Beatriz Baker-Méio and Robert Marquis have a paper in volume 100:1 in the Journal titled “Context-dependent benefits from ant-plant mutualism in three sympatric varieties of Chamaecrista desvauxii“.  Their topic of variation in species interaction outcomes is close to my heart, so I am glad to see more attention on this topic.

Beatriz has written a nice lay summary of their research presented in the paper, which we provide below along with pictures of the study organisms.

Figure 1. A caterpillar feeding on Chamaecrista desvauxii var. brevipes' flower bud being attacked by a Crematogaster ant.

Many plants produce nectar in extrafloral nectaries that are not directly involved in pollinator attraction. These secretions attract foraging ants that patrol the entire individual and often attack the herbivores they encounter. Compared to ant-plant mutualisms in which ants nest on the plants, benefits from this facultative mutualism are more variable and depend on both the biotic and abiotic context. The mechanisms that lead to this variation, however, are still unclear.

Several species in the legume genus Chamaecrista have extrafloral nectaries. We studied three varieties of C. desvauxii that coexist in the cerrado reserve of the Clube Caça e Pesca Itororó in Uberlândia, Brazil. One of the varieties, C. desvauxii var. brevipes, has larger nectaries, produces more nectar, and attracts more ants than the others. Both small-bodied ants like those in genus Crematogaster (Figure 1), and larger-bodied ants like those in genus Pachycondyla (Figure 2) are frequently found in this variety.

Figure 2. A Pachycondyla ant feeding on the extrafloral nectary of C. desvauxii var. brevipes.

By removing all nectaries from individuals of the three varieties, we confirmed that having extrafloral nectaries only increases seed production in var. brevipes, the variety with the largest nectaries. However, this benefit disappears if the fruits are bagged to prevent attack by weevils and other seed predators (Figure 3). If extrafloral nectar production is costly, locally isolated populations of C. desvauxii that receive low benefits from ant attraction may be selected for reduced extrafloral nectar production, ultimately reinforcing reproductive isolation among varieties.

Figure 3. A weevil puncturing the base of a fruit of C. desvauxii var. brevipes.