Essential genes encoded by the mating-type locus of the human fungal pathogen Cryptococcus neoformans
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Fungal sexual reproduction is controlled by the mating-type ( MAT ) locus. In contrast to a majority of species in the phylum Basidiomycota that have tetrapolar mating-type systems, the opportunistic human pathogen Cryptococcus neoformans employs a bipolar mating-type system, with two mating types ( a and α) determined by a single MAT locus that is unusually large (∼120 kb) and contains more than 20 genes. While several MAT genes are associated with mating and sexual development, others control conserved cellular processes (e.g. cargo transport and protein synthesis), of which five ( MYO2 , PRT1 , RPL22 , RPL39 , and RPO41 ) have been hypothesized to be essential. In this study, through genetic analysis involving sporulation of heterozygous diploid deletion mutants, as well as in some cases construction and analyses of conditional expression alleles of these genes, we confirmed that with the exception of MYO2 , both alleles of the other four MAT genes are indeed essential for cell viability. We further showed that while MYO2 is not essential, its function is critical for infectious spore production, faithful cytokinesis, adaptation for growth at high temperature, and pathogenicity in vivo . Our results demonstrate the presence of essential genes in the MAT locus that are divergent between cells of opposite mating types. We discuss possible mechanisms to maintain functional alleles of these essential genes in a rapidly-evolving genomic region in the context of fungal sexual reproduction and mating-type evolution.
IMPORTANCE
Sexual reproduction is essential for long-term evolutionary success. Fungal cell type identity is governed by the MAT locus, which is typically rapidly evolving and highly divergent between different mating types. In this study, we show that the a and α alleles of four genes encoded in the MAT locus of the opportunistic human fungal pathogen C. neoformans are essential. We demonstrate that a fifth gene, MYO2 , which had been predicted to be essential, is in fact dispensable for cell viability. However, a functional MYO2 allele is important for cytokinesis and fungal pathogenicity. Our study highlights the need for careful genetic analyses in determining essential genes, which is complementary to high-throughput approaches. Additionally, the presence of essential genes in the MAT locus of C. neoformans provides insights into the function, maintenance, and evolution of these fast-evolving genomic regions.
