Academic journal article Genetics

Roles for Receptors, Pheromones, G Proteins, and Mating Type Genes during Sexual Reproduction in Neurospora Crassa

Academic journal article Genetics

Roles for Receptors, Pheromones, G Proteins, and Mating Type Genes during Sexual Reproduction in Neurospora Crassa

Article excerpt

ABSTRACT Here we characterize the relationship between the PRE-2 pheromone receptor and its ligand, CCG-4, and the general requirements for receptors, pheromones, G proteins, and mating type genes during fusion of opposite mating-type cells and sexual sporulation in the multicellular fungus Neurospora crassa. PRE-2 is highly expressed in mat a cells and is localized in male and female reproductive structures. Δpre-2 mat a females do not respond chemotropically to mat A males (conidia) or form mature fruiting bodies (perithecia) or meiotic progeny (ascospores). Strains with swapped identity due to heterologous expression of pre-2 or ccg-4 behave normally in crosses with opposite mating-type strains. Coexpression of pre-2 and ccg-4 in the mat A background leads to self-attraction and development of barren perithecia without ascospores. Further perithecial development is achieved by inactivation of Sad-1, a gene required for meiotic gene silencing. Findings from studies involving forced heterokaryons of opposite mating-type strains show that presence of one receptor and its compatible pheromone is necessary and sufficient for perithecial development and ascospore production. Taken together, the results demonstrate that although receptors and pheromones control sexual identity, the mating-type genes (mat A and mat a) must be in two different nuclei to allow meiosis and sexual sporulation to occur.

DISTINCTION between self and nonself is a fundamental requirement for various biological events in eukaryotes, ranging from somatic growth and mate selection in fungi (Janeway and Medzhitov 2002; Aanen et al. 2010; Casselton and Feldbrugge 2010; Debuchy et al. 2010) to immune defense in vertebrates (Janeway and Medzhitov 2002; Boehm 2006). In heterothallic ascomycete filamentous fungi, non- self-recognition is important during vegetative growth and sexual reproduction (see reviews in Glass and Dementhon 2006; Aanen et al. 2010). During vegetative growth, fusions between cells are constantly formed, generating a network of interconnected hyphae that is important for cell-cell communication and homeostasis in an individual colony. Hyphal fusion between individuals that differ at certain loci results in rejection of heterokaryon formation, leading to programmed death of the fusion cell. This process is referred to as heterokaryon incompatibility and is regulated by genetic differences at het loci (Glass and Dementhon 2006; Aanen et al. 2010).

As opposed to self-fertile homothallic species, which do not require a mating partner to complete sexual reproduction, heterothallic fungi are self-sterile and are only able to mate with a haploid cell of opposite mating type. The genetic barrier and sexual identity of heterothallic strains are established by the mating type (mat) genes. mat genes encode transcriptional regulators that control the expression of many genes required for sexual compatibility and reproduction, including the mating-type-specific pheromone and its G-protein-linked receptor (reviewed by Debuchy et al. 2010). The mating process of heterothallic fungi is best understood in the yeast Saccharomyces cerevisiae (Kurjan 1992; Bardwell 2005). Morphologically identical haploid cells of opposite mating type (MATa and MATa) mutually stimulate each other by secreting small peptide pheromones that are detected by cell-surface receptors. The pheromone-receptor interaction activates a mitogen activated protein kinase (MAPK) cascade, triggering several cellular responses, including cell cycle arrest in G1, elongation toward cells of opposite mating type, and transcription of genes involved in cellular and nuclear fusion. Under appropriate environmental conditions, the resulting diploid cell then undergoes meiosis to form four haploid spores (Cross et al. 1988; Herskowitz 1988).

Neurospora crassa is a heterothallic filamentous fungus with two mating types, mat a and mat A. N. crassa undergoes a more complex process of asexual and sexual reproduction than unicellular yeasts (Raju 1992b; Springer 1993). …

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