The role of mating genes in the speciation processes of smut fungi
Final Report Abstract
Understanding speciation processes is still one of the most challenging research topics in biology. For highly specialized plant parasites, this includes the development of specific pathogen-host interactions as well as the genetic separation of populations by the establishment of breeding barriers. In this project, we analysed both aspects in the basidiomycete model organism Ustilago maydis and in closely related non-model species of the order Ustilaginales. In particular, our investigations concentrated on two different aspects, namely the intraspecific diversity of putative virulence factors and the specificity of the pheromone-pheromone receptor (PR) system. We analysed the intraspecific diversity of 33 putative virulence factors from 16 strains of a world-wide collection of U. maydis. In comparison to housekeeping genes, genes coding for secreted proteins of unknown function revealed a significantly increased intraspecific diversity. In general, the phylogeny of each of these genes reflects the large-scale population structure of U. maydis showing a clear separation of strains from Latin America from strains that originate from North America or Europe. The overall intraspecific diversity is lower than expected by the comparison of the same genes from U. maydis and Sporisorium reilianum. However, despite the young age of recent populations, we could identify a significant heterogenic distribution of variation within the studied virulence gene clusters 2A and 19A, which supports the polygenic nature of the U. maydis-Zea mays parasite host interaction. Furthermore our data might reflect differential selection of genes with different functions during the parasitic interaction of pathogen and host. Whether these genes were involved in coevolution by host tracking or even were differentially selected during maize domestication remains to be studied. Additionally, we focused on the specificity of the pheromone-pheromone receptor (PR) system and its genetic diversity in several Ustilaginaceae species spanning about 100 million years of mating type evolution. Our data suggest a triallelic PR system as the plesiomorphic character state for the Ustilaginaceae giving rise to convergent evolution of biallelic states. Moreover, the genetic diversity of the pheromone receptors is much lower than expected and interspecific mating assays revealed a high potential for hybridisation that is directly linked to pheromone signalling. Thus, interspecific sexual compatibility spans the whole range of tested species albeit hybrid filament formation was only observed within Ustilaginaceae. As the PR system rather diverged by genetic drift than by positive selection and sexual compatibility preserved throughout speciation events, hybridisation could have played an important role in the radiation of grass smuts. The comparison of genes involved in virulence and mating suggests positive selection of genes involved in parasite-host interaction. In other words, speciation seems to involve host specificity dependent factors as major drivers followed by genetic separation due to the establishment of breeding barriers. Although, our data shows a high degree of mating compatibility among Ustilaginaceae enabling hybridisation between even distantly related species, further studies are needed to understand the role of hybridisation in natural populations of U. maydis and other plant parasites.
Publications
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(2008) Koevolution – Zur Bedeutung organismischer Interaktion für die Evolution. In Betz & Köhler (Hrsg.) Die Evolution des Lebendigen. Attempto-Verlag. 173-186
Begerow D
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(2010) Current state and perspectives of fungal DNA barcoding and rapid identification procedures. Applied Microbiology and Biotechnology 87: 99-108
Begerow D, Nilsson H, Unterseher M, Maier W
