Abstract
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.
Highlights
Reproduction is an essential component of fitness and is subjected to strong selective forces
In this study we aim to describe O. spectabilis anthesis phenology and pollination ecology, and to experimentally study the intra- and interspecific reproductive barriers and mechanisms of incompatibility by controlled intraspecific crosses, self-pollination, and apomixis in O. spectabilis and through interspecific crosses with the close co-generic species O. hexasperma and O. floribunda
Pollen viability estimates were high for all three species, averaging 99.97 ± 0.03 % of viable pollen grains for all three species, which did not differ in their pollen viability rates (t = −0.772, df = 8, p = 0.469, t value derived from the division of the estimates by the standard error)
Summary
Reproduction is an essential component of fitness and is subjected to strong selective forces. A marked dry season, nutrient-poor soils, and periodic fires have been considered the determinants of this vegetation type (Oliveira and Marquis, 2002) It is the secondmost important vegetation type in Brazil, after the Amazon rain forests, and covers approximately one-fifth of Brazilian territory. Its flora is very rich, comprising more than 800 species of trees, with perhaps 4 times that number of herbs and shrubby species (Oliveira and Gibbs, 2000); the few studies on reproductive biology of savanna communities have covered less than 10 % of the species (Oliveira and Gibbs, 2002) Those savannah areas are characterized by a high beta diversity of plant species with a turnover of species from area to area (Bridgewaters et al, 2004)
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