However, these copies of plants, made over several generations, bring a major risk: a reduction in genetic diversity, which can in turn lead to the loss of the plant's adaptability to its environment. Nevertheless, Amerindian communities have managed to preserve the biodiversity of the cassava they have been growing for thousands of years. The research conducted by Doyle McKey's team thus centred on observing and modelling cropping practices in Amazonia.
Clonal and sexual propagation, not forgetting ants…
"We already had a hunch about these good cropping methods, as a result of studies by anthropologists", Doyle McKey says right away. What was needed were field observations. With his students, he thus spent time with cassava farmers in Guyana and French Guiana proving that the hunch was right. The Amerindians combine reproduction from cuttings (clonal propagation) and from seed (sexual propagation) in their cassava plots. They let plants obtained from seed grow among others obtained from cuttings, before choosing the most vigorous, which they then include in their clone reserve.
Ants play a far from negligible role too, spreading cassava seeds through the soil. This applies to both wild populations of parent plants and the Amerinidian cassava fields. "A seed bank develops a few centimetres below the soil surface and remains dormant during the fallow period," says Doyle McKey, "after slash-and-burn, the seeds from the previous crop cycle germinate and a new genotype may be added to the clone reserve".
In Africa and Vanuatu
This mixed cassava reproduction system led researchers from CIRAD and the CNRS along various lines of study: ethnobotany and ecology in the field, followed by genetics to characterize the existing diversity and understand its dynamics. Research is now under way in Africa to compare the Amerinidian results with those obtained on different soils and with other cropping practices. Have African cassava growers developed similar practices of incorporating the results of sexual reproduction?
Doyle McKey is due to launch a research project on cassava in Vanuatu in July 2007, in conjunction with Vincent Lebot, who knows the country and its plant diversity well. Again, the aim will be to study the know-how held by farmers, who have a long tradition of growing other clonally propagated plants such as taro and yam. The scientists involved hope to find clues to how to maintain biodiversity in areas in which clonally propagated crops have been introduced. As Doyle McKey concludes, "it is up to us to find the synergy between our various observations and develop the clonally propagated crop reproduction method".
Doyle McKey teaches at the University of Montpellier II and works with the CEFE (Centre d’écologie fonctionnelle et évolutive) at the CNRS and with the CIRAD "Management of Genetic Resources and Social Dynamics" Research Unit.
Vincent Lebot is a researcher with the CIRAD "Genetic Improvement of Vegetatively Propagated Crops" Research Unit.
Jean-Louis Noyer is a researcher with the "Polymorphisms of Interest in Agriculture" Joint Research Unit (UMR PIA)
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