|
The
genetics of virulence of the maize streak mastrevirus
(MSV)
R. Edema, D.T. Gordon & D.M. Bisaro
Maize Streak Mastrevirus (MSV) isolates differ in the severity of
symptoms they incite on maize. Virulent forms of the virus incite severe
stunting and chlorosis on infected plants and drastically reduce yield. Of major importance to breeding programs
is the knowledge of whether current resistance is durable when
challenged with the most virulent MSV isolates. However, the relative
virulences of MSV isolates across the geographic range of MSV is
unclear. A reliable method for identifying MSV virulent types would,
therefore, be of considerable value. We focused on developing a means of
identifying virulent MSV isolates based upon genetic variation. The genomes of four isolates were cloned and sequenced. Dimeric cloned
DNA of each isolate was infective when inoculated to susceptible maize
by vascular puncture inoculation. Clone symptom severity was classified
as mild (MSV-KL), moderately mild (MSV-Z), moderate (MSV-KH, or severe (MSV-Km).
Virulence determinants were mapped using
chimeric clones constructed from parental clones pMSV-KL (mild) and pMSV-Km
(severe) employing restriction endonuclease fragments. Complementary
exchange of restriction fragments between the genomes of pMSV-Km
(severe) and pMSV-KL (mild) demonstrated that determinants of virulence
were present in several regions of the genome, including those encoding
the coat protein, replication associated and replication initiator
proteins and the non-coding long and short intergenic regions. Analyses
of these sequences, along with MSV sequences from the GenBank for
isolates with reported symptom phenotypes, were conducted for motifs
associated with virulence. The analysis involved various on-line tools
and databases. Phylogenetic analyses of the amino acid sequences of the
replication-associated (Rep and RepA), movement and coat proteins failed
to reveal distinct African regional strains or symptom phenotype motifs.
It was concluded that MSV isolates from Africa exist as a common,
multi-allelic genome population without a simple genetics of virulence.
|
