Genetic Variability of Maize Streak Virus (Msv) for Virulence and Resistance in Maize

Abstract

Maizestreakdisease (MSD), caused by Maize streak virus (MSV), reduces maize yields in an infectedmaize plant by up to 100% even in high potential zones. The geographical range of MSVis restricted to Africa and its neighbouring islands. Recurrent outbreaks of MSV have been reported in several Sub-Saharan African countries, including Kenya. The causes of outbreakshave inadequately been investigated and remain unresolved. A significant influence on the appearance of virus epidemics is virus diversity. Very few natural sources of resistance/toleranceamongst maize genotypes have been developed and therefore there is need to screen more genotypes for resistance/tolerance. The objective of this study was to survey and characterise MSV genetic diversity in Kenya, to understand the population genetic, phylogenetic, and phylogeographic characteristics of MSVs from Kenya and elsewherein Africa; characterize patterns of natural selection in the protein coding regions of maize streak virus so as to identify genomic sites that have undergone adaptive changes facilitatingthe infection of maize. To genetically characterise a streak causing mastrevirus fromLa Reunion isolated from maize and to evaluate the levels of resistance/tolerance of selectedmaize genotypes. The virus genomes were cloned using Phi29 DNA polymerase and sequencedby primer walking. These sequences together with those obtained from public sequencedatabases were subjected to various bioinformatic tools implemented in Molecular EvolutionaryGenetic Analysis (MEGA 5), Recombination Detection Programme (RDP 3) and datamonkey web server. Maize inbred genotypes were screened under controlled agroinoculationconditions using three cloned MSV genotypes on a split plot arranged on a completely randomised design. Disease severity in growth rooms was scored using a completely objective computer based image analysis system and the data subjected to Analysisof variance (ANOVA), and means separated by Duncan's multiple range test. The studyidentified seven recombinant lineages circulating in Kenya (MSV-Al-II, III, IV, V, VI, VII, XIII), and revealed geographical clustering of recombinant lineages, highlighting the importanceof the preventing movement of lineages between these regions and targeting breedingof maize varieties against the specific lineages in the regions. It also revealed that no particularMSV coding region is evolutionarily constrained however, breeding work can focus on nine sites identified as evolving under purifying selection distributed across its three genes (cp, mp & rep); and that the novel virus named: Maize Streak Reunion Virus (MSRV) is geneticallyunique and can pose a new threat to maize production. The Maseno University developedmaize genotypes: AB2, ABLEP and EX44/42-2 were found to possess degrees of MSV tolerance similar to previously described resistant genotypes such as Pan77 and are potentialsources of new MSV tolerance genes in Kenya.