A Mutant Defective in Sexual Development Produces Aseptate Ascogonia

Abstract

The transition from the vegetative to the sexual cycle in filamentous ascomycetes is initiated with the formation of ascogonia. Here, we describe a novel type of sterile mutant from Sordaria macrospora with a defect in ascogonial septum formation. This mutant, named pro22, produces only small, defective protoperithecia and carries a point mutation in a gene encoding a protein that is highly conserved throughout eukaryotes. Sequence analyses revealed three putative transmembrane domains and a C-terminal domain of unknown function. Live-cell imaging showed that PRO22 is predominantly localized in the dynamic tubular and vesicular vacuolar network of the peripheral colony region close to growing hyphal tips and in ascogonia; it is absent from the large spherical vacuoles in the vegetative hyphae of the subperipheral region of the colony. This points to a specific role of PRO22 in the tubular and vesicular vacuolar network, and the loss of intercalary septation in ascogonia suggests that PRO22 functions during the initiation of sexual development.The formation of fruiting bodies in filamentous ascomycetes is a process of multicellular differentiation controlled by many developmentally regulated genes. The self-fertile ascomycete Sordaria macrospora represents an excellent model for studying cell differentiation during fungal fruiting body development (reviewed in references 9 and 24).During the life cycle of S. macrospora, a mature haploid ascospore germinates and produces a mycelium composed of multinucleate hyphal compartments. After 2 to 3 days, coiled female reproductive hyphae, called ascogonia, are formed. Each ascogonium develops further into a more-or-less spherical protoperithecium composed of pseudoparenchymatous tissue surrounding the original ascogonium (24, 47). Ascogenous hyphae emerge from the ascogonium inside the protoperithecium. Within the ascogenous hyphae, the nuclei pair up to form the dikaryotic state, even though S. macrospora is selffertile and does not require fertilization with an opposite mating type (11,49). The transition from the spherical protoperithecial to the flask-shaped perithecial stage, is believed to be stimulated by the formation of the dikaryon, although this has not been experimentally verified (24). The dikaryotic state in individual hyphal compartments of the growing ascogenous hyphae is maintained by precisely orchestrated crozier formation from the tip of an ascogenous hypha, fusion of the crozier tip with the penultimate cell of the ascogenous hypha, conjugate mitotic divisions, and highly regulated septation (50, 69). Karyogamy of two nuclei in the penultimate cell results in the formation of a diploid ascus mother cell. The diploid state is very short-lived because the diploid nuclei which are formed immediately undergo meiosis, followed by an extra mitotic division resulting in an ascus containing eight haploid ascospores (11,49). An identical process has been observed in the heterothallic fungus Neurospora crassa, with the exception that cell fusion between opposit...

A Fungal Sarcolemmal Membrane-Associated Protein (SLMAP) Homolog Plays a Fundamental Role in Development and Localizes to the Nuclear Envelope, Endoplasmic Reticulum, and Mitochondria

“…All Sordaria STRIPAK components characterized so far are involved in sexual development, i.e., the formation of sexual fruiting bodies (perithecia) (35,56,57). We therefore analyzed the pro45 deletion strain for defects in the sexual cycle.…”

“…It can be hypothesized that a correctly assembled STRIPAK complex tethers PRO45 to the ER, while the localization of PRO45 to mitochondria might be STRIPAK independent. STRIPAK-independent functions of distinct STRIPAK subunits have been described for other systems (reviewed in reference 21), and one example is the function of S. macrospora PRO22, but not other STRIPAK subunits, in ascogonial septation (57). However, PRO45 might still need interaction with STRIPAK to carry out its function at mitochondria.…”

Identification and Characterization of Genes Required for Cell-to-Cell Fusion in Neurospora crassa

“…HAM-2 is a homolog of yeast Far11p, which has been shown to regulate the cell cycle during cell fusion events in mating yeast. The S. macrospora homolog of HAM-2, PRO22, is required for protoperithecium development and has been shown to be localized to the vesicular vacuolar network near the tips of the growing hyphae (5). HAM-3 is a homolog of striatin and Far8p and is a known cell fusion protein (12,47).…”

TORC2 Signaling Is Antagonized by Protein Phosphatase 2A and the Far Complex in Saccharomyces cerevisiae

“…In Neurospora crassa, a mutation in ham-2, the ortholog of yeast FAR11, leads to defects in hyphal fusion (Xiang et al 2002). In Sordaria macrospora, mutations in PRO22, encoding the yeast Far11 ortholog, generate a novel type of sterile mutant with a defect in ascogonial septum formation (Bloemendal et al 2010). In yeast, mutations in FAR9/ VPS64 and FAR11/YNL127w result in vacuolar sorting defects (Bonangelino et al 2002), and mutations in FAR3, FAR7, FAR8, FAR10, and FAR11 create long-lived mutants (Fabrizio et al 2010).…”