|Goodwin, Stephen - Steve|
Submitted to: Fungal Genetics and Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/10/2006
Publication Date: 3/30/2007
Citation: Ware, S.B., Verstappen, E.C., Breeden, J., Cavaletto, J.R., Goodwin, S.B., Waalwijk, C., Crous, P.W., Kema, G.H. 2007. Discovery of a functional Mycosphaerella teleomorph in the presumed asexual barley pathogen Septoria passerinii. Fungal Genetics and Biology. 44:389-397. Interpretive Summary: Speckled leaf blotch of barley, caused by the fungus Septoria passerinii, is an economically important disease in regions with cool, wet summers. This fungus is known only as an asexual Septoria stage, even though recent evolutionary studies have shown that it is closely related to the wheat pathogen Septoria tritici, which has an active sexual stage in the genus Mycosphaerella. Furthermore, S. passerinii has two mating types, which have been found on the same leaves in North Dakota, and high genetic variation within populations, both of which indicate that it may be sexual. To test the hypothesis of an unknown sexual stage for S. passerinii, isolates of different mating type were crossed on barley leaves using the same protocol as for its relative S. tritici. Putative hybrid progeny were obtained from two crossing experiments in the Netherlands and the USA. These isolates were confirmed as hybrids by molecular markers that were present in both parents and that segregated according to Mendelian expectation in the progeny. The spores and sexual structures were as expected for the sexual genus Mycosphaerella, but were not preserved well enough to be named separately. These experiments confirmed that S. passerinii has an active sexual stage. This contradicts previous assumptions but explains the existing pattern of genetic variation within populations. Knowledge of the sexual stage of S. passerinii will be used by geneticists to make crosses and to develop this organism as a genetic system for comparative analyses with S. tritici from wheat. Plant pathologists will use this information to devise more effective disease-management strategies, as all previous control methods were designed using the incorrect assumption that the fungus was asexual.
Technical Abstract: We studied the possibility of a teleomorph associated with the genotypically diverse barley septoria speckled leaf blotch (SSLB) pathogen, Septoria passerinii. A teleomorph in the genus Mycosphaerella had been predicted previously based on phylogenetic analyses. Isolates with opposite mating types were co-inoculated onto susceptible barley cultivars, and leaves were monitored for the presence of sexual structures and for the discharge of ascospores. Ascospores from the hitherto unknown telomorph of S. passerinii were generated both in Europe (the Netherlands) and in the United States. Characterization of the segregating population was by both molecular (ITS sequencing, mating type PCR, RAPD, and AFLP) and phenotypic (spore morphology and ability to cause SSLB symptoms on barley) analyses. The sexual stage of S. passerinii has been discovered nearly 130 years after the description of the anamorph. A previously unknown sexual cycle explains the high degree of genetic variation among isolates found in nature. The rapid identification of a predicted teleomorph for S. passerinii indicates that cryptic sexual cycles may be common for thousands of other “asexual” fungi with high levels of genotypic diversity.