Hyphal Homing, Fusion and Mycelial Interconnectedness

Review TRENDS in Microbiology Vol.12 No.3 March 2004 Hyphal homing, fusion and mycelial interconnectedness N. Louise Glass1, Carolyn Rasmussen1, M. Gabriela Roca2 and Nick D. Read2 1Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA 2Fungal Cell Biology Group, Institute of Cell and Molecular Biology, University of Edinburgh, Rutherford Building, Edinburgh, UK EH9 3JH Hyphal fusion is a ubiquitous phenomenon in filamen- fusion, or whether common molecular mechanisms are tous fungi. Although morphological aspects of hyphal associated with fusion events during vegetative growth fusion during vegetative growth are well described, and sexual development. molecular mechanisms associated with self-signaling Hyphal fusion in filamentous fungi is comparable to cell and the cellular machinery required for hyphal fusion fusion events in other eukaryotic organisms. Examples are just beginning to be revealed. Genetic analyses include fertilization events between egg and sperm or suggest that signal transduction pathways are con- somatic cell fusion resulting in syncytia formation (e.g. served between mating cell fusion in Saccharomyces between myoblasts during muscle differentiation), fusion cerevisiae and vegetative hyphal fusion in filamentous between osteoclasts in bone formation, and also in fungi. However, the mechanism of self-signaling and placental development [3–5]. Although molecular mech- the role of vegetative hyphal fusion in the biology of anisms of non-self fusion (e.g. between Saccharomyces filamentous fungi require further study. Understanding cerevisiae cells of opposite mating types) have been well hyphal fusion in model genetic systems, such as Neuro- characterized, molecular mechanisms associated with spora crassa, provides a paradigm for self-signaling fusion between somatic cells in eukaryotes are not as mechanisms in eukaryotic microbes and might also pro- well analyzed. Understanding the molecular basis of vide a model for somatic cell fusion events in other hyphal fusion during vegetative growth in filamentous eukaryotic species. fungi provides a paradigm for self-signaling mechanisms in eukaryotic microbial species, and might also provide a Hyphal fusion (anastomosis) occurs at crucial stages useful model for somatic cell fusion events in other during the life cycle of filamentous fungi and serves eukaryotes. In this review, we focus on hyphal fusion in many important functions. During the vegetative phase, filamentous ascomycetes, particularly in the model system fusion initially occurs between spore germlings, and later Neurospora crassa [6,7]. on in the interior of the mature vegetative colony. Figure 1 shows the interconnected mycelial network of a fungal Fusion between conidial germlings colony that results from multiple fusion events. It is widely Hyphal fusion between conidial germlings has been assumed that vegetative hyphal fusion is important for previously observed in numerous species [8–10]. In the intra-hyphal communication, translocation of water and plant pathogen Colletotrichum lindemuthianum and in nutrients, and general homeostasis within a colony [1,2]. N. crassa, it has been observed that conidia also form However, these predicted roles still need to be analyzed specialized hyphae, called ‘conidial anastomosis tubes’, experimentally. which are morphologically distinct from germ tubes and Entry into the sexual cycle in out-crossing species function to anastomose with other conidia in close involves the fusion of hyphae with other hyphae or spores proximity [10] (Figure 2). Conidia from C. lindemuthia- of the opposite mating type. Maintenance of the dikaryotic num that have undergone fusion show a higher rate of state, which is a prelude to karyogamy, also requires the germination suggesting that fusion between conidial fusion of hyphae, and this commonly involves specialized germlings might serve to increase or pool resources structures called croziers (in the ascogenous hyphae of between genetically identical individuals. Even early ascomycetes) or clamp connections (associated with the publications provided evidence for positive autotropism vegetative hyphae of basidiomycetes). Hyphal fusion is being involved in fusion between conidial germlings [8]. also involved in the formation of multihyphal aggregates Attraction between conidia of different species was also from which the multicellular fruitbodies are derived. demonstrated, suggesting that mechanisms associated with Although morphological aspects that are associated signaling, homing and the cellular machinery of hyphal with hyphal fusion during the life cycle of filamentous fusionareconservedamongfilamentousascomycetespecies. fungi have been well characterized, little