Bsn-t alleles from french field strains of agaricus bisporus

Abstract

In the Agaricus bisporus desert population in California, the dominant Bsn-t allele determines the production of tetrasporic basidia and homokaryotic spores (n) that characterize a heterothallic life cycle. Strains belonging to a French population have the Bsn-b/b genotype that results in bisporic basidia that produce heterokaryotic spores (n + n) which characterize a pseudohomothallic life cycle. More recombination occurs in the tetrasporic population than in the bisporic population. In France, tetrasporic strains are rare. For two such isolates, Bs 261 and Bs 423, we determined the life cycle, the heritability of the tetrasporic trait, the amount of variation in the recombination rate, and the haploid fruiting ability. We found that (i) Bs 261 was heterothallic, (ii) Bs 423 was homokaryotic and homothallic, (iii) Bs 261 was Bsn-t/b, (iv) recombination on a segment of chromosome I depended on the genotype at BSN, (v) some of the homokaryotic offspring of Bs 261 and all of the progeny of Bs 423 were able to fruit, (vi) Bs 261 and Bs 423 were closely related, and (vii) Bs 423 was partially intersterile with other strains of the species.

FIG. 1

Schematic representation of the life cycles of A. bisporus. Heterokaryotic spores (n + n) receive two compatible meiotic products. Homokaryotic spores from heterothallic strains give rise to mycelia that are usually sterile but sometimes fertile in fruiting tests (3). During plasmogamy and following anastomosis, two homokaryotic and sexually compatible primary mycelia give rise to a heterokaryotic secondary mycelium. The three percentages given for the type of basidia are the mean percentages of basidia that produce, respectively, four or more spores, three spores, and one or two spores (5).

FIG. 2

Pedigrees of the hybrids. (A) First-generation hybrids derived from crosses between U 1-2 (a homokaryotic isolate produced via protoplasting from bisporic strain U 1) and SSIs from Bs 261. [T], tetrasporic; [B], bisporic. (B) Second-generation hybrids derived from crosses between U 1-2 and SSIs from the TT hybrid (a hybrid between the tetrasporic Bs 261 and JB 3 strains). (C) Hybrids derived from crosses between U 1-2 and either Bs 423 or Bs 423-2 (an SSI from Bs 423). Presumed BSN genotypes are indicated.

FIG. 3

Graphic representation of the distribution of the four basidial spore number variables and frequency distribution of ASN for 36 first-generation hybrids (Bs 261-x × U 1-2). The ASN values for progenitors Bs 261 and U 1 are 3.77 and 2.12, respectively. The bisporic and tetrasporic groups lie below and above, respectively, the ASN = 3.0 point. BSC, 3SC, and TSC are the percentages of basidia bearing, respectively, one or two spores, three spores, and four or more spores.