Pollination ecology and spatial genetic population structure of wild and cultivated species of cacao (Theobroma) in Bolivia

Abstract

Tropical forest in southwestern Amazonia is threatened by increasing human populations, economic exploitation, deforestation and burning activities for cattle raising and agriculture. Tropical rainforests are characterized by a high level of tree species diversity and consequently low population densities and large distances among conspecifics. The spatial distribution of individuals has substantial consequences for the movement of their pollinators and intraspecific gene flow, hence affecting sexual reproduction. Thus knowledge of gene flow is elementary for understanding the reproductive success and management of tropical tree species. In this study I explored different aspects of the sexual reproduction of wild and cultivated cacao (Theobroma cacao) and two wild relatives of cacao (T. speciosum and T. subincanum) in Bolivia. The hypothesis addressed were: 1) Wild and cultivated plants of T. cacao in Bolivia differ genetically; 2) Pollen dispersal distances of wild cacao trees are larger than in cultivated plants; 3) There is some level of gene flow between wild and cultivated plants of T. cacao; 4) Pollinator assemblages differ between wild and cultivated populations of T. cacao both with respect to taxonomic composition and abundance of floral visitors; 5) Small-scale spatial genetic structure of Theobroma species is moderate due to the dependence on animals for both, pollination and seed dispersal. For this purpose, three common species of the genus Theobroma of the lowlands of Bolivia were selected, Theobroma cacao L. (wild and cultivated), T. speciosum Willd. ex Spreng. and T. subincanum Mart. 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 estimated as well as gene flow between wild and cultivated cacao. I also explored whether wild and cultivated cacao trees differ genetically. To identify cacao pollinators I documented floral visitor assemblages on wild and cultivated cacao flowers. I further assessed genetic diversity and small-scale spatial genetic structure (SGS) of the three Theobroma species and gene dispersal distances were inferred from the patterns of SGS. The results show that wild and cultivated cacao differs genetically, suggesting that “wild” populations of cacao in lowland Bolivia might correspond to truly wild forms or at least to ancient cultivars. Wild populations showed lower genetic diversity than the cultivated cacao, probably because cultivated forms come from a variety of origins (e.g. Costa Rica, Trinidad and Tobago). In contrast to prior studies, I 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. The overall mean pollen dispersal distance was 867 m. Pollen dispersal distances of wild cacao was documented in a range of 10 m to 3007 m, whereas pollen of cultivated cacao was dispersed over distances of 13 m to 2360 m. Pollination distances in cacao were larger than those typically reported in tropical understory tree species, considering the minute sizes of the pollinating midges. The relatively high pollen exchange from cultivated to wild cacao (20%) found, compromises genetic identity of wild populations. Further, a wide range of flower visitors were documented as well as considerable differences between the visitors of wild and cultivated cacao. The findings suggest that midges alone were probably too rare to act as main or even sole pollinators of cacao in our study region. Potential additional pollinators would be 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 largest 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. In conclusion, improved management of cacao populations should consider the following points: the large pollen dispersal distances reported in cacao and the relatively high pollen exchange from cultivated to wild cacao pointed out the necessity for the protection of extensive natural forests tracts to protect the genetic identity of wild cacao in Bolivia and, in addition, to promote genetic exchange among wild populations to maintain the genetic variability of viable populations. The small-scale spatial genetic structure found in Theobroma pointed out the importance of the protection of seed dispersal animals to maintain the genetic resources of this tropical tree genus.