A team of international scientists
that has been studying DNA sequence information from cassava varieties grown
all over the world have provided clear evidence on the ancestry of cassava.
They have also published a high-quality genome assembly of the hardy crop which
will be very useful in efforts to improve cassava.
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. The new DNA sequence resources will help scientists
develop varieties that are resistant to these virus diseases.
The team comprised scientists from the
International Institute of Tropical Agriculture (IITA), the University of
California, Berkeley, the United States Department of Energy Joint Genome
Institute (DOE JGI), and from National Agricultural Research Organizations of
Tanzania, Nigeria, Fiji, and Micronesia, among others.
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 paper is titled “Sequencing wild and cultivated
cassava (Manihot esculenta) and related species reveal extensive interspecific
hybridization and genetic diversity”.
“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. 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.
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. This 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 Peter Kulakow, Head of IITA cassava breeding program, Ibadan, Nigeria.
History
of the cassava breeding program in Africa
The study provided useful insights into
cassava varieties in Africa and their breeding history. The scientists studied
varieties from a past cassava breeding program in Amani, Tanga Province in
Tanzania, 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.
After screening cassava from many parts of
the world, they were unable to find varieties with high levels of resistance to
these diseases, so they started making crosses with wild species including M.
glaziovii and “tree cassava”. M. glaziovii, was introduced to
Tanzania during the time of German colonization for rubber production. These
trees now grow in the wild, particularly in coastal areas of Tanzania and are
an ancestor of “tree cassava” found in many homesteads and on the edge of
fields where their leaves are used as a vegetable.
Sequence 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 program; these include
Nachinyaya, Albert, and Kibaha.
When the Amani program closed, many of the
varieties were transferred to Mtwapa in Kenya and Moor Plantation in Nigeria.
IITA have used the varieties from Moor Plantation extensively in their breeding
programs. Evidence of this is shown by trace sections of M. glaziovii
DNA that were found in many IITA developed varieties. These sections are rare,
however, in farmer varieties from Southeast and Central Africa.
No comments:
Post a Comment