We have sequenced the coat protein gene of nine isolates of papaya ringspot virus (PRSV) including six Australian and three Asian isolates and compared these with four previously reported sequences of PRSV. There was up to 12 % sequence variation between isolates at the nucleotide level. However, there was no significant difference between the sequences obtained from Australian isolates irrespective of whether they were PRSV type P (cucurbit or papaya infecting) or PRSV type W (cucurbit infecting) and these isolates were more closely related to one another than to any other isolate. These results imply that PRSV-P, first recorded in Australia in 1991, arose locally from PRSV-W (first recorded in Australia in 1978) rather than being introduced. Further, there was no consistent sequence difference between PRSV-P and PRSV-W isolates that would obviously account for their host range difference.

The potyvirus Papaya ringspot virus (PRSV) is found throughout the tropics and subtropics. Its P biotype is a devastating pathogen of papaya crops and its W biotype of cucurbits. PRSV-P is thought to arise by mutation from PRSV-W. However, the relative impact of mutation and movement on the structure of PRSV populations is not well characterized. To investigate this, we have determined the coat protein sequences of isolates of both biotypes of PRSV from Vietnam (50), Thailand (13), India (1) and the Philippines (1), and analysed them together with 28 PRSV sequences already published, so that we can better understand the molecular epidemiology and evolution of PRSV. In Thailand, variation was greater among PRSV-W isolates (mean nucleotide divergence 7n6 %) than PRSV-P isolates (mean 2n6 %), but in Vietnamese populations the P and W biotypes were more but similarly diverse. Phylogenetic analyses of PRSV also involving its closest known relative, Moroccan watermelon mosaic virus, indicate that PRSV may have originated in Asia, particularly in the Indian subcontinent, as PRSV populations there are most diverse and hence have probably been present longest. Our analyses show that mutation, together with local and longdistance movement, contributes to population variation, and also confirms an earlier conclusion that populations of the PRSV-P biotype have evolved on several occasions from PRSV-W populations.

Recently published evidence for sequence repetition in potyvirus genomes prompted us to analyse the published complete genome sequences and coat protein gene sequences of viruses of this family for evidence of replication slippage. Five of nine complete genomic sequences and 17 of 3 2 coat protein genes had significant sequence repetitions. Most of these were in coat protein genes, although the 5' region of the turnip mosaic virus genome also showed evidence of slippage. The results suggest that replication slippage may be involved in the evolution of viruses, as well as prokaryotes and eukaryotes, and that slippage can occur in both RNA and DNA when it is being replicated.

No abstract

No abstract