RNA viruses propagate as a population of genetically related
entities composing a quasi-species. Specific representatives
are the result of both a high mutation rate during replication
and competition between the continuously arising sequence
variants. Similar to other RNA pathogens, potato spindle
tuber viroid (PSTVd) propagates as a population of similar
but nonidentical sequences. The sequence of progeny molecules
derived from cloned molecular variants of PSTVd were studied
after one and six consecutive plant passages. Although
the severe parental sequence S23 was found to be genetically
stable, all five other parental sequences analyzed, irrespective
of their pathogenicity, led to the appearance of complex
populations. Divergence of the progeny was observed at
the sequence level, but also, more surprisingly, at the
level of the pathogenicity of individual progeny molecules.
In two cases, the parental sequence was retained in the
progeny population. In the other cases, it was completely
out-competed and eliminated, sometimes in as little as
one plant passage. Although it has been observed previously
that artificially mutated PSTVd molecules may revert rapidly
to the wild-type sequence, this study presents direct evidence
for the rapid evolution of naturally occurring PSTVd sequence
variants.