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Incompatibility Factors and Mating Competence of Two Laccaria spp. (Agaricales) Associated with Black Spruce in Northern Minnesota. R. L. Doudrick, Department of Forest Resources, University of Minnesota, St. Paul 55108; N. A. Anderson, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Phytopathology 79:694-700. Accepted for publication 20 January 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-694.

Sporocarps of Laccaria were collected in conjunction with a survey of potential ectomycorrhizal fungi associated with black spruce in northern Minnesota. Homokaryons were grown from single spores, and dikaryons were isolated from sporocarp tissue and from surface-disinfested ectomycorrhizae. Pairings of sibling homokaryons indicated a bifactorial (tetrapolar) sexual incompatibility system for all sporocarps. In only six of 37 cases were 15–22 homokaryons insufficient to find the four mating-type factors from a single sporocarp. All pairings between homokaryons obtained from sporocarps collected on peatlands with those from mineral soils were negative. Sporocarps collected in black spruce stands on peatlands were members of a freely interbreeding population, Laccaria laccata var. moelleri. Sporocarps collected on mineral soils represented several populations of Laccaria bicolor, with reduced outbreeding efficiency. The reduced outbreeding efficiency in L. bicolor (evident as a decrease in the number of clamp-connections produced in pairings of nonsibling sexually compatible homokaryons) apparently is due to a heterogenic incompatibility system, superimposed on the sexual incompatibility system. Preliminary evidence suggested a possible relationship between site-related factors for collections of L. bicolor identified by the survey and their mating competence. Di-mon pairings of L. l. moelleri and L. bicolor homokaryons with dikaryons isolated from black spruce ectomycorrhizae synthesized in aseptic culture indicated that precise genetic identification of the dikaryotic strains of both species was possible when known A and B mating-type factors were used as markers.