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ARS Home » Midwest Area » West Lafayette, Indiana » Crop Production and Pest Control Research » Research » Publications at this Location » Publication #131772
Title: Cloning and analysis of the mating-type idiomorphs from the barley pathogen Septoria passerinii

Author
item Goodwin, Stephen - Steve
item WAALWIJK, CEES - PLANT RES INTL, NL
item KEMA, GERRIT - PLANT RES INTL, NL
item Cavaletto, Jessica
item ZHANG, GUODONG - PURDUE UNIVERSITY

Submitted to: Molecular Genetics and Genomics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2002
Publication Date: 3/11/2003
Citation: Goodwin, S.B., Waalwijk, C., Kema, G.H., Cavaletto, J.R., Zhang, G. 2003. Cloning and analysis of the mating-type idiomorphs from the barley pathogen Septoria passerinii. Molecular Genetics and Genomics. 269:1-12.

Interpretive Summary: Speckled leaf blotch of barley is an economically important disease caused by the fungus Septoria passerinii. This pathogen has no known sexual stage and is assumed to reproduce exclusively through asexual spores. One hypothesis to explain the lack of a known sexual stage for S. passerinii could be that it is a recent derivative of a sexual species with only one of the two mating types so has lost the ability to mate. To test this hypothesis, the mating-type region of S. passerinii was cloned, sequenced, and compared to that reported recently for its close relative Mycosphaerella graminicola. Surprisingly, both mating types of S. passerinii were found at approximately equal frequencies and often on the same leaf. Analyses with molecular markers revealed that each isolate had a unique genotype, which is expected for sexual, but not asexual, species. Comparative genetic analyses revealed that the mating-type genes of S. passerinii and M. graminicola are evolving rapidly, approximately ten time faster than control sequences. The common occurrence of both mating types on the same leaf and high levels of genetic diversity indicate that S. passerinii almost certainly is not an asexual derivative of M. graminicola. Instead, sexual reproduction probably plays an integral role in the life cycle of S. passerinii and may be much more important than believed previously in this and possibly other "asexual" species of Septoria. This information will be of great interest to scientists studying fungal mating types, evolution and speciation mechanisms, and to plant pathologists trying to manage the disease. Previous control strategies assumed that the fungus is asexual. Knowing that this is not correct will help plant pathologists develop more effective approaches to combat this disease.

Technical Abstract: One hypothesis to explain the lack of a known sexual stage (teleomorph) for species of the fungal genus Septoria could be that many of them are recent, asexual derivatives of sexual species that have lost the ability to mate. To test this hypothesis, the mating-type region of S. passerinii, a species with no known teleomorph, was cloned, sequenced, and compared to that reported recently for its close relative Septoria tritici (teleomorph: Mycosphaerella graminicola). Each of the S. passerinii mating-type idiomorphs was approximately 3 kb in size and contained a single open reading frame. Comparisons between S. passerinii and M. graminicola revealed that the mating-type genes of these species are evolving rapidly, approximately ten times faster than the internal transcribed spacer region of the ribosomal DNA. Phylogenetic analyses of mating-type gene sequences indicated that S. passerinii and M. graminicola are not closely related to Cochliobolus or other Loculoascomycetes in the order Pleosporales. A three-primer multiplex-PCR test allowed rapid identification of the mating types of isolates of S. passerinii from fields in Minnesota and North Dakota. Both mating types were present in approximately equal frequencies, often among multiple isolates from the same leaves. Analyses with isozyme and random amplified polymorphic DNA markers revealed that each isolate had a unique genotype. The common occurrence of both mating types on the same leaf and high levels of genotypic diversity indicate that S. passerinii almost certainly is not an asexual derivative of M. graminicola. Instead, sexual reproduction probably plays an integral role in the life cycle of S. passerinii and may be much more important than believed previously in this and possibly other "asexual" species of Septoria.