Genetics, Vol 141, 503-512, Copyright © 1995


INVESTIGATIONS

Genetic Mapping and Non-Mendelian Segregation of Mating Type Loci in the Oomycete, Phytophthora infestans

H. S. Judelson, L. J. Spielman and R. C. Shattock
Department of Plant Pathology, University of California, Riverside, California 92521

DNA markers linked to the determinants of mating type in the oomycete, Phytophthora infestans, were identified and used to address the genetic basis of heterothallism in this normally diploid fungus. Thirteen loci linked to the A1 and A2 mating types were initially identified by bulked segregant analysis using random amplified polymorphic DNA markers (RAPDs) and subsequently scored in three crosses as RAPD markers, restriction fragment length polymorphisms (RFLPs), single-strand conformational polymorphisms (SSCP), cleaved amplified polymorphisms (CAPS), or allele-specific polymerase chain reaction markers (AS-PCR). All DNA markers mapped to a single region, consistent with a single locus determining both mating types. Long-range restriction mapping also demonstrated the linkage of the markers to one region and delimited the mating type locus to a 100-kb region. The interval containing the mating type locus displayed non-Mendelian segregation as only two of the four expected genotypes were detected in progeny. This is consistent with a system of balanced lethal loci near the mating type locus. A model for mating type determination is presented in which the balanced lethals exclude from progeny those with potentially conflicting combinations of mating type alleles, such as those simultaneously expressing A1 and A2 functions.


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