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Title: Frequency of Sexual Recombination by Mycosphaerella graminicola in Mild and Severe Epidemics

Author
item Cowger, Christina
item BRUNNER, PATRICK - ETH ZURICH
item MUNDT, CHRIS - OREGON ST UNIVERSITY

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2007
Publication Date: 7/1/2008
Citation: Cowger, C., Brunner, P.C., Mundt, C.C. 2008. Frequency of Sexual Recombination by Mycosphaerella graminicola in Mild and Severe Epidemics. Phytopathology. 98:752-759.

Interpretive Summary: The fungus Mycosphaerella graminicola causes a wheat foliar disease of worldwide importance called Septoria tritici blotch. The fungus reproduces both sexually and asexually during the wheat growing season. Researchers have used mathematical modeling and field experiments to estimate the proportions of M. graminicola descendants that are sexual and asexual at the end of a growing season. In this study, field plots of the susceptible wheat cultivar Stephens were inoculated early in each of three successive growing seasons (1998-2000) with M. graminicola isolates that had rare alleles at two or three RFLP loci. Near harvest time, leaves were randomly sampled from the same plots, and a population of over 100 isolates was created each year. Natural populations were also sampled from noninoculated plots in two of the seasons. Using molecular markers, isolates from the inoculated plots were categorized as the same as the inoculated strains (inoculants), sexual offspring, or members of the natural population (immigrants). In 1998, a high-disease year, and 1999, a low-disease year, inoculants comprised 35%-36% and 23% of end-of-season samples, respectively. In those two years, sexual offspring as a percentage of all inoculant descendants were 30%-35% and 24%-27%, respectively. By comparison, an earlier study of M. graminicola reproductive structures, or fruiting bodies, had found 93% and 32% of fruiting bodies were ascocarps in 1998 and 1999, respectively. Our findings support the hypothesis that sexual recombination makes a relatively consistent contribution to M. graminicola population structure, despite year-to-year differences in epidemic severity. Our estimates of the proportion of the M. graminicola population that results from sexual recombination are in line with previously reported estimates from modeling and field research.

Technical Abstract: The importance of sexual recombination in determining fungal population structure cannot be inferred solely from the relative abundance of sexual and asexual spores and reproductive structures. To complement a previously reported study of proportions of Mycosphaerella graminicola ascocarps and pycnidia, we investigated the share of sexual recombinants among isolates randomly derived from the same field at the same time. Early in three successive growing seasons (those ending in 1998, 1999, and 2000), field plots of the susceptible winter wheat cultivar Stephens were inoculated with suspensions of two M. graminicola isolates that each had rare alleles at two or three RFLP loci. Near harvest time, leaves were randomly sampled from the same plots, and a population of over 100 monopycnidial isolates was created for each year of the experiment. Natural populations were also sampled from noninoculated plots in the 1999 and 2000 seasons. Based on RFLP haplotypes and DNA fingerprints, isolates from the inoculated plots were categorized by both inspection and Bayesian methods as inoculant clones, recombinants, or immigrants. Inoculation in the 2000 season was delayed, and the recovery rate of inoculant types was just 1%. In 1998, a high-disease year, and 1999, a low-disease year, inoculants comprised 35%-36% and 23% of end-of-season samples, respectively. In those two years, recombinants as a percentage of inoculant descendents were 30%-35% and 24%-27%, respectively. By comparison, the earlier study of fruiting bodies had found 93% and 32% of M. graminicola fruiting bodies were ascocarps in 1998 and 1999, respectively. These findings support the hypothesis that sexual recombination makes a relatively consistent contribution to M. graminicola population structure, despite differences in epidemic severity and ascocarp proportions.