Raspberry ringspot virus has two RNA species, of mol. wt. about 2.4 × 10 (RNA-1) and 1.4 × 10 (RNA-2). In experiments with four naturally occurring strains, virus hybrids were made by mixing RNA-1 and RNA-2 preparations from different strains. Parent strains were regenerated by crossing appropriate hybrids. In the crosses, both serological specificity and transmissibility by the nematode were determined by RNA-2, suggesting that the protein surface of the virus particles is involved in the transmission process. Ability of isolates to cause systemic yellowing in , previously found to be also controlled by RNA-2, was shown to be associated with distinctive ultrastructural changes in the chloroplasts. Severity of systemic symptoms in and other herbaceous hosts, ability to infect Lloyd George raspberry and ability to invade the non-inoculated leaves of , were all determined by RNA-1. Both RNA species played a part in determining lesion type in inoculated leaves of and , and in some crosses the two kinds of hybrid were respectively less virulent and more virulent than either parent. The determinant for systemic symptoms in that is carried by RNA-2 was not expressed when in association with RNA-1 from the strain able to infect Lloyd George raspberry. Some genes of raspberry ringspot virus are probably pleiotropic.


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