Reproductive biology of Canna species naturalized in southern Africa.
Sibiya, Mvana Simon.
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Invasive species have many deleterious effects, including ecological and economic impacts. Invasiveness of exotic plant species has been linked to various attributes, including reproductive traits such as potential for uniparental reproduction, co-option of pollinators in the invasive range, and hybridization with congeners. Therefore, studies of the reproductive biology of invasive plant species can provide invaluable information about the risks species may pose and assist in their management. Several taxa of Canna L. native to the Americas have been introduced into South Africa, but their modes of reproduction have remained largely unknown. In this study, I investigated the breeding systems, extent of inbreeding depression, pollinator assemblages, pollinator effectiveness, extent of pollen limitation, and hybridization potential in populations of Canna indica and Canna glauca in KwaZulu-Natal, South Africa. Results showed that C. indica is partially autogamous and self-compatible, whereas C. glauca is allogamous, but the orangeflowered form (OF) of this species is partially self-compatible while the yellow-flowered form (YF) of this species is fully self-compatible. Inbreeding depression was detected in self-fertilized progeny of C. indica, but not in C. glauca OF. I found that seeds of C. indica are sired readily by C. glauca YF and partially by C. glauca OF, and the two colour forms of C. glauca are completely interfertile. Despite the apparent adaptation for pollination by birds, I found that honeybees are the most frequent and important pollinators of all taxa. Sunbirds rob flowers of nectar without effecting pollination. In C. indica, I found that honeybees and social bees are frequent visitors. Honeybee pollination in C. indica resulted in significantly higher seed set than did autonomous selfpollination. Both forms of C. glauca are pollinated effectively by pollen-collecting honeybees. I found that C. indica experienced pollen limitation in two of the three years of study. Canna glauca YF experienced severe pollen limitation in one year of the study, whereas C. glauca OF did not experience pollen limitation in any of the study years. Both study taxa exhibit vegetative reproduction. The results of this study highlight the potential for honeybees to promote reproduction of invasive plant species adapted to other pollinators (primarily hummingbirds in this case). While C. indica is already a declared invasive species, this study suggests that C. glauca should also be prioritized in management programs due to its prolific seed production resulting from co-option of local bee pollinators, absence of inbreeding depression, and ease of hybridization with other species.