Cassava crop origin mystery ‘demystified’

MASEMBE TAMBWE

THE mystery of where the cassava crop comes from has been demystified thanks to a team of international scientists that has been studying DNA sequence information from cassava varieties grown all over the world.

According to a statement issued by the International Institute of Tropical Agriculture (IITA) yesterday, the scientists have also published a high-quality genetic material of the hardy crop, which will be very useful in efforts to improve cassava production.

“The development of genomic resources, such as this chromosome scale reference sequence, has increased understanding of the genetic diversity of cassava and its wild relatives and given insights into its population structure,’’ noted the statement.

It is expected this will accelerate progress in basic biological research and genetic improvement,” says Morag Ferguson, Molecular Geneticist at IITA-Kenya and one of the scientists in this effort and co-author of the paper.

Cassava is a very important food staple in sub-Saharan Africa, yet its yield and quality are compromised by two damaging virus diseases: cassava mosaic disease, causing disfiguration, curling, and yellowing of leaves, and cassava brown streak disease, causing severe browning in the storage roots.

It is further expected that the new DNA sequence resources will help scientists develop varieties that are resistant to these virus diseases.

The team sequenced 58 wild and cultivated cassava varieties, including related species such as Caera or Indian rubber (Manihot glaziovii), and genotyped 268 African cassava varieties. The team has published the results in the April issue of the journal Nature Biotechnology.

The study also provided clear evidence to support the hypothesis that cassava was domesticated from a wild species, known as M. esculenta subspecies flabellifolia, in the western part of the southern Amazon region in Brazil, and went through a “genetic bottleneck”, restricting the amount of genetic diversity in cassava. Many of the common cassava varieties in many countries were found to be nearly identical.

This reduction in cassava genetic diversity, especially in Africa, was a result of the use of a limited number of parents in the development of new varieties. It is important knowledge to guide breeding decisions to restore lost variation.

“We are excited about the results of this study. As a clonally propagated crop, genetic improvement of cassava through conventional breeding is a complicated and lengthy affair.

Therefore understanding the genetic diversity of cassava and its wild relatives will support genome-enabled breeding efforts,” says Mr Peter Kulakow, Head of IITA cassava breeding programme in Ibadan, Nigeria.

The study provided useful insights into cassava varieties in Africa and their breeding history. The scientists studied varieties from a past cassava breeding programme at Amani in Tanga Region, which was started under the Germans, closed during World War II and later re-opened by the British. It focused on breeding cassava for resistance to cassava mosaic disease, and later Cassava Brown Streak Disease.

Sequential information revealed that some of the popular cassava varieties in Tanzania, including Namikonga and Muzege, still contain sections of M. glaziovii in their genomes. These varieties are also among the most tolerant to CBSD.

Other popular varieties are related and are likely to have been derived from the Amani programme. These include Nachinyaya, Albert, and Kibaha.

The team comprised scientists from the IITA, the University of California, Berkeley, the United States Department of Energy Joint Genome Institute (DOE JGI), and from National Agricultural Research Organisations of Tanzania, Nigeria, Fiji, and Micronesia, among others