Genome Formula Changes During Whitefly Transmission of Two Bipartite Cassava Mosaic Begomoviruses

Cassava mosaic disease is caused by a complex of whitefly-transmitted begomovirus species, which often occur in mixed infections. These viruses have bipartite genomes consisting of DNA-A and DNA-B that are encapsidated into separate virions. Individual virus species exist in plants and whitefly vectors as populations comprising both genome segments, which can occur at different frequencies. Both segments are required for infection, and both must be transmitted for virus spread to occur. Cassava plants infected with both cognate segments of African cassava mosaic virus (ACMV) and/or East African cassava mosaic Cameroon virus (EACMCV) were used to examine how titers of the segments in a plant relate to their respective probabilities of acquisition by whiteflies and to the titers of each segment acquired and subsequently transmitted by whiteflies. The relationship between the DNA-A:DNA-B ratio in the plant and the relative likelihood of acquiring each segment differed between ACMV and EACMCV. However, for both viruses, DNA-A:DNA-B ratios acquired by whiteflies differed from those in the source plant and the ratios transmitted by the whitefly were “1” – the ratio at which the highest probability of transmitting both segments is expected.