It is becoming increasingly evident that adoption of different reproductive strategies, such as sexual selfing and asexuality, greatly impacts genome evolution. In this study, we test theoretical predictions on genomic maladaptation of selfing lineages using empirical data from the model fungus Neurospora. We sequenced the genomes of four species representing distinct transitions to selfing within the history of the genus, as well as the transcriptome of one of these, and compared with available data from three outcrossing species. Our results provide evidence for a relaxation of purifying selection in protein-coding genes and for a reduced efficiency of transposable element silencing by Repeat Induced Point mutation. A reduction in adaptive evolution was also identified in the form of reduced codon usage bias in highly expressed genes of selfing Neurospora, but this result may be confounded by mutational bias. Potentially counteracting these negative effects, the nucleotide substitution rate and the spread of transposons is reduced in selfing species. We suggest that differences in substitution rate relate to the absence, in selfing Neurospora, of the asexual pathway producing conidia. Our results support the dead-end theory and show that Neurospora genomes bear signatures of both sexual and asexual reproductive mode.

Abstract Background The 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. Results In 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. Conclusions Taken 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.

The order Sordariales is taxonomically diverse, and harbours many species with different lifestyles and large economic importance. Despite its importance, a robust genome-scale phylogeny, and associated comparative genomic analysis of the order is lacking. In this study, we examined whole-genome data from 99 Sordariales, including 52 newly sequenced genomes, and seven outgroup taxa. We inferred a comprehensive phylogeny that resolved several contentious relationships amongst families in the order, and cleared-up intrafamily relationships within the Podosporaceae. Extensive comparative genomics showed that genomes from the three largest families in the dataset (Chaetomiaceae, Podosporaceae and Sordariaceae) differ greatly in GC content, genome size, gene number, repeat percentage, evolutionary rate, and genome content affected by repeat-induced point mutations (RIP). All genomic traits showed phylogenetic signal, and ancestral state reconstruction revealed that the variation of the properties stems primarily from within-family evolution. Together, the results provide a thorough framework for understanding genome evolution in this important group of fungi.