Scientists unravel genome and diversity of whitefly vector behind viral diseases devastating cassava in Africa

14 June 2019

As part of efforts to control the spread of the two viral diseases attacking cassava in Africa, cassava mosaic disease (CMD) and cassava brown streak disease (CBSD)—scientists have announced a breakthrough whole genome sequencing of Bemisia tabaci, the African whitefly that is spreading the diseases.

Scientists unravel genome  and diversity of whitefly vector behind viral diseases devastating cassava in Africa
The African whitefly, Bemisia tabaci, on cassava leaves.

To develop the genome map of the African whitefly, the team drawn from IITA with partners at Cornell University, USA and the United States Department of Agriculture (USDA), collected cassava whiteflies from a single field in Chato, northwestern Tanzania, a region characterized by a super-abundance of whiteflies on cassava and severe CMD and CBSD epidemics. These were confirmed as sub-Saharan Africa-East and Central Africa (SSA-ECA) using single nucleotide polymorphism (SNP)-genotyping.

The Bemisia tabaci, a global pest and vector of damaging plant viruses in agriculturally important crops, is a complex species comprising many morphologically indistinguishable species. Cassava B. tabaci, confined to sub-Saharan Africa, specialize on cassava, and are rarely found on other host plants. Two other important B. tabaci species, Middle East-Minor Asia 1 (MEAM1) and Mediterranean genetic group (MED), colonize many host plants and have spread as invasive populations throughout the tropical regions of the world. They were the first B. tabaci species to have
their whole genomes sequenced. This adds African cassava B. tabaci to this list, making it the third whole genome to be sequenced for this complex species.

The draft genome assembled from Illumina short reads has 78% of core genes fully covered, which is comparable to figures for the other two B. tabaci genomes. It has a total size of 513.7 Mb and contains 15,084 predicted protein-coding genes. The SSA-ECA genome has fewer detoxification genes (509) than those of MEAM1 (667) and MED (680). This may be because SSA-ECA specializes on cassava and is not under pressure from insecticides, whereas the polyphagous MEAM1 and MED are under extreme pressure to evolve resistance due to heavy use of insecticides on their preferred annual crops. The divergence between MEAM1 and SSA-ECA appears recent and is estimated at 5.26 million years.

The findings of this paper have been published in the paper Genome of the African cassava whitefly Bemisia tabaci and distribution and genetic diversity of cassava-colonizing whiteflies in Africa, a draft published in the insect Biochem journal.

“This is a major step forward for whitefly research in Africa. The availability of the SSA-ECA genome resource presents important new opportunities for further research to underst and the evolution of cassava B. tabaci. It will be particularly interesting to unravel how these whiteflies adapted to cassava, a crop that cannot be colonized by the
other types of B. tabaci,” says IITA Senior Plant Health Specialist, James Legg.

He adds that underst anding the mechanisms, which allow these whiteflies to utilize cassava will help in the design and development of novel control strategies that could involve the deployment of products that disrupt these mechanisms.

Another area of focus will be to underst and the mechanisms responsible for virus acquisition and transmission. Disrupting these could reduce or eliminate virus transmission in cassava. Gene sequences obtained from the genome may also allow for the development of high affinity insecticides that target specific pathways in the whitefly, which would make them more environmentally friendly and less harmful to non-target insects/organisms.

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