Submitted to: Fungal Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2012
Publication Date: 2/14/2012
Citation: Bolton, M.D., Secor, G.A., Rivera, V., Weiland, J.J., Rudolph, K., Birla, K., Rengifo, J., Campbell, L.G. 2012. Evaluation of the potential for sexual reproduction in field populations of Cercospora beticola from USA. Fungal Biology. 116:511-521. Interpretive Summary: Cercospora beticola causes leaf spot disease of sugar beet. Although previous studies have shown that C. beticola populations in Europe have many characteristics of sexual reproduction, no sexual stage has been found for this fungus. Knowledge regarding sexual reproduction is important because sexually reproducing pathogens are more likely to defeat resistance genes or develop fungicide resistance than pathogens that are strictly asexual. We found that genes necessary for sexual reproduction called mating type genes were equally distributed in field populations, which suggests that sexual reproduction is occurring. Individual lesions always consisted of a single mating-type, but both mating-types could be found on a single leaf. We analyzed mating-type gene expression and found that the MAT1-2 gene turned on during late stages of colonization. Finally, we found evidence that a portion of the MAT1-2-1 gene was in MAT1-1-1 strains, which may provide clues on the evolutionary history of sexual reproduction in this fungus. Taken together, this study suggests that C. beticola has the potential for sexual reproduction.
Technical Abstract: Cercospora leaf spot, caused by the hemibiotrophic fungal pathogen Cercospora beticola, is the most economically damaging foliar disease of sugarbeet worldwide. Although most C. beticola populations display characteristics reminiscent of sexual recombination, no teleomorph has been described. To assess whether populations in northern United States have characteristics consistent with sexual reproduction, 1,024 isolates collected over a three-year period were analyzed for frequency and distribution of mating type genes. After clone correction, an approximately equal distribution of mating types was found for each sampling year. Mating type frequency was also assessed in individual lesions. Lesions always consisted of isolates with a single mating type and microsatellite haplotype, but both mating types and up to five microsatellite haplotypes could be found on an individual leaf. The MAT1-1-1 and MAT1-2-1 genes were sequenced from 28 MAT1-1 and 28 MAT1-2 isolates, respectively. Three MAT1-1-1 nucleotide haplotypes were identified that encoded a single amino acid sequence. For MAT1-2-1, five nucleotide haplotypes were identified that encoded four protein variants. MAT1-1-1 and MAT1-2-1 gene expression analyses were conducted on plants inoculated with either or both mating types. MAT1-1-1 expression remained low, but MAT1-2-1 spiked during late stages of colonization. A segment of the MAT1-2-1 coding sequence was also found in MAT1-1 isolates. Taken together, these results suggest that C. beticola has the potential for sexual reproduction.