Reproductive ecology of buzz-pollinated Ouratea spectabilis trees (Ochnaceae) in Brazilian Cerrados

Abstract. Ouratea spectabilis is a ubiquitous tree species in the Brazilian savannas, or Cerrados, where it plays an important ecological role. We studied its anthesis phenology, pollination biology, pollen viability, and pollen tube growth, and executed a set of intra- and interspecific experimental crosses to determine its mechanisms of incompatibility and reproductive ecology. The species presents a specialized buzz pollination syndrome and is served by a small array of specialized pollinator species. It is a mostly self-incompatible species, and although self-pollination is possible, it strongly reduces fertility, with reproductive outputs for hand self-pollination similar to those of interspecific crosses with the co-generic species O. hexasperma. Incompatibility with another commonly co-occurring co-generic species, O. floribunda, was complete, with a null fruit set, as occurred for the autonomous apomixis tests. Our pollen tube growth observations indicate that incompatibility occurs at the style, and is thus pre-zygotic. All three Ouratea species presented very high pollen viability. Manual pollen supplementation did not increase seed sets. Nevertheless, even after excess manual pollen supplementation, seed-to-ovule ratios were only 30%. Such limits are common in stressful environments, and fruit production for most Cerrado species is reported to be regularly under those levels. The apparent ubiquity of this fertility limit among Cerrado species poses interesting ecological questions, such as the role of environmental stress on reproduction and the potential overproduction of ovules as an evolutionary strategy to deal with seed predation – questions which deserve further research in the future.