Frequent flyers, bottle gourds crossed the ocean many times

"Beginning in the 1950s we thought that bottle gourds floated across the ocean from Africa," said Logan Kistler, post-doctoral researcher in anthropology, Penn State. "However, a 2005 genetic study of gourds suggested an Asian origin."

Domesticated bottle gourds are ubiquitous around the world in tropical and temperate areas because, while they are edible when young, the mature fruit make ideal lightweight, waterproof, liquid-carrying vessels. They were popular in areas either before development of ceramics or where ceramics never developed.

The 2005 study looked only at targeted sequence markers that turned out not to be informative on the population level, said Kistler. Now we can do sequencing of the large single copy area of the plastid genome, which is about 86,000 base pairs of DNA, he added.

Unlike animals, which have two types of DNA -- nuclear and mitochondrial -- plant cells have three types of DNA -- nuclear, mitochondrial and plastid. Animal mitochondrial DNA is often used in genetic studies because it is completely inherited from the mother and changes very slowly, but in plants, mitochondrial DNA is prone to structural rearrangements, while often carrying a slow mutation rate. The DNA found in plastids such as the organelles that perform photosynthesis and create starch or pigments, is a better choice for plants because it does not recombine, but it mutates fast enough for population-level studies.

The researchers, who report their results in a recent issue of the Proceedings of the National Academy of Sciences, looked at 36 modern samples of bottle gourd and 9 ancient samples. They found that all the ancient bottle gourds from the Americas that were tested fell within the normal variation of the African bottle gourds and not the Asian bottle gourds.

"The best explanation for this is that they came directly from Africa," said Kistler. "However, we wanted to test the possibility that gourds did float from Africa to form New World populations."

Previous studies found that gourds do float in the oceans and that after long periods of time in the ocean they still have viable seeds. The researchers developed an ocean-current drift model that showed that wild African gourds could have simply floated across the Atlantic during the Late Pleistocene. They suggest that large mammals like the mammoth helped naturalized populations establish in the neotropics, because these large mammals were known to eat various members of the family that includes gourds. The seeds are found in ancient deposits of large mammal dung.

Bottle gourds in Africa exist today mostly in the domesticated form, with only small populations of the wild variety in Kenya and Zimbabwe. In the ocean drift model, the researchers looked at realistic, unrealistically conservative and optimistic scenarios. They divided the western coast of Africa by latitude and simulated 12 years of gourd release where one gourd per month entered the ocean in each latitude division. The shortest amount of time it took for a gourd to arrive was 100 days, with an average arrival time of about nine months.

According to the researchers, it is feasible that gourds did float across the Atlantic Ocean frequently. This is especially true of gourds growing near rivers that flow into the ocean.

"It wasn't one gourd that came over and gave rise to all New World gourds," said Kistler. "Populations show up in Florida and Mexico around 10,000 years ago and in Central America a little later."