Abstract
Cassava is a major staple food for about 800 million people in the tropics and sub-tropical regions of the world. Production of cassava is significantly hampered by cassava brown streak disease (CBSD), caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). The disease is suppressing cassava yields in eastern Africa at an alarming rate. Previous studies have documented that CBSV is more devastating than UCBSV because it more readily infects both susceptible and tolerant cassava cultivars, resulting in greater yield losses. Using whole genome sequences from NGS data, we produced the first coalescent-based species tree estimate for CBSV and UCBSV. This species framework led to the finding that CBSV has a faster rate of evolution when compared with UCBSV. Furthermore, we have discovered that in CBSV, nonsynonymous substitutions are more predominant than synonymous substitution and occur across the entire genome. All comparative analyses between CBSV and UCBSV presented here suggest that CBSV may be outsmarting the cassava immune system, thus making it more devastating and harder to control.
Highlights
Serious yield losses due to cassava mosaic disease (CMD) were first observed on mainland East Africa in the 1920s6
It has been shown that plants of cassava brown streak disease (CBSD) tolerant or resistant cultivars graft-inoculated with Ugandan cassava brown streak virus (UCBSV) developed milder symptoms and a significantly higher proportion of the progenies were virus-free compared to those infected with CBSV27
CBSD Field Symptoms Associated with Cassava brown streak virus (CBSV) and UCBSV Isolates
Summary
Serious yield losses due to CMD were first observed on mainland East Africa in the 1920s6. In the early 2000s, outbreaks of CBSD were reported over 1000 km inland at mid-altitude locations (above 1000 m) in multiple countries all around Lake Victoria in Uganda[10], western Kenya[11] and northern Tanzania[4] Where it is already established in eastern Africa, the current CBSD epidemic prevails as the main cause of losses in cassava production. Symptoms of CBSD on cassava vary with cultivar, virus or plant age, but typically include leaf veinal chlorosis, brown stem lesions, as well as constrictions, fissures and necrosis of the tuberous storage roots[22,23] Overall, both CBSV and UCBSV cause similar symptom types, infection with CBSV tend to result in more severe sumptoms. It has been shown that plants of CBSD tolerant or resistant cultivars graft-inoculated with UCBSV developed milder symptoms and a significantly higher proportion of the progenies were virus-free (reverted) compared to those infected with CBSV27. The underlying reasons for this more aggressive nature of CBSV compared with UCBSV are not known
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