It was hypothesized and subsequently demonstrated that RNA viruses exist as a quasi-species. In essence, a quasi-species distribution describes a metastable equilibrium which will disintegrate and be replaced by another metastable equilibrium when a mutant appears that is more competitive. Thus, a quasi-species is defined as a mixture of viral sequences containing a predominant sequence and a variable mixture of related, but molecularly distinct, sequences. There is, however, little information on the plant viruses, yet this biological phenomenon raises a number of important questions: What is the relationship between a plant virus quasi-species distribution of molecular sequences and its fitness; How do these characteristics change as specific environmental conditions change, or perhaps change if the virus infects a different host; Can we observe evidence of adaptation at the sequence level; This proposal is aimed at obtaining information on sequence diversity in certain plant virus populations, and determining the frequency of mutations in these populations. Specific Aim 1 is to examine the sequence diversity of a virus population. It is hypothesized that 1) a virus population exists as a distribution of molecular sequences (i.e., a quasi-species), 2)the frequency of mutations is genome position dependent, and 3) the frequency of mutations will vary depending on selective constraints. Specific Aim 2 is to determine how the population structure changes over time. It is hypothesized that 1)the distribution of molecular sequences in the absence of selective conditions will remain relatively stable over time, 2) the distribution of molecular sequences in the presence of selective conditions, will change over time before reaching a new equilibrium, 3) adaptation of virus to new conditions will result in the same or similar changes in amino acid sequences in several lines studied simultaneously, and 4) there will be a measurable change in relative fitness in adapted virus populations. The overall goals are to obtain an understanding of virus variability in individual populations of virus, to determine if we can detect changes that may become fixed in this population over time, and to determine the effect of selective conditions. It is hoped that a fundamental understanding of potyvirus evolution will be obtained.