Rapid generation of genetic heterogeneity in progenies from individual cDNA clones of peach latent mosaic viroid in its natural host

Viroids, small single-stranded circular RNAs endowed with autonomous replic ation, are unique systems to conduct evolutionary studies of complete RNA g enomes. The primary structure of 36 progeny variants of peach latent mosaic viroid (PLMVd), evolved from inoculations of the peach indicator GF-305 wi th four individual PLMVd cDNAs differing in their pathogenicity, has been d etermined. Most progeny variants had unique sequences, revealing that the e xtremely heterogeneous character of PLMVd natural isolates most probably re sults from the intrinsic ability of this RNA to accumulate changes, rather than from repeated inoculations of the same individual trees under field co nditions. The structure of the populations derived from single PLMVd sequen ces differed according to the observed phenotype. Variant gds6 induced a re producible symptomatic infection and gave rise to a more uniform progeny th at preserves some parental features, whereas variant gds15, which induced a variable phenotype, showed a more complex behaviour, generating two distin ct progenies in symptomatic and asymptomatic individual plants. Progenies d erived from variants esc10 and Is11,which incited latent infections, follow ed a similar evolutionary pattern, leading to a population structure consis ting of two main groups of variants, one of which was formed by variants cl osely related to the parental sequence. The evolution rate exhibited by PLM Vd, considerably higher than that reported for potato spindle tuber viroid, may contribute to the fluctuating symptomatology of the severe PLMVd natur al isolates. However, the polymorphism observed in PLMVd progenies does pre serve some structural and functional elements previously proposed for this viroid, supporting the fact that they act as constraints limiting the genet ic divergence of PLMVd quasispecies generated de novo.