Odds of compatibility among gametes of a diploid individual in basidiomycete fungi. Gametes are fully compatible only if they carry different alleles at both mating-type loci, the PR (including pheromone receptor and pheromone genes, with a1 and a2 alleles) and HD (including homeodomain genes, with b1 and b2 alleles) loci. (A) With PR and HD mating-type loci unlinked from each other and from the centromeres (shown here located in different chromosomes in blue and red), the percentage of compatibility of a given gamete among the other gametes produced by the same diploid individual is 25% across multiple meioses (a given gamete is compatible with one of every four gametes), and the percentage is 33% within tetrad (a given gamete is compatible with one of the other three gametes in the tetrad) or 67% (a given gamete is compatible with two of the three remaining gametes in the tetrad), depending on segregation of the mating-type alleles. The different types of gametes produced are tetratypes (TT), parental ditypes (PD), or nonparental ditypes (NPD), which depend on allele segregation and on whether a crossing-over occurred between one of the two loci and the centromere. (B) With PR and HD mating-type genes linked to the centromeres of different chromosomes (blue and red), the percentage of compatibility of a given gamete among the other gametes produced by the same diploid individual is 25% across multiple meioses but 67% within a tetrad (a given gamete is compatible with two of the three other gametes in the tetrad) due to the segregation of the variation occurring only at meiosis I for both mating-type loci. The different types of gametes produced are parental ditypes (PD) or nonparental ditypes (NPD), which depend on segregation. (C) With HD and PR loci fully linked to each other on the same chromosome, the percentage of compatibility of a given gamete among the other gametes produced by the same diploid individual is 50% across multiple meioses (a given gamete is compatible with one of every two gametes), and 67% within a single meiotic tetrad (a given gamete is compatible with two of the three other gametes in the tetrad). The light blue background shows the opposite cells of ordered tetrads, both in a Microbotryum linear tetrad representation (bottom) and in the different types of possible tetrads depicted depending on mating-type locus linkage.
