AN EMPIRICAL STUDY OF THE EVOLUTION OF VIRULENCE UNDER BOTH HORIZONTAL AND VERTICAL TRANSMISSION

Abstract According to current thinking, a parasite's transmission mode will be a major determinant of virulence, defined as the harm induced by parasites to their hosts. With horizontal transmission, virulence will increase as a byproduct of a trade‐off between fitness gained through increased among‐host transmission(infectivity) and fitness lost through increased virulence. With vertical transmission, virulence will decrease because a parasite's reproductive potential will be maximized only by decreasing harm to te host, allowing parasite transmission to more host offspring. To test both predictions, we transmitted barley stripe mosaic virus (BSMV) horizontally and then vertically in its host, barley (Hordeum vulgare). After four genrations of horizontal transmission, we observed a nearly two fold increase in horizontal infectivity and nearly tripled virulence. After three generation of subsequent vertical transmission, we observed a modest (16%) increase in vertical transmissibility and a large (40%) reduction in virulence. Increased horizontal transmission is often due to increased pathogen replication which, in turn, causes increased virulence. However, we found no correlation between within‐host virus concentration and virulence, indicating that the observed changes in virulence were not due to changes in viral titer. Finally, horizontally transmitted BSMV had reduced vertical transmission and vertically transmitted BSMV had reduced horizontal infectivity. These two observations suggest that, in nature, in different host populations with varying opportunities for horizontal and vertical transmission, different viral strains may be favored.

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