Genotypic and Phenotypic Analyses of Hybridization in a Basidiomycete Yeast

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Hybridization is pervasive in nature, e.g. an estimated 25% of plants and 10% of animals are believed to be of hybrid origins. However, the patterns and genetic bases of hybridization and hybrid performance remain poorly understood. The long-term goal of this research is to understand how hybridization impact fungal population dynamics and evolution. The short-short objectives are to use the human pathogenic Cryptococcus species complex (CSC) as a model to investigate the frequencies of hybridization in nature (Aim 1); the phenotypic diversity of laboratory hybrids (Aim 2); the molecular bases for hybrid vigor (Aim 3); and the stability (Aim 4) and fertility of hybrids (Aim 5). CSC contains at least seven divergent haploid lineages. However, natural hybrids have been found, and laboratory hybrids can be created in CSC. AIM 1: The current CSC hybrids have been identified primarily based on a cell surface feature that only distinguishes four serotypes, and with an unknown genetic basis. Aim 1 will use discriminatory genetic markers and genomic tools to screen the >800 global CSC strains in our lab. Our data will robustly identify natural CSC hybrids, including their potential origins. AIM 2: How do the phenotypes of hybrids differ from parental strains? Is genetic divergence between parental strains positively correlated to phenotypic diversity among hybrid progeny? Here we will cross haploid strains from different CSC lineages and analyze both the genotypes and phenotypes of hybrids under select conditions. AIM 3: What are the genetic bases of hybrid vigor? How do environmental factors impact the transcriptome differently for progeny showing hybrid vigor and those that do not? We will analyze the genomes and transcriptomes of selected hybrids showing representative and extreme phenotypes. AIM 4: How stable are hybrids? Are hybrids derived from evolutionary more distant lineages more likely to be genetically unstable? Are certain chromosomes more unstable than others in hybrids? We will use experimental evolution to investigate the genetic and phenotypic stabilities of selected hybrids over thousands of mitotic generations. AIM 5: How interfertile are hybrids with each other and/or with parental strains? Inter-sterile with parental strains while interfertile with each other would be consistent with immediate hybrid speciation. The fertility pattern of CSC hybrids will be compared with their genotypes to help reveal the genetic bases for hybrid speciation. Our proposed research will generate significant new information about the patterns, mechanisms, and consequences of hybridization in CSC, contributing to novel understanding of fungal population dynamics and evolution and to training the next generation of microbiologists, geneticists, and infectious disease researchers in Canada.