Pollination ecology and genetic population structure of three species of cacao (Theobroma spp.) in Bolivia
Final Report Abstract
Tropical rainforests are characterized by a high level of tree species diversity and consequent low population densities and large distances between conspecifics. The spatial distribution of individuals has substantial consequences for the movement of pollinators and intraspecific gene flow, which affects sexual reproduction. As such, knowledge of gene flow is elementary for understanding the reproductive success and management of tropical tree species. In this study, different aspects of the sexual reproduction of wild and cultivated cacao (Theobroma cacao) populations and two wild relatives of cacao (T. speciosum and T. subincanum) were studied. These species are cauliflorus understory trees, with numerous small flowers and heavy fruits with fleshy edible seeds. To study the pollen flow of cacao, the degree of self-pollination and pollen dispersal distances were assessed as well as the level of gene flow between wild and cultivated cacao. The extent to which wild and cultivated cacao trees differ genetically was also explored. To identify cacao pollinators, we documented floral visitor assemblages on wild and cultivated cacao flowers. In addition, we assessed the genetic diversity and small-scale spatial genetic structure (SGS) of the three Theobroma species, while gene dispersal distances were inferred from the patterns of SGS. The results show that wild and cultivated cacao populations differ genetically. Wild populations showed lower genetic diversity than cultivated ones, probably because cultivated examples originate from a variety of cultivars. In contrast to prior studies, we found evidence for self-pollination in both wild and cultivated cacao, even though cross-pollination clearly predominates. Self-pollination events were more frequent in cultivated than in wild cacao populations. The overall mean pollen dispersal distance was 867 m. Pollen dispersal distances of wild cacao were recorded within a range of 10 m to 3007 m, whereas pollen of cultivated cacao was dispersed over distances of 13 m to 2360 m. The relatively high pollen exchange recorded from cultivated to wild cacao (20%) compromises the genetic identity of wild populations. Further, a wide range of flower visitors were documented with considerable differences being noted between the visitors of wild and cultivated cacao populations. The findings suggest that midges alone are probably too rare to act as the main or sole pollinators of cacao in our study region. Potential additional pollinators include small Diptera (e.g. Chloropidae and Phoridae) and Hymenoptera (e.g. Eulophidae and Platygasteridae). The wild relatives of T. cacao, T. speciosum and T. subincanum, showed relatively high gene diversity. Theobroma species displayed low but significant small-scale spatial genetic structure. T. speciosum showed the greatest pollen dispersal distance, suggesting a more efficient seed and pollen dispersal, whereas T. subincanum was the more structured species and had the lowest gene dispersal distance.
Publications
- 2013. Gene flow and genetic diversity in cultivated and wild cacao (Theobroma cacao L.) in Bolivia. American Journal of Botany 100(11): 2271-2279
Chumacero de Schawe, C., Durka, W., Tscharntke, T., Hensen, I., Kessler, M.
(See online at https://doi.org/10.3732/ajb.1300025)
