Communicate and Fuse: How Filamentous Fungi Establish and Maintain an Interconnected Mycelial Network

Abstract

Cell-to-cell communication and cell fusion are fundamental biological processes across the tree of life. Survival is often dependent upon being able to identify nearby individuals and respond appropriately. Communication between genetically different individuals allows for the identification of potential mating partners, symbionts, prey, or predators. In contrast, communication between genetically similar (or identical) individuals is important for mediating the development of multicellular organisms or for coordinating density-dependent behaviors (i.e., quorum sensing). This review describes the molecular and genetic mechanisms that mediate cell-to-cell communication and cell fusion between cells of Ascomycete filamentous fungi, with a focus on Neurospora crassa. Filamentous fungi exist as a multicellular, multinuclear network of hyphae, and communication-mediated cell fusion is an important aspect of colony development at each stage of the life cycle. Asexual spore germination occurs in a density-dependent manner. Germinated spores (germlings) avoid cells that are genetically different at specific loci, while chemotropically engaging with cells that share identity at these recognition loci. Germlings with genetic identity at recognition loci undergo cell fusion when in close proximity, a fitness attribute that contributes to more rapid colony establishment. Communication and cell fusion also occur between hyphae in a colony, which are important for reinforcing colony architecture and supporting the development of complex structures such as aerial hyphae and sexual reproductive structures. Over 70 genes have been identified in filamentous fungi (primarily N. crassa) that are involved in kind recognition, chemotropic interactions, and cell fusion. While the hypothetical signal(s) and receptor(s) remain to be described, a dynamic molecular signaling network that regulates cell-cell interactions has been revealed, including two conserved MAP-Kinase cascades, a conserved STRIPAK complex, transcription factors, a NOX complex involved in the generation of reactive oxygen species, cell-integrity sensors, actin, components of the secretory pathway, and several other proteins. Together these pathways facilitate the integration of extracellular signals, direct polarized growth, and initiate a transcriptional program that reinforces signaling and prepares cells for downstream processes, such as membrane merger, cell fusion and adaptation to heterokaryon formation.

Keywords: MAP kinase signaling; Neurospora; ROS signaling; STRIPAK; cell fusion; chemotropic interactions.

FIGURE 1

The life-cycle of Neurospora crassa. N. crassa is a heterothallic Ascomycete species with a distinct sexual cycle and asexual cycle. Conidia are clonal asexual propagules that can either generate a new colony on their own, or serve as a “male” partner to the “female” trichogyne during mating. The trichogyne is a specialized hypha that emerges from the protoperithecium and chemotropically grows toward a conidium of opposite mating type (shown here expressing H1-GFP). Ascospores are the result of meiosis, which occurs inside the perithecium. This lifecycle specifically highlights chemotropic interactions and cell fusion events. Stars indicate chemotropic interactions and fusion. Germlings, hyphae, and the trichogynes all undergo chemotropism and cell fusion. Protoperithecium image is from Lichius et al. (2012b), and the image showing many ascospores is from Raju (2009), with permission.

FIGURE 2

Diagram of crosstalk between the MAK-2 pathway, the CWI/MAK-1 pathway and the STRIPAK complex. A cyan “P” indicates canonical MAPK cascade phosphorylation, and a yellow “P” indicates MAK-2-dependent phosphorylation. MAPK pathway proteins are purple, the STRIPAK complex is blue, and pink transcription factors (PP-1 and ADV-1) are downstream of all three signaling components. MAK-2 is required for activation or de-repression of PP-1, and MAK-1 regulates ADV-1 in a PP-1-independent manner.

FIGURE 3

All currently known proteins required for germling communication and fusion in N. crassa. Proteins are color-coded by function. Red proteins are components of the NOX complex that produces superoxide by reducing NADPH and oxidizing molecular oxygen. Purple proteins make up two different MAPK pathways; the Cell Wall Integrity/MAK-1 pathway, or the MAK-2 signal response pathway. Yellow proteins are involved in actin dynamics, vesicle trafficking, endocytosis/exocytosis, or secretion. Green proteins are involved in long-distance non-self recognition. Blue proteins compose the STRIPAK complex. Pink proteins are transcription factors. Brown proteins are involved in membrane fusion. Orange proteins are uncharacterized or have an unknown function. Gray proteins are associated and relevant, but are dispensable for communication or fusion. Proