MBE Advance Access originally published online on November 17, 2007
Molecular Biology and Evolution 2008 25(1):168-178; doi:10.1093/molbev/msm253
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Research Articles |
The Evolution of the Pheromonal Signal System and Its Potential Role for Reproductive Isolation in Heterothallic Neurospora
Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden
E-mail: hanna.johannesson{at}ebc.uu.se.
Accepted for publication October 26, 2007.
Comparative sequencing studies among a wide range of taxonomic groups, including fungi, provide the overall pattern that reproductive genes evolve more rapidly than other genes, and this divergence is believed to be important in the establishment of reproductive barriers between species. In this study, we investigated the molecular evolution of the pheromone receptor genes pre-1 and pre-2 of strains belonging to 12 and 13 heterothallic taxa, respectively, of the model genus Neurospora. Furthermore, we examined the regulatory pattern of both pheromone precursor and receptor genes during sexual crosses of Neurospora crassa and Neurospora intermedia, for which reinforcement of interspecific reproductive barriers in sympatry previously has been documented. We conclude that the part encoding the C-terminal intracellular domain of pre-1 and pre-2 genes evolves rapidly. Both stochastic and directional processes drive this divergence; both genes contain neutrally evolving codons, and in addition, pre-1 contains codons evolving under positive selection, whereas in pre-2 we found highly variable regions with numerous repeats encoding glycine, threonine, or aspartic acid. In addition, we found regulatory changes of the pheromone and receptor genes during crosses between N. crassa and N. intermedia with different reproductive success. Gene expression levels are higher in the interspecific sympatric crosses with low reproductive success than in their intraspecific and/or allopatric equivalents, both at the stage of initial communication and contact and later at postfertilization stages. Taken together, our data indicate that pheromones and receptors are important key players during reproductive isolation between Neurospora species, and this study provides a general framework for future studies on the role of reproductive proteins for reproductive isolation.
Key Words: Neurospora • pheromone • pheromone receptor • reproductive protein • speciation • reinforcement
1 Present address: Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, Uppsala, Sweden.