DNA as Clue

"In nature," says Swensen, "begonias — about 1,000 species — occupy tropical habitats on every continent except Australia, and most continents contain their own unique set of species. The begonias provide an interesting case study in how plants migrate to colonize new lands and how they diverge in the process. Our investigations are aimed at elucidating the evolutionary relationships among plants within each continent and among continental groups. We do this by comparing the DNA sequences of genes located in the plant cells. This information will help us understand the center of origin for these species and by what routes they came to be distributed around the globe."

That is not always an easy task. Swensen’s research has uncovered a fascinating biogeographic contradiction — upsetting conventional theories and requiring the most careful of biological sleuthing. It involves a native Hawaiian begonia called Hillebrandia sandwichensis, named after the Sandwich Islands, the original English name for the Hawaiian Islands.

Swensen explains the dilemma: "The Hawaiian Islands are relatively recent, formed about 15 million years ago by volcanic eruption, and all the plants found there were introduced from main-land floras. The endemic Hillebrandia, with its minute seeds, might have been a stowaway in the mud on a bird’s foot."

But, she points out, Hillebrandia is found nowhere else on earth. "The intriguing problem with Hillebrandia," Swensen says, "is that it appears to be the most ancestral of all the begonias, making its mainland predecessor either extinct or as yet undiscovered. But if the Hawaiian islands are ‘only’ 15 million years old, and Hillebrandia didn’t exist on the mainland before then, where did it come from?"

But, she stresses, the primary goal of the re-search is to understand the biogeography and systematics of the entire family, not just the Hillebrandia issue. "We want to know," says Swensen, "what the center of origin is for all the species of Begonia — right now it looks like Africa, according to molecular analyses — and where each of the begonia groups on the different continents likely came from. Our efforts at Ithaca College are focused on begonias found in the Americas, including Hawaii."