Sequence analysis of shorter than genome length episomal Banana streak OL virus like sequences isolated from banana in India

Abstract

Electron microscopy and sequencing of reverse transcriptase and ribonuclease H (RT/RNase H) region of Badnavirus genome from two banana cultivars: Poovan (triploid: AAB) and Safed velchi (diploid: AB), exhibiting leaf streak symptoms, confirmed the association of Banana streak OL virus (BSOLV). As per ICTV species demarcation threshold of 80 % identity in RT/RNase H region, both the isolates were identified as BSOLV. Rolling circle and end-to-end amplification showed the association of two short episomal BSOLV variants: BSOLV-IN1 and BSOLV-IN2 from Poovan and Safed velchi banana, respectively. The genome sizes of both isolates were 6,950 nucleotides long, but shorter than the typical BSOLV genome of 7,389 bp. Open reading frames (ORFs) 1 and 2 of shorter BSOLV isolates shared almost complete nucleotide identity (>99 %) to that of BSOLV. However, the ORF 3 (5,130 bp) and intergenic region (IGR), 886 bp, showed deletions compared with ORF 3 (5,499 bp) and IGR (956 bp) of BSOLV. In phylogenetic analysis for ORF 3 polyprotein, both the isolates clustered with BSOLV, Banana streak CA virus (BSCAV), and Sugarcane bacilliform GA virus (SCBGAV). Identical ORF 1, ORF 2, and the presence of all the conserved domains in short ORF 3 and promoter elements in IGR indicated that these isolates represent replicationally competent shorter variants of BSOLV. These two shorter-than-BSOLV genome sequences and two other identical banana streak virus sequences in GenBank (BSV–TRY; DQ859899 and BSV–GD; DQ451009) might have evolved due to error-prone reverse transcription and splicing or excision from the integrated sequences by homologous recombination in natural banana hybrids under field conditions.