This past Friday, Elizabeth Borer of the University of Minnesota spoke at the weekly biology department seminar at Dartmouth College. Borer focused on the role of barley and cereal yellow dwarf viruses (BCYDV) in determining species dominance in California grasslands. More specifically, she called on her own “BigFert” experiment in California to outline how BCYDV enabled the invasion of California grasslands by exotic annual grasses.
BCYDV is a multi–host pathogen that is capable of infecting over 150 different species, including important cereal crops, as well as annual and perennial grasses. Borer noted that those species affected by BCYDV display variable susceptibility to the pathogen, and respond to infection with different symptoms. BCYDV is a “persistent viral infection of phloem” that is present only in the vegetative tissue of an infected plant. As such, the seeds are not infected, and there is no vertical transmission of the BCYD virus.
Borer called on this property of the BCYD virus in her BigFert experiment to help explain the differential successes of perennial and annual grass species in West Coast grasslands. The most important difference between perennial and annual grasses is the lifespan of individual organisms. Perennial grasses have a much longer lifespan, and retain their vegetative biomass anywhere from five to 100 years, while annual grasses only retain their biomass for one growing season. Since the BCYD virus is only present in the biomass of the plants it infects, annual grasses effectively rid themselves of the infection and start anew each growing season, while perennial grasses struggle because they are characterized by a much longer life cycle.
Before the 17th century, the California grasslands were almost entirely dominated by perennial grasses. Perennial grasses are better competitors for light, nitrogen, and water than annual grasses and were therefore far more successful than their annual counterparts. However, in the last 200 years annual grasses have established dominance on the West Coast, and have resisted even concerted efforts to increase perennial prevalence.
Borer suspected disease, especially BCYDV, to be the largest contributing factor to the recent successes of annual grass species. She noted that in environments unaffected by BCYDV, perennials invade and replace annual grasses. Conversely, in areas with high disease prevalence, annual grasses invade and dominate the natural system.
To support her hypothesis, Borer outlined the “BigFert” experiment, which spanned seven degrees of latitude along the Pacific Coast of the United States and included more than 5,100 individual grasses. In each of her test plots, Borer introduced three pairs of annual and perennial grasses side–by–side and observed the impact of the BCYD virus on each population. Through the course of the experiment, 25 percent of all individuals were infected with BCYDV. Borer observed a much higher prevalence of the virus in test plots with higher perennial grass cover.
Borer’s results in this experiment led her to conclude that “BCYDV could have enabled the invasion of California grasslands by exotic annuals” and that other similar invasions may also be controlled by disease. In the future, Borer hopes to apply her findings on a larger scale by beginning a similar experiment near her home in Minnesota to observe whether the impact of disease on species distribution is similar in different parts of the world.