Tomato yellow leaf curl virus in Australia: distribution, detection and discovery of naturally occurring defective DNA molecules

Tomato yellow leaf curl virus (TYLCV) was detected for the first time in Australia in March 2006 in field-grown tomatoes in Brisbane, Queensland. Surveys showed that the virus was confined to south-east Queensland. Virus transmission studies carried out using Bemisia tabaci (B biotype) verified that resistant tomato lines containing the Ty-1 or Ty-5 genes displayed tolerance to infection by TYLCV isolates from Australia. A PCR assay specific for TYLCV was designed and optimised to confirm the presence of the virus in samples that tested positive in begomovirus-specific double-antibody sandwich enzyme-linked immunosorbent assay. Eight isolates ofTYLCVfrom various sites were cloned and sequenced, and were shown to have near-identical sequences and a high nucleotide sequence similarity (
98%) to the monopartite Tomato yellow leaf curl virus-Israel (TYLCV-IL). No DNA-B, DNA-1 nor DNA-b satellite molecules were detected using degenerate PCR assays. Phylogenetic analysis revealed that Australian isolates of TYLCV separated into two sequence groups, TYLCV-IL[Au:Bri:06] and TYLCV-IL[Au:Bun:06], that showed a defined geographic segregation. Naturally occurring defective DNA molecules containing partial, rearranged segments of the native DNA-A, were present in one isolate. To our knowledge, this is the first report of an incursion of a begomovirus into Australia, and the first report of the characterisation of naturally occurring defective DNAs of TYLCV.