Cassava: reconquering diversity
An international team has compared the cassava genome with that of its closest relatives. This work, which involved CIRAD, has lifted the veil on the history and genetic diversity of a plant that currently feeds 800 million people. It paves the way for work to improve yields via the development of more productive or disease-resistant varieties.
A vital but fragile crop
Cassava (Manihot esculenta), a member of the family Euphorbiaceae, originated in South America, and has been cultivated for almost 9000 years, which makes it one of the oldest known crops. It was exported to Africa by Spanish and Portuguese settlers, and has since become a major food resource, thanks to its starch-rich tubers. It is now found in numerous countries from sub-Saharan Africa to Southeast Asia, via the Indian subcontinent. However, the fact that it is propagated clonally, through cuttings, makes it susceptible to diseases.
"Cassava is an extremely important species for people in the South, particularly in Africa. However, very little has yet been done in the way of genetic improvement, despite the urgent need to produce new varieties tailored to climate change and new types of parasite pressure, particularly from viruses", Vincent Lebot, a CIRAD researcher who was part of the international team behind this work, explains. Establishing effective improvement programmes means knowing the genome of the target plant as precisely as possible. This is why, seven years after its sequencing in 2009, the cassava genome is still a hot topic for biologists.
Comparing so as to decipher better
The authors of this new study compared the reference genetic sequence for cultivated cassava with those of the genomes of 58 other wild or domesticated varieties from various countries, and other Euphorbiaceae such as castor, Hevea and manicoba (Manihot glaziovii), a Brazilian rubber tree. They also analysed the genome of 268 African varieties.
This work, which was published in the journal Nature Biotechnology, resulted in a top quality genome assembly that provides information on the chromosome location of 97% of cassava's 30 000 or so genes. Its analysis showed that numerous gene transfers have occurred in the past between manicoba and cultivated cassava. Those exchanges, which have boosted the genetic diversity of cassava and improved its resistance to viruses in particular, were the result not only of improvement programmes in Africa, but also of natural cross-pollination between the two species. Moreover, these results also showed that a South American species, as yet unidentified but related to cassava, contributed to its genome.
Responding to growing demand
It would therefore be in breeders' interests to look into crosses between cassava and related species, particularly since the genetic diversity of the varieties used in hybridization programmes has shrunk considerably, notably in Africa. Introducing genes from other species could serve to make cassava less vulnerable and boost yields. There is a burning necessity: since the early 2000s, production worldwide has increased by 100 million tonnes, driven in Asia by the demand for starch from the livestock production sector and industry, and in Africa by the emergence of new food markets resulting from urbanization. Furthermore, its high starch content is also of interest to agrofuel manufacturers. To satisfy that fast-growing demand, the millions of small-scale producers in tropical countries will have to intensify production.
More information:
Bredeson VJ et al. (2016) Sequencing wild and cultivated cassava and related species reveals extensive interspecific hybridization and genetic diversity. Nature Biotechnology, 2016; DOI: 10.1038/nbt.3535
Provided by CIRAD
