HELPER-DEPENDENT VECTOR TRANSMISSION OF PLANT-VIRUSES

A variety of noncirculatively transmitted viruses have evolved a vecto r transmission strategy that involves, in addition to virions, virus-e ncoded proteins that are not constituents of virions. These ''helpers' ' and the genes encoding them have been characterized for viruses in t he genera Potyvirus and Caulimovirus. Several lines of evidence suppor t the hypothesis that these helpers act by mediating retention of viri ons in regions of the vector's alimentary tract from which they subseq uently can be egested to initiate an infection. The possible advantage this convergently evolved strategy could confer to noncirculatively t ransmitted virus quasispecies is discussed.