1.11.31
THE RELATIONSHIP BETWEEN TRANSMISSION MODE AND VIRULENCE AMONG BARLEY STRIPE MOSAIC VIRUS STRAINS

SE KELLEY, AD STEWART and CM MORAYTIS

Dept. Biology, Emory University, Atlanta, GA, 30322, USA

Background and objectives
The mode of pathogen transmission has been hypothesized to affect the evolution of virulence. According to theory, pathogens with predominant vertical (parent to offspring) transmission will evolve reduced virulence, whereas pathogens with predominant horizontal (among unrelated individuals) transmission will evolve enhanced virulence [1]. This hypothesis is based on the assumption that with vertical transmission, enhanced virulence will reduce the fitness of pathogen strains, but with horizontal transmission, enhanced virulence will increase pathogen fitness, if higher viral titre causes increased horizontal transmission rates. Among strains of barley stripe mosaic hordeivirus, different seed transmission frequencies and symptom expression have been reported. But the relationship between transmission mode and virulence has not been elucidated. Other hypotheses do not predict a correlation between the relative frequency of vertical vs. horizontal transmission and virulence. Reduced virulence in vertically transmitted viruses may be caused by (i) bottlenecking effects, whereby mutants deleterious to viral fitness accumulate as a consequence of sampling effects when small viral populations are transmitted to seed offspring, or (ii) small viral populations in seeds inducing systemic plant defenses, which reduce the fitness consequences of the virus for the plant fitness.

Methods and materialsbr> Ten BSMV strains (PV 43, the type strain; PV 305, PV 306, PV 309, mild strains; PV 313, PV 314, PV 316, moderate strains; and PV 319, PV 320, PV 326, severe strains) were each inoculated into three barley varieties (Ga-acton, Black hulless and Dickson) using different inoculum concentrations. Horizontal transmissibility was estimated as the viral concentration which effected infection in 25% of inoculated plants. Vertical transmission was estimated as the proportion of infected seed offspring. Individuals of each of the barley varieties were either mechanically inoculated with a single BSMV strain or mock-inoculated with water. Virulence was estimated as the difference in total plant dry weight between infected and mock inoculated plants.

Results and conclusions
Preliminary results indicate considerable variation in horizontal transmissibility, vertical transmission, and virulence among the ten BSMV strains. However, the particular characteristics of the BSMV strain depended on the barley variety in which it was grown. The relationship between relative rates of vertical and horizontal transmissibility and virulence were complicated, which may indicate that reduced virulence in vertically transmitted viruses may arise (at least in part) as a result of either bottleneck effects in seed offspring, or the stimulation of plant defense responses by small seedborne viral populations.

References
1. Cooper JI, Kelley SE, Massalski PR, 1988. Advances in Vector Research 5, 221-49.