Wednesday, August 8, 2007: 3:20 PM
San Carlos II, San Jose Hilton
What ecological factors are important in the early stages of the speciation process? Chamaecrista desvauxii (Leguminosae) is a neotropical subshrub that includes seventeen varieties. When two or more varieties occur within the same region, they often differ in ecological traits such as timing of flowering and fruiting, attraction of ants to extrafloral nectaries, and presence of herbivores. Our goal was to determine if interactions with herbivores, ants, and pollinators differ among co-occurring varieties in ways that could maintain them as separate evolutionary lineages, eventually leading to speciation. We studied these interactions in three sympatric varieties of C. desvauxii that are very common in a savanna area in Central Brazil, and often are found within less than a meter from each other. Although those plants are closely related, we established that leaves of the variety where ants are more active produce 7-fold more nectar than those of the variety with the lowest activity. We cut extrafloral nectaries and bagged developing fruits to quantify the relative importance of predispersal seed predators and ants for the reproductive success of each variety. Bagging fruits significantly increased fruit and seed set in all cases, but nectary excision only affected seed production in the variety for which nectar production and ant activity is highest. For the other varieties, nectar production represented a cost, thus individuals with low nectar production may have higher fitness. For two of the varieties there is some overlap in the identity of bees visiting flowers, but the third one is able to self-pollinate and rarely receives any visits. Crosses among varieties are ongoing but only one combination of pollen donor and recipient appears to yield fruits. Our results indicate that, despite the overlap in their distribution, those varieties are likely to represent independent lineages.
