Identification of neutral mutants surrounding two naturally occurring variants of Potato spindle tuber viroid.
Single point mutations in the pathogenicity domain of Potato spindle tuber viroid (PSTVd) can have a dramatic effect on disease expression, and only three substitutions are required for the spontaneous conversion of the type strain PSTVd-Intermediate to the rapidly replicating, highly pathogenic variant RG1 (Gruner et al., Virology 209, 60-69, 1995). To identify available evolutionary pathways linking these two variants, we mutagenized five positions in an infectious cDNA copy of PSTVd-Intermediate and screened the resulting mixture of 768 sequences for neutral or near-neutral mutants. Numerical simulations based on the bioassay data indicate that the 23 variants recovered represent >80 % of all such sequences. RG1 was the only naturally occurring variant recovered, and the overall pattern of sequence changes observed indicates that PSTVd-Int occupies a comparatively steep peak within the fitness landscape.
Owens
R A
RA
Molecular Plant Pathology Laboratory, USDA/ARS,Beltsville, MD 20705, USA.
Thompson
S M
SM
Molecular Plant Pathology Laboratory, USDA/ARS,Beltsville, MD 20705, USA.
Kramer
M
M
Biometrical Consulting Service, USDA/ARS,Beltsville, MD 20705, USA.
D001483:Base Sequence; D018076:DNA, Complementary / Q000032:analysis; D014644:Genetic Variation; D008969:Molecular Sequence Data; D009154:Mutation; D011198:Solanum tuberosum / Q000821:virology*; D014772:Viroids / Q000235:genetics*
D018076:DNA, Complementary