VARIATION IN BIOLOGICAL PROPERTIES OF CAULIFLOWER MOSAIC-VIRUS CLONES

Infectous clones were prepared from virion DNA of three cauliflower mo saic virus (CaMV) isolates, 11/3, Xinjiang (XJ), and Aust, to investig ate pathogenic variation in virus populations. Of 10 infectious clones obtained for isolate 11/3, four pathotypes were identified, each prod ucing symptoms in turnip that differed from those of the 11/3 wild-typ e. Virus from two clonal groups of 11/3 was transmissible by aphids wh ereas that from two others was not. Of the five infectious clones obta ined from isolate XJ, two groups were identified, one of which differe d symptomatically from the wildtype. Only one infectious clone was obt ained from isolate Aust and this had properties similar to the wildtyp e. Restriction enzyme polymorphisms were found in some clonal groups a nd these correlated with symptoms. Other groups with different pathoge nic properties could not be distinguished apart by restriction site po lymorphisms. Further variation was observed in the nucleotide sequence s of gene II (coding for aphid transmission factor) from these viruses as compared with other CaMV isolates. In the aphid non-transmissible clones of isolate 11/3, one had a Gly to Arg mutation in gene II simil ar to that of other non-deleted non-transmissible CaMV isolates. The s econd had a 322 bp deletion at the site of a small direct repeat simil ar to that of isolate CM4-184 although occurring in a different positi on. The gene II deletion of isolate 11/3 produced a frame-shift that s eparated genes II and III by 60 bp. Most CaMV clones studied remained biologically stable producing similar symptoms during subsequent passa ges. However, one clone (11/3-7) produced two new biotypes during its first passage suggesting that it was relatively unstable. Our results show that wild-type populations of CaMV contain a range of infectious genome variants with contrasting biological properties and differing s tability. We suggest that a variety of significant viral phenotypic ch anges can occur during each infection cycle resulting from relatively small genome changes.