is known about the signals or molecular machinery involved in hyphal Fusion in vegetative colonies Hyphal fusion occurs within a single vegetative colony and Corresponding author: N. Louise Glass ([email protected]). also between fungal colonies to form heterokaryons, www.sciencedirect.com 0966-842X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tim.2004.01.007 136 Review TRENDS in Microbiology Vol.12 No.3 March 2004 (a) (b) Figure 1. (a) Drawing showing an interconnected fungal colony resulting from a single germinated spore. (b) Projection of confocal images of the interconnected colony interior of Neurospora crassa showing hyphal fusions (f). Scale bar represents 50 mm. (a) and (b) reproduced, with permission, from Ref. [17]. whereby genetically distinct nuclei coexist in a common associated with the site of the future pore (Figure 4b), cytoplasm [11]. Different stages in vegetative hyphal suggesting that it might target vesicles involved in the fusion are shown in Figure 3. In a mature filamentous secretion of extracellular adhesives and cell-wall-degrading fungal colony, a criterion for whether a hypha can enzymes to the future pore site. Vesicle trafficking to and anastomose is its presence in the colony interior rather from the fusion site therefore plays a central role in than the periphery. The developmental switch for gen- regulating many of the processes involved in hyphal fusion. erating fusion-competent hyphae is unknown. It has been After fusion of plasma membranes occurs, the cyto- hypothesized that the attraction of hyphae involved in plasms of the two participating hyphae mix. In N. crassa, fusion is mediated by diffusible substances, which results the Spitzenko¨rper remains associated with the fusion pore in re-directed polarized hyphal tip growth [8,12–14] as it enlarges [17]. Dramatic changes in cytoplasmic flow (Figure 3). We propose that these unidentified diffusible are often associated with hyphal fusion, and nuclei and signals regulate the behavior of the Spitzenko¨rper, which organelles, such as vacuoles and mitochondria, also pass is a complex of organelles and proteins predominated by through the fusion pore (Figure 5). Septum formation near secretory vesicles. The Spitzenko¨rper is found in growing the hyphal fusion site is also often observed. hyphal tips or at sites of branch initiation, and its behavior plays an important role in hyphal morphogenesis [15,16]. Fusion during sexual reproduction Hyphal fusion occurs in the fruiting bodies of basidiomy- Live-cell imaging of hyphal homing and fusion in N. crassa cete species [18]. However, it is unclear whether hyphal has shown that these processes are also intimately fusion is also required for the formation of female associated with the dynamic behavior of the Spitzenko¨rper reproductive structures (protoperithecia) in filamentous (Figure 4) [17]. After making contact, hyphae involved in ascomycetes such as N. crassa, although the hyphae that fusion switch from polar to isotropic growth, resulting in form the walls of these sexual organs become tightly swelling of hyphae at the fusion point [17]. However, the adhered to each other [19]. Hyphal fusion is also essential ¨ Spitzenkorper persists in these hyphae and is invariably for fertilization during mating. During fertilization, reproductive hyphae, known as trichogynes, protrude from protoperithecia; trichogynes are attracted to, and fuse with, male cells (microconidia or macroconidia) of the opposite mating type [20,21]. Following fertilization, nuclei of the opposite mating types proliferate in a common cytoplasm and eventually pair off and migrate into a hook- shaped structure called a crozier [22].InN. crassa, karyogamy occurs in the penultimate cell of the crozier, whereas hyphal fusion occurs between the terminal cell Figure 2. Differential interference contrast images showing two germlings of Neu- rospora crassa undergoing homing and fusion of conidial anastomosis tubes at 8 and the hyphal compartment nearest to the penultimate and 40 minutes after the onset of imaging (time 0). Scale bar represents 5 mm. cell [22,23]. Although fusion events occur during www.sciencedirect.com Review TRENDS in Microbiology Vol.12 No.3 March 2004 137 1 ? plasma membrane nucleus cell wall Spitzenkörper 2 3 Pre-contact stage 4 adhesive 5 Post-contact stage 6 7 8 Post-fusion stage 9 TRENDS in Microbiology Figure 3. Diagram showing the pre-contact, post-contact and post-fusion stages involved in vegetative hyphal fusion. Two types of pre-contact behavior are shown: (i) a hyphal tip induces a branch and they subsequently fuse (shown in stages 1 and 2); and (ii) two hyphal tips grow towards each other and subsequently fuse (shown in stage 3). A third type of pre-contact behavior,

Load more