Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68588, UNITED STATES OF AMERICA.
It has been established that RNA viruses should be genetically diverse, due to the high error rate of their RNA-dependent RNA polymerases and the lack of proofreading capabilities. Plant RNA viruses are not as genetically diverse as expected. Evolutionary factors, such as purifying selection and bottlenecks that favor genetic stability, might be affecting plant viral populations. Otherwise RNA virus populations, with their potential for extreme diversity, might acquire a lethal number of mutations leading to the collapse of the population.
Triticum mosaic virus (TriMV) populations maintained in a controlled greenhouse environment displayed genetic stability. The mutation frequency per nucleotide of TriMV protein 1 (P1) and coat protein (CP) cistrons was discovered to be half that of similar plant viruses. Variations within serial passaged TriMV populations were dominated by singletons within both cistrons examined. Thus, stochastic processes such as bottlenecking are impacting the populations observed. The founding inoculum type sequence was preserved throughout the serial passages therefore some level of genetic stability was being maintained.
The mutation frequencies observed within the CP cistron of two TriMV isolates within wheat, rye, barley, and triticale, were not significantly different, suggesting that the level of TriMV population diversity is maintained across these hosts. The lack of preference for any particular host examined suggests that both TriMV isolates are equally adapted. The variation of the P1 and CP regions of TriMV populations, horizontally transmitted by wheat curl mites, were determined. The alternative host, jointed goatgrass, caused no shift from the founding haplotype of the P1 or CP within the TriMV populations observed, indicating that the bottlenecks occurring within this host are similar to those within wheat. TriMV populations maintain genetic stability, even with the constraints of genetic bottlenecking by horizontal transfer by wheat curl mites.
The TriMV populations observed maintained genetic stability by the evolutionary forces of selection and genetic bottlenecking. This leads us to conclude that selection and drift are not exclusive, and may occur concurrently within a virus population. This research shows that the combined effect of both forces acting simultaneously on different regions of the genome ultimately regulates the degree of sequence variation within virus populations.