Abstract
Heterobasidion irregulare and H. occidentale are two closely related conifer root rot pathogens in the H. annosum sensu lato (s.l.) species complex. The two species H. irregulare and H. occidentale have different host preference with pine and non-pine tree species favored, respectively. The comparison of transcriptomes of H. irregulare and H. occidentale growing in Norway spruce bark, a susceptible host non-native to North America, showed large differences in gene expression. Heterobasidion irregulare induced more genes involved in detoxification of host compounds and in production of secondary metabolites, while the transcriptome induced in H. occidentale was more oriented towards carbohydrate degradation. Along with their separated evolutionary history, the difference might be driven by their host preferences as indicated by the differentially expressed genes enriched in particular Gene Ontology terms.
Highlights
In plant pathogens, selection has been shown to contribute to the diversification of genes involved in infection, colonization and specialization on host plants, such as genes involved in biosynthesis of secondary metabolites or toxins and genes encoding cell wall degrading enzymes[2,6,7]
The closely related species H. irregulare and H. occidentale, in the species complex H. annosum s.l., showed surprisingly small difference in gene content, a majority of genes could be found as one-to-one orthologous or in groups of gene families
The difference in enriched Gene ontology (GO) terms of gene expression in spruce bark between H. irregulare and H. occidentale indicate that the differences found are likely to be a result of adaption to their particular infection strategy
Summary
Selection has been shown to contribute to the diversification of genes involved in infection, colonization and specialization on host plants, such as genes involved in biosynthesis of secondary metabolites or toxins and genes encoding cell wall degrading enzymes[2,6,7]. Heterobasidion annosum s.l. consist of three European and two North American species with partly overlapping geographic distribution and host preferences[10]. After the Atlantic land bridge disappeared the two species H. annosum s.s. and H. irregulare evolved in allopatry for several millions of years in Europe and North America, respectively. While the ancestor of H. parviporum, H. abietinum and H. occidentale may originate in east Asia or west North America[10]. After original speciation between pine and non-pine infecting species, H. irregulare and H. occidentale have had a long period to evolve in allopatry before today’s overlapping geographic distribution in the western North America. To analyze the genetic bases for the difference in adaption we performed massive gene expression profiling using deep RNA sequencing of the two pathogens during infection of Norway spruce, a susceptible conifer host not native to North America
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