First report of Maize yellow mosaic virus infecting maize in Ecuador

Maize yellow mosaic virus (MaYMV) was first described in maize (Zea mays) from China and proposed as a new virus in the genus Polerovirus (Chen et al., 2016). Recently, MaYMV was also detected in symptomatic maize plants in Burkina Faso (Palanga et al., 2017) indicating a global distribution for the virus. In May 2016 maize plants showing leaf chlorosis and downward curling of leaf tips were collected from a large production area (c. 125,000 hectares in maize cultivation) in the Los Rios Province in Ecuador. It has been estimated that the disease causes yield losses of 10-30%. 

Total RNA from symptomatic leaf samples was extracted using the RNeasy Plant Mini Kit (Qiagen, USA) and subjected to deep sequencing analysis using the Illumina platform. Quality filtered paired end reads (8,777,028) were aligned with reference genomes of known plant viruses using standard bioinformatic tools. The transcriptome was highly enriched with viral RNA reads (>5%) showing significant similarity with Sugarcane mosaic virus (SCMV) and other MaYMV sequences. These short reads were assembled into contigs and full-length and nearly full-length genome sequences of SCMV (9632 nt; Genbank Accession No. K006657) and MaYMV (5488 nt; KY052793) were identified. Alignments and phylogenetic trees were performed using SeaView software (Gouy et al., 2010).

The Ecuadorian MaYMV isolate had the highest nucleotide identity (96%) with a Chinese isolate (KU291103). MaYMV has a typical Polerovirus organisation with six open reading frames (ORFs; 0-5) including ORF3a. ORF3a is translated from a non-standard start codon (in this case AUA) and encodes a protein responsible for long distance movement of the virus in plants (Smirnova et al., 2015). ORF3a (nt 3389-3523) shows a single amino acid substitution compared with the Chinese isolate (KU291103). ORF0 (nt 42-836) shows 97% amino acid identity with MaYMV isolate KU291100 (Table 1) and harbours an essential F-box like element (Pazhouhandeh et al., 2006). The phylogenetic tree (Fig. 1) constructed from ORF0 amino acid sequences indicates significant similarity to the Chinese MaYMV isolate and high variability between different species of the genus Polerovirus. Phylogenetic analysis of ORF3 from selected species of the genera Enamovirus, Luteovirus and Polerovirus places the Ecuadorian isolate within a group of poleroviruses that infect a wide range of host plants (Fig. 2).

MaMYV has been reported to be generally present in mixed infections with other viruses, including Maize chlorotic mottle virus, Southern rice black-streaked dwarf virus or SCMV, but has also been found in single infections in China (Chen et al., 2016) displaying characteristic disease symptoms. The significance of mixed infections with SCMV in disease pathology in Ecuador is unclear and requires further investigation.