evolutionary maintenance of mutualisms, then, depends on the ability to “sanction”, i.e., limit investment into poor-quality partners.

The mutualism between legumes and nitrogen-fixing rhizobia is an essential part of the N cycle, and reduces the fertilisation requirements of crops like peas, soybeans, and lentils. The genetic cost of domestication has reduced genetic diversity in legumes, and when selection is focused on a particular suite of traits, trade-offs against other traits can occur. In agricultural conditions, where plants are more likely to receive N via fertilisation, selection on the ability to maximise N gain through the rhizobial mutualism could be relaxed. We investigated the possibility that domesticated peas are less able to sanction against non-fixing rhizobia that their wild relatives.

In a greenhouse experiment, we grew domesticated pea (Pisum sativum), and two wild pea species (Pisum sativum subsp. elatius and Pisum fulvum) with an inoculation of rhizobia (Bradyrhizobium leguminosarum) capable of N fixation, an isogenic strain incapable of N fixation, a mix of the two, or no rhizobia. We used fluorescent labelling to track the presence of these rhizobia in the root nodules of the peas. To alter the resource context of the mutualism, we also applied a 50% shade treatment and a high or low N fertilisation. 5 weeks post-inoculation, we harvested the plants and weighed their biomass. We photographed the fluorescent root nodules and used automated image analysis to count and measure their size. Additionally, we crushed mixed-strain nodules and grew the released rhizobia on agar, and counted the colonies that grew.

Preliminary analysis indicates that despite a greater benefit from inoculation for domesticated pea, and higher overall nodule occupancy in the wild pea species, the three species do not differ in their ability to sanction against non-fixing rhizobia. Nodules containing only non-fixing rhizobia yielded fewer colonies than effective nodules, and mixed nodules contained significantly fewer non-fixing rhizobia than fixing rhizobia. Domestication, then, has not compromised the sanctioning abilities of peas.