Genetic variability of the wild incompatibility alleles of the tetrapolar basidiomycete Agrocybe aegerita

EVIDENCE FOR LIMITED INTERCONTINENTAL GENE FLOW IN THE COSMOPOLITAN MUSHROOM, SCHIZOPHYLLUM COMMUNE

The genetic structure of populations of Schizophyllum commune was inferred from electrophoretic variation among 136 individuals at 11 polymorphic allozyme loci to determine the extent of geographic differentiation in this widespread mushroom species. The majority of the genetic variation was contained within populations; however, considerable genetic differentiation was observed among populations (global G_ST = 0.214). Clustering analysis demonstrated that genetic distance was correlated with geographic distance and that a large component of the genetic variation was due to allele frequency differences among populations from the eastern and western hemispheres. Our results also suggest that populations are large and geographically widespread. The lack of fixed genetic differences among intercontinental populations at any of the allozyme loci suggests that long-distance spore dispersal may counter the effects of genetic drift in this cosmopolitan species. These results are contrasted with a previous description of the same collection, in which the mating allele distribution of the species displayed no population substructure at any geographic scale (Raper et al. 1958). Broader implications of this study are that both species and mating allele distributions may not be correlated with long-distance gene flow in basidiomycete fungi.

Genetic evidence for nonrandom sorting of mitochondria in the basidiomycete Agrocybe aegerita

We studied mitochondrial transmission in the homobasidiomycete Agrocybe aegerita during plasmogamy, vegetative growth, and basidiocarp differentiation. Plasmogamy between homokaryons from progeny of three wild-type strains resulted in bidirectional nuclear migration, and the dikaryotization speed was dependent on the nuclear genotype of the recipient homokaryon. Little mitochondrial migration accompanied the nuclear migration. A total of 75% of the dikaryons from the fusion lines had both parental mitochondrial haplotypes (mixed dikaryons), and 25% had only a single haplotype (homoplasmic dikaryons); with some matings, there was a strong bias in favor of one parental haplotype. We demonstrated the heteroplasmic nature of mixed dikaryons by (i) isolating and subculturing apical cells in micromanipulation experiments and (ii) identifying recombinant mitochondrial genomes. This heteroplasmy is consistent with the previously reported suggestion that there is recombination between mitochondrial alleles in A. aegerita. Conversion of heteroplasmons into homoplasmons occurred (i) during long-term storage, (ii) in mycelia regenerated from isolated apical cells, and (iii) during basidiocarp differentiation. Homokaryons that readily accepted foreign nuclei were the most efficient homokaryons in maintaining their mitochondrial haplotype during plasmogamy, long-term storage, and basidiocarp differentiation. This suggests that the mechanism responsible for the nonrandom retention or elimination of a given haplotype may be related to the nuclear genotype or the mitochondrial haplotype or both.

Heterologous transformation of Agrocybe aegerita with a bacterial neomycin-resistance gene fused to a fungal promoter-like DNA sequence

DNA sequences of the basidiomycete Agrocybe aegerita were cloned in E. coli based on their ability to drive the expression of the bacterial promoterless tetracycline (Tc)-resistance gene. A 0.48% frequency of the cloned sequences promoted antibiotic-resistance. The sequence conferring the highest Tc resistance (40 μg/ml) was selected to drive the expression in E. coli of two other promoterless genes encoding chloramphenicol and neomycin resistance. One of the derivative vectors, pN13-A2, carrying a chimeric neomycin-resistance gene, was used to transform an A. aegerita neomycin-sensitive strain by protoplast electroporation. Transformation frequencies ranged from 1 to 2.8 transformants per μg of DNA per 10(3) viable cells, in a relatively high background of spontaneous-resistant colonies (2% of the surviving protoplasts). Molecular analyses showed that transformation had occurred by the integration of pN13-A2 sequences, either ectopically or at the resident locus carrying the A. aegerita promoter-like sequence, with probable molecular rearrangements. The nucleotide sequence of the promoter-like fragment revealed the presence of a CT motif that is known to be involved in a promoter function in some highly expressed genes of filamentous fungi.

Wide Distribution of Mitochondrial Genome Rearrangements in Wild Strains of the Cultivated Basidiomycete Agrocybe aegerita

We used restriction fragment length polymorphisms to examine mitochondrial genome rearrangements in 36 wild strains of the cultivated basidiomycete Agrocybe aegerita, collected from widely distributed locations in Europe. We identified two polymorphic regions within the mitochondrial DNA which varied independently: one carrying the Cox II coding sequence and the other carrying the Cox I, ATP6, and ATP8 coding sequences. Two types of mutations were responsible for the restriction fragment length polymorphisms that we observed and, accordingly, were involved in the A. aegerita mitochondrial genome evolution: (i) point mutations, which resulted in strain-specific mitochondrial markers, and (ii) length mutations due to genome rearrangements, such as deletions, insertions, or duplications. Within each polymorphic region, the length differences defined only two mitochondrial types, suggesting that these length mutations were not randomly generated but resulted from a precise rearrangement mechanism. For each of the two polymorphic regions, the two molecular types were distributed among the 36 strains without obvious correlation with their geographic origin. On the basis of these two polymorphisms, it is possible to define four mitochondrial haplotypes. The four mitochondrial haplotypes could be the result of intermolecular recombination between allelic forms present in the population long enough to reach linkage equilibrium. All of the 36 dikaryotic strains contained only a single mitochondrial type, confirming the previously described mitochondrial sorting out after cytoplasmic mixing in basidiomycetes.

Homologous transformation of the edible basidiomycete Agrocybe aegerita with the URA1 gene: characterization of integrative events and of rearranged free plasmids in transformants

The URA1 gene, encoding dihydroorotate dehydrogenase of the pyrimidine pathway, cloned into pUC18 (pUra1-1) was used to develop an homologous transformation system for the cultivated mushroom Agrocybe aegerita. Protoplasts of a ura1 auxotrophic strain were transformed by electroporation with efficiencies ranging from 1 to 26 transformants per micrograms of DNA. The phenotype of the stable Ura+ transformants suggested a strong nuclear heterogeneity further confirmed by Southern-blot analysis. All transformants acquired extrachromosomal forms derived from pUra1-1. Integration of pUra1-1 into chromosomal DNA occurred for some transformants. Plasmids containing the integrant of pUC18 recombined to different parts of the URA1 gene were rescued from A. aegerita transformants through transformation of E. coli. Their molecular analysis indicated that they represent products of the continuous excision of primary-integrated vector sequences rather than ARS-dependent autoreplicative forms.

Involvement of a large inverted repeated sequence in a recombinational rearrangement of the mitochondrial genome of the higher fungus Agrocybe aegerita

Southern hybridization of the total DNA of Agrocybe aegerita with cloned mitochondrial (mt) probes revealed a sequence homology between two distant mitochondrial restriction fragments. From the mtDNA restriction map and the distribution of restriction sites on the cross-hybridizing mitochondrial fragments, two copies of a large inverted repeated sequence (IR) of 3 kbp were located on the mitochondrial genome. These IR sequences divided the 80 kbp mtDNA into two single-copy regions of 24 kbp (SSC) and 50 kbp (LSC). For the first time in higher fungi, this IR sequence has been shown to be involved in an intramolecular homologous recombinational event. Such a rearrangement led to an inversion of the orientation of the two unique-copy regions, without any change in mtDNA complexity. The location of the recombinational event was compared with previously reported plant and fungal mitochondrial rearrangements and the potential role of the IR sequence was discussed.