RNA-RNA recombination was first described in cucumoviruses isolated from tobacco plants after multiple passage of a pseudorecombinant (reassortant) virus containing RNAs 1 and 2 from CMV, and RNA 3 from TAV (30) . The progeny RNA 3 consisted mostly of TAV RNA 3 but with the 3 0 terminal sequence from CMV RNA 2. In addition, in an earlier passage, there was an intermediate RNA 3 present containing a duplicated 21-nt sequence at the crossover site (30) . Subsequently, several other cucumoviral recombinant RNAs were also discovered in plants infected with a so-called quadripartite hybrid virus having RNAs 2 and 3 from CMV, and RNAs 1 and 2 from TAV (31) , with a mixture of wild-type (wt) CMV and wt TAV (32, 33) , with mixed CMV isolates (34) (35) (36) , or within a single wt CMV (37) . These characterized recombinant RNAs had non-templatederived nucleotides at the crossover site (31) and had CMV sequences at one end of the crossover site and TAV sequence at the other end (32) or had the same 3 0 -terminal NTR sequence for all three genomic RNAs (37) . Some of the recombinants had crossover sites localized within a short region of high sequence similarity (32) . Others were predicted to occur in regions of high secondary structure (33, 34, 38) . TAV RNA 3 itself has a direct repeat of 163 nt (differing by one nucleotide) in the 3 0 NTR, which is not essential for infection (39) and in some cases, was deleted during long-term incubation (40) . While some of the above studies examined recombinants formed in the directly inoculated leaves and thus avoided detecting only those replicable recombinants that might be selected (32, 33) , other studies examined those recombinants detected after slow selection (31, 38) and found that some recombinants could co-exist in the same plants (38) . However, what factors may have led to the selection of particular recombinants has not previously been examined.