The evolution of mating-type genes in the mushroom fungi (Homobasidiomycetes).

摘要

The separation of the individuals within a species into two sexes is one solution to ensuring outcrossing and recombination during reproduction. In organisms that do not have morphologically distinguishable sexes, such as most fungi, individuals are of different mating-types, and the mating-type of an individual is determined by one or more incompatibility loci. Although most fungi have only two sexes or mating-types, the mushroom fungi (homobasidiomycetes) have adopted a system with numerous mating-types within a species. The mushrooms are further distinguished from other fungi by their use of two unlinked mating-type loci (termed bifactorial incompatibility), which are often divisible into closely linked, but functionally redundant, subloci. Although most mushrooms have a bifactorial mating system, within the group numerous reversals to a single-locus (termed unifactorial) mating system have occurred.; How the tremendous variability of the mushroom mating-type loci is generated and maintained is a fascinating question, but the variation itself is a hindrance to the study of mating-type genes. Specifically, balancing selection on mating-type genes increases the amount of DNA sequence divergence between alleles, creating a problem in using standard approaches to clone these genes from novel species. In addition, the duplication of mating-type genes into subloci makes delimiting homologous loci difficult when comparing multiple species. In this thesis, I propose the use of conserved gene order for both cloning mating-type genes and studying their evolution. A strategy for cloning mating-type genes from mushrooms is explored by testing the cosegregation of the gene encoding for a mitochondrial intermediate peptidase (MIP) and the A mating-type locus (Chapter 1) and the cosegregation of a serine-threonine kinase (CLA4) with the B mating-type locus (Chapter 2). MIP is demonstrated to be tightly linked to the A mating-type locus throughout the history of the mushrooms. A combination of genetic crosses, DNA sequencing, and phylogenetic analyses are then used to describe the genetic architecture of the mating-type loci in three species: Pleurotus djamor (Chapter 3), Coprinellus disseminatus (Chapter 4), and Phanerochaete chrysosporium (Chapter 5). Two of these species are unifactorial, and information on mating-type organization in these species is used to address the question of what genetic changes happened during the switch from a bifactorial to a unifactorial mating system. There is a striking similarity in the mating system of these two fungi. Both species appear to have switched to determinining mating-types using only the homeodomain transcription factors of the A locus, while the pheromones and receptors of the B locus have not been lost but presumably have been co-opted to perform a novel function. By understanding the origins of the unifactorial mating system we can begin to understand how the A and B loci work in concert to coordinate sexual development in this remarkable group of fungi.