Spore behaviors reveal a category of mating-competent infertile heterokaryons in the offspring of the medicinal fungus Agaricus subrufescens

Abstract

Strain breeding is much less advanced in the edible and medicinal species Agaricus subrufescens than in Agaricus bisporus, the button mushroom. Both species have a unifactorial system of sexual incompatibility, a mating type locus tightly linked to a centromere, and basidia producing both homokaryotic (n) and heterokaryotic (n + n) spores. In A. bisporus, breeding is mainly based on direct selection among the heterokaryotic offspring and on hybridization between homokaryotic offspring. The parental heterozygosity is highly maintained in the heterokaryotic offspring due to suppression of recombination and preferential pairing in the spores of nuclei, each one per second meiotic divisions; such "non-sister nuclei" heterokaryons are fertile. In A. subrufescens, recent studies revealed that recombination is not suppressed and that nuclei from the same second meiotic division can also be paired in a spore that give rise to a "sister nuclei" heterokaryon in which the nuclei bear the same mating type allele. The objective of the present work was to investigate the potential function of the different categories of spores in A. subrufescens and their possible use in a genetic breeding program. Using eight co-dominant molecular markers, we found that half of the offspring of the A. subrufescens strain WC837 were heterokaryotic, one quarter of them being sister nuclei heterokaryons. These heterokaryons were infertile and behaved like homokaryons, being even able to cross between each other. In contrast, non-sister nuclei heterokaryons could fruit but inconsistently due to inbreeding depression. Potential roles of these two categories of heterokaryons in nature and consequences for strain breeding are discussed.

The Genetic Linkage Map of the Medicinal Mushroom Agaricus subrufescens Reveals Highly Conserved Macrosynteny with the Congeneric Species Agaricus bisporus

“…The availability of A. subrufescens EST catalogs as a source of molecular markers will greatly enhance the development of targeted loci related to specific pathway or traits of interest. Our results confirm also that the recombination events occur normally in A. subrufescens , as already suggested ( Thongklang et al 2014a ; Rocha da Brito et al 2016 ). Such recombination ability is of particular interest for mushroom breeding purposes since it will facilitate the creation of favorable allelic combinations in breeding schemes.…”

“…In the light of our results, the overall gene collinearity between the two species was evidenced, but local structural changes could not be completely excluded. Possible rearrangement has been already suggested in a previous mapping study based on another progeny analysis to explain the loss of synteny for the rDNA (ITS) locus chromosome assignment between the two species ( Rocha de Brito et al 2016 ).…”

“…The recent development of molecular markers and available expressed sequence tag (EST) resources have enriched our toolbox for studying the biology of this mushroom ( Foulongne-Oriol et al 2012c , 2014 ). The clarification of its amphithallic life cycle, and of the interfertility between field specimens from different continents ( Rocha de Brito et al 2016 ; Thongklang et al 2014a ), provide a sound basis to exploit the genetic diversity of this species, and to conduct controlled mating of selected genotypes.…”

Genetic Analyses of the Internal Transcribed Spacer Sequences Suggest Introgression and Duplication in the Medicinal Mushroom Agaricus subrufescens

“…subrufescens such as CA487, the basidia produce not only classical homokaryotic spores but also a variable percentage of heterokaryotic spores. Because at least 99% of the basidia were tetrasporic, it was hypothesized that ability to produce both types of spores was likely due to one additional round of post-meiotic mitosis occurring in a variable proportion of basidia and generating eight haploid nuclei [ 16 , 26 ]. In such basidia each basidiospore receive two nuclei would be heterokaryotic.…”

“…In a theoretical random tetrasporic model proposed by Rocha de Brito et al [ 26 ], the nuclei are paired at random and the expected loss rate of parental heterozygosity among the heterokaryotic offspring is 33%. This applies to any locus including the centromeres; subsequently, two-third receive non-sister nuclei (i.e.…”