Evidence of multiplication of sowthistle yellow vein virus in an inefficient aphid vector, Macrosiphum euphorbiae

Abstract

The aphid Macrosiphum euphorbiae was able to acquire sowthistle yellow vein virus (SYVV) from infected sowthistle, Sonchus oleraceus, as readily as did the efficient vector, Hyperomyzus lactucae. After acquisition, the virus survived in an infective state in the hemocoel for the life of the aphid but was rarely transmitted to plants. The recovery of virus from M. euphorbiae at intervals after injection with infective hemolymph showed a low level of infective virus 1 day after injection followed by a much higher level 4 days later. The initial low level phase was more marked than in H. lactucae. Evidence for the multiplication of SYVV in M. euphorbiae was obtained by serial passage of the virus by injection without access to infected source plants. Four of fifteen lines established from aphids injected with hemolymph from viruliferous H. lactucae still contained virus particles after 16 passages. Unless virus multiplication is assumed, this was equivalent to a dilution of 10 −32 based on an estimated 100-fold dilution at each passage. Infectivity was lost after dilution in vitro to ca. 10 −3. The maintainence of an approximately constant median incubation period in test aphids suggested that the level of virus in the hemolymph of M. euphorbiae remained approximately constant at each passage. In three lines, H. lactucae assay insects inoculated with virus, passaged in M. euphorbiae beyond the seventh passage, did not transmit virus infective to test plants, although virus particles were readily detected by electron microscopy in both M. euphorbiae and nontransmitting H.lactucae.