Comparison of random amplified polymorphic DNA markers and morphological characters in identification of homokaryon isolates of white button mushroom (Agaricus bisporus)

Distinguishing Homokaryons and Heterokaryons in Medicinal Polypore Mushroom Wolfiporia cocos (Agaricomycetes) Based on Cultural and Genetic Characteristics

The sclerotia of Wolfiporia cocos are a kind of traditional medicine and food with excellent benefits and are widely used in China, Japan, and other Asian countries. The mating system of fungi is not only of practical importance for breeding but also has profound effects on genetic variability and molecular evolution. However, the lack of clamp connections in W. cocos increases the difficulty of research on mating systems. In this study, homokaryons and heterokaryons were distinguished by comparing the characteristics of culture, fruiting tests, and molecular markers, which was further demonstrated by k-mer analysis based on Illumina sequencing. Uninucleate, binucleate, and nuclei-free condition basidiospores of W. cocos were observed, and binucleate basidiospores were the most predominant. Brown-type colonies, slow growth rates in both PDA medium and sawdust substrate, and neutral pH after the growth of mycelia and unfruiting were found to be the morphological and growth characteristics of homokaryotic strains. Primers SSR37 and 38 were screened to identify homokaryons. K-mer analysis based on Illumina sequencing exhibited different heterozygous ratios for homokaryons and heterokaryons. The results revealed that pseudo-homothallism was the predominant mode of reproduction in the Chinese population of W. cocos, and heterothallism also existed in all probability. This study will be helpful for the cross-breeding of this precious medicinal mushroom and for understanding its evolution and population structure.

Characterization of single spore isolates of Agaricus bisporus (Lange) Imbach using conventional and molecular methods

Strains A-15, S11, S-140, and U3 of Agaricus bisporus (Lange) Imbach, were used as parent strains for raising single spore homokaryotic isolates. Out of total 1,642 single spore isolates, only 36 single spore isolates were homokaryons and exhibited slow mycelial growth rate (≤2.0 mm/day) and appressed colony morphology. All these SSIs failed to produce pinheads in Petri plates even after 65 days of incubation, whereas the strandy slow growing SSIs along with parent strains were able to form the fructification in petriplates after 30 days. Out of 24, six ISSR primers, exhibited scorable bands. In the ISSR fingerprints, single spore isolates, homokaryons, lacked amplification products at multiple loci; they grow slowly and all of them had appressed types of colony morphology. The study revealed losses of ISSR polymorphic patterns in non-fertile homokaryotic single spore isolates compared to the parental control or fertile heterokaryotic single spore isolates.

Differentiation of homokaryons and heterokaryons of Agaricus bisporus with inter-simple sequence repeat markers

Morphologically different fifteen slow growing single spore isolates (SSIs) were screened from germinated basidiospores of Agaricus bisporus; assumed to be homokaryons, and subjected to growth rate, spawn run, fruiting and inter-simple sequence repeat (ISSR) analysis for confirmation of true homokaryons. The present studies are the first report of fingerprinting on differentiating homo- and hetero-karyotic SSIs using ISSR markers. The patterns were highly polymorphic and very reproducible. Among 40 primers tested, 7 ISSR primers were selected for the analysis of genomic DNA and generated a total of 54 ISSR fragments, sufficient to differentiate the 15 isolates from each other. ISSR fingerprinting detected 46.30% polymorphic loci. All appressed homokaryons carried a subset of ISSR markers found in the heterokaryons, and these were not able to produce a fruiting body. A test of cross-fertility and the following fruiting trial proved that 3 of the 15 SSIs with different ISSR fingerprints were homokaryons. None of the discarded SSIs have any missing bands present in the parental heterokaryotic control; these are heterokaryons derived from non-sister nuclear pairs. It is revealed that homokaryons are probably restricted to the assumed morphological classes of SSIs. These results demonstrate that ISSR markers provide an efficient alternate for identification of homokaryons and suggest these markers be considered as new tools for the survey of Agaricus species.