Nicklas Samils

Sex-linked transcriptional divergence in the hermaphrodite fungus Neurospora tetrasperma

In the filamentous ascomycete Neurospora tetrasperma, a large (approx. 7 Mbp) region of suppressed recombination surrounds the mating-type (mat) locus. While the remainder of the genome is largely homoallelic, this region of recombinational suppression, extending over 1500 genes, is associated with sequence divergence. Here, we used microarrays to examine how the molecular phenotype of gene expression level is linked to this divergent region, and thus to the mating type. Culturing N. tetrasperma on agar media that induce sexual/female or vegetative/male tissue, we found 196 genes significantly differentially expressed between mat A and mat a mating types. Our data show that the genes exhibiting mat-linked expression are enriched in the region genetically linked to mating type, and sequence and expression divergence are positively correlated. Our results indicate that the phenotype of mat A strains is optimized for traits promoting sexual/female development and the phenotype of mat a strains for vegetative/male development. This discovery of differentially expressed genes associated with mating type provides a link between genotypic and phenotypic divergence in this taxon and illustrates a fungal analogue to sexual dimorphism found among animals and plants.

Nuclear interactions in a heterokaryon: insight from the model Neurospora tetrasperma

A heterokaryon is a tissue type composed of cells containing genetically different nuclei. Although heterokaryosis is commonly found in nature, an understanding of the evolutionary implications of this phenomenon is largely lacking. Here, we use the filamentous ascomycete Neurospora tetrasperma to study the interplay between nuclei in heterokaryons across vegetative and sexual developmental stages. This fungus harbours nuclei of two opposite mating types (mat A and mat a) in the same cell and is thereby self-fertile. We used pyrosequencing of mat-linked SNPs of three heterokaryons to demonstrate that the nuclear ratio is consistently biased for mat A-nuclei during mycelial growth (mean mat A/mat a ratio 87%), but evens out during sexual development (ratio ranging from 40 to 57%). Furthermore, we investigated the association between nuclear ratio and expression of alleles of mat-linked genes and found that expression is coregulated to obtain a tissue-specific bias in expression ratio: during mycelial extension, we found a strong bias in expression for mat A-linked genes, that was independent of nuclear ratio, whereas at the sexual stage we found an expression bias for genes of the mat a nuclei. Taken together, our data indicate that nuclei cooperate to optimize the fitness of the heterokaryon, via both altering their nuclear ratios and coregulation genes expressed in the different nuclei.

Analyses of expressed sequence tags in Neurospora reveal rapid evolution of genes associated with the early stages of sexual reproduction in fungi

BackgroundThe broadly accepted pattern of rapid evolution of reproductive genes is primarily based on studies of animal systems, although several examples of rapidly evolving genes involved in reproduction are found in diverse additional taxa. In fungi, genes involved in mate recognition have been found to evolve rapidly. However, the examples are too few to draw conclusions on a genome scale.ResultsIn this study, we performed microarray hybridizations between RNA from sexual and vegetative tissues of two strains of the heterothallic (self-sterile) filamentous ascomycete Neurospora intermedia, to identify a set of sex-associated genes in this species. We aligned Expressed Sequence Tags (ESTs) from sexual and vegetative tissue of N. intermedia to orthologs from three closely related species: N. crassa, N. discreta and N. tetrasperma. The resulting four-species alignments provided a dataset for molecular evolutionary analyses. Our results confirm a general pattern of rapid evolution of fungal sex-associated genes, compared to control genes with constitutive expression or a high relative expression during vegetative growth. Among the rapidly evolving sex-associated genes, we identified candidates that could be of importance for mating or fruiting-body development. Analyses of five of these candidate genes from additional species of heterothallic Neurospora revealed that three of them evolve under positive selection.ConclusionsTaken together, our study represents a novel finding of a genome-wide pattern of rapid evolution of sex-associated genes in the fungal kingdom, and provides a list of candidate genes important for reproductive isolation in Neurospora.

Impact of the biological control agent Phlebiopsis gigantea on its resident genetic structure in the Baltic Sea area

Abstract The basidiomycete Phlebiop sis gigantea (Fr.) Jülich has been used in Swedish forestry as a biocontrol agent against the root and butt-rot pathogen Heterobasidion annosum s.l. on freshly cut Picea abies stumps since the early 1990s. Until 2005, the commercial preparation of this biological stump treatment, Rotstop®, has been based on a single strain of P. gigantea that has been applied on more than 47,000 ha annually in Fennoscandia. This paper reports on the spread of genetic material from the Rotstop® biocontrol strain of P. gigantea to resident populations of P. gigantea. We conclude that the inoculated fungus remained to a large extent restricted to the treated plots and did not spread to the adjacent areas, dominated by the natural spore deposition from resident populations of the fungus. Furthermore, the study demonstrates high genetic diversity and low geographic differentiation in P. gigantea populations in the geographical area around the Baltic Sea.

Microsatellite markers for the wood decay fungus Phlebiopsis gigantea

Nine polymorphic microsatellite markers were developed for the wood-decay basidiomycete Phlebiopsis gigantea, which is used commercially as a biocontrol agent for annosum root disease on conifers. Microsatellite sequences were isolated from repeat-enriched genomic libraries. Primers flanking these sequences were screened on P. gigantea isolates from Europe and North America. The number of alleles per locus ranged from 5 to 15, and gene diversity ranged from 0.72 to 0.90. These markers should be useful for studies of P. gigantea natural population structure and for making predictions about the impact of P. gigantea application in conifer forests.