Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2007
Publication Date: 12/31/2007
Citation: Covert, S.F., Aoki, T., O Donnell, K., Starkey, D.E., Holliday, A., Geiser, D.M., Cheung, F., Town, C., Strom, A.L., Juba, A., Scandiani, M., Yang, X.B. 2007. Sexual reproduction in the soybean sudden death syndrome pathogen Fusarium tucumaniae. Fungal Genetics and Biology. 44(8):799-807. Interpretive Summary: Sudden death syndrome (SDS) is an economically devastating disease of soybean, resulting in hundreds of millions U.S. dollar losses in North and South America annually. The primary objective of this study was to determine whether the two most important SDS pathogens, Fusarium tucumaniae and F. virguliforme, possess a sexual reproductive mode. To accomplish this objective, we conducted laboratory mating experiments by pairing different strains of each species on an agar medium. Results of these experiments showed that some paired strains of F. tucumaniae, but none of the F. virguliforme tested, were able to produce a sexual state in which abundant ascospores were present. Molecular genetic analysis of the ascospore progeny demonstrated that sexual recombination had occurred. These basic and applied studies will benefit plant breeders in the development of soybean varieties with broad-based resistance to the SDS pathogens.
Technical Abstract: The symptoms of soybean sudden death syndrome (SDS) include leaf chlorosis and necrosis, root rot, defoliation and death. Four members of the Fusarium solani species complex are known to cause these symptoms on soybean. Thus far, three of these pathogens have only been found in South America (i.e., F. tucumaniae, F. brasiliense, and an as yet unnamed species). The fourth SDS pathogen (F. virguliforme) has been found in Argentina and the U.S. Mating experiments pairing F. tucumaniae isolates in different combinations frequently were highly fertile, making it possible to assign mating type and assess female fertility in 24 representatives of the species. Genotyping of progeny from three separate F. tucumaniae crosses confirmed that sexual recombination had occurred in these mating experiments. The red, warty perithecia and two-celled, oblong-elliptical ascospores produced by F. tucumaniae demonstrate that it produces a sexual stage typical of the F. solani species complex. In contrast, pairings among 17 U.S. F. virguliforme isolates never produced perithecia. Inter-species crosses between F. tucumaniae and F. virguliforme, in which infertile perithecia were induced only in one of the two F. tucumaniae mating types, suggest that all U.S. F. virguliforme isolates are of a single mating type. We conclude that the F. tucumaniae life cycle in S. America includes a sexual reproductive mode, and thus this species has greater potential for rapid evolution than the F. virguliforme population in the U.S., which may be exclusively asexual.