The ability to form a hyphal network is a hallmark of filamentous fungi. In filamentous ascomycete species such as Neurospora crassa, an individual hypha (a multinucleate, multicellular filament with incomplete crosswalls, or septa) grows by hyphal tip extension and branching. Behind the growing colony margin, fusions between hyphae are continuously formed (a process called anastomosis), yielding a network of interconnected hyphae, or mycelium, that makes up the fungal individual. Although the capacity to form a hyphal network is ubiquitous in filamentous fungi, little is known about the mechanism or function of an interconnected hyphal network. Our observations on the process of hyphal fusion by live cell microscopy (http://www.neurospora.org/ and (http://plantbio.berkeley.edu/~glass/Glasslab_site/hyphalfusion/movies.html) has revealed a complex and carefully regulated biological process.
Fig. 1: Left panel: Drawing of fungal colony (Buller 1931). Note differences in hyphal morphology in the periphery versus the interior of the colony. Right panel: Confocal image of hyphae at periphery of a colony (A) versus morphology of hyphae in the interior of a colony (B). f: indicates fusion events (From Hickey et al., FGB 2002).
Hyphal fusion
While hyphae at the periphery of a fungal colony avoid each other, hyphae involved in anastomoses in the interior of a colony show attraction and fusion. Chemotropic interactions between fusion hyphae often results in the initiation of new hyphal tips (pegs) in response to the proximity of a fusion. The nature of the self-signaling chemotropic ligand for hyphal fusion is not known, nor is it understood how signaling results in the initiation of fusion pegs. Live-cell imaging of hyphal homing and fusion in Neurosporea has shown that these processes are intimately associated with dynamic behavior of the Spitzenkörper. Fusion is often associated with dramatic alterations in cytoplasmic flow and organelles, including nuclei, pass through fusion pores. Such dramatic changes in cytoplasmic flow and movement of organelles suggest that the filamentous fungi must adapt to the physiological consequences of hyphal fusion within a fungal colony. Adaptation to hyphal fusion events or the physiological consequences of the failure to form a hyphal network are virtually uncharacterized in filamentous fungi.
Fig. 2. Hyphal fusion events a colony of Neurospora crassa. Colony stained with FM4-64. (From Hickey et al., 2002)
Germling fusion
In addition to anastomosis within a mature fungal colony, cell fusion events occur during conidial germination in filamentous ascomycete species (Fig. 2). Germinating conidia often produce specialized hyphae called conidial anastomosis tubes (CATs) which fuse with each other to establish a hyphal network that rapidly develops into a colony. Fusion events are also required during sexual reproduction (Fig. 3). Whether common or different signaling and fusion machinery is required for each of these cell fusion events in the lifecycle of filamentous fungi is unclear. Hyphal anastomosis in filamentous fungi is comparable to somatic cell fusion events in other eukaryotic organisms. Examples include somatic cell fusion events resulting in syncytia formation (e.g. between myoblasts during muscle differentiation), fusion between osteoclasts in bone formation, and also in placental development. Although molecular mechanisms of sexual fusion (e.g. between Saccharomyces cerevisiae cells of opposite mating types) have been well characterized, molecular mechanisms associated with fusion between somatic cells in eukaryotes are not as well analyzed. Understanding the molecular basis of hyphal fusion during vegetative growth in filamentous fungi provides a paradigm for self-signaling mechanisms in eukaryotic microbial species, and may also provide a useful model for somatic cell fusion events in other eukaryotic species.
Fig. 3. Fusion events between germinating conidia of Neurospora crassa of identical genotype.
Molecular and genetic analysis of germling and hyphal fusion
We have characterized a number of hyphal and germling fusion mutants in N. crassa. A MAP kinase signal transduction pathway is essential for the initiation of both germling and hyphal fusion (Pandey et al., 2004). In addition, we identified a WW protein, SOFT, which is required for chemotropic interactions during germling fusion (Fleissner et al., 2005) and a putative transmembrane protein, HAM-2, which is required for both hyphal and germling fusion (Xiang et al., 2002). Our current research objectives are to identify the receptor and ligands associated with self signaling, understand how chemotropic interactions result in polarization of cytoskeletal elements and the relationship between the MAP kinase pathway, so and ham-2.
Fig. 4. Model for hyphal fusion in Neurospora crassa
Fusion publications |
Rasmussen, C. G. and N. L. Glass, 2007. Localization of RHO-4 indicates differential regulation of conidial versus vegetative septation in the filamentous fungus Neurospora crassa Eukaryot Cell 6:1097-107. Click here for abstract
Fleissner, A. and N. L. Glass, 2007. SO, a protein involved in hyphal fusion in Neurospora crassa, localizes to septal plugs. Eukaryot Cell 6:84-94. Click here for abstract
Fleissner, A. and N. L. Glass, 2006. Re-wiring the network: understanding the mechanism and function of anastomosis in filamentous ascomycete fungi. In The Mycota I: Growth Differentiation and Sexuality. Eds. U. Kües and R. Fischer, Springer-Verlag, Berlin Heidelberg, pp.123-139.
Rasmussen C. G and N. L. Glass, 2005. A Rho-Type GTPase, rho-4, is required for septation in Neurospora crassa. Eukaryot Cell. 4:1913-25. Click here for abstract
Fleißner, A., S. Sarkar, D. J. Jacobson, M. G. Roca, N. D. Read and N. L. Glass (2005) Identification and characterization of so, a hyphal fusion mutant of Neurospora crassa. Eukaryot Cell 4:920-30. Click here for abstract.
Pandey A., Roca M.G., Read N.D. and N.L. Glass (2004) Role of a mitogen-activated protein kinase pathway during conidial germination and hyphal fusion in Neurospora crassa. Euk. Cell 3: 348-358. Click here for abstract.
Glass N.L., Rasmussen C., Roca M.G. and N.D. Read (2004) Hyphal homing, fusion and mycelial interconnectedness. Trends Microbiol. 12:135-141. Click here for abstract.
Kroken S, Glass N.L., Taylor J.W., Yoder O.C. and B.G. Turgeon (2003) Phylogenomic analysis of type I polyketide synthase genes in pathogenic and saprobic ascomycetes. Proc Natl Acad Sci USA 100:15670-15675. Click here for abstract.
Galagan JE, Calvo SE, Borkovich KA, Selker EU, Read ND, Jaffe D, FitzHugh W, Ma LJ, Smirnov S, Purcell S, et al., (2003) The genome sequence of the filamentous fungus Neurospora crassa. Nature 422:859-68. Click here for abstract.
Hickey, P. C., D. J. Jacobson, N. D. Read and N. L. Glass (2002) Live-cell imaging of vegetative hyphal fusion in Neurospora crassa. Fungal Genet. Biol. 37:109-119. Click here for abstract.
Xiang, Q., Rasmussen, C. and N.L. Glass (2002) The ham-2 locus, encoding a putative transmembrane protein, is required for hyphal fusion in Neurospora crassa. Genetics 160:169-180. Click here for abstract.
Glass, N. L., Jacobson, D. J. and P.K.T. Shiu (2000) The genetics of hyphal fusion and vegetative incompatibility in filamentous ascomycete fungi. Annu. Rev. Genet. 34:165-186. Click here for abstract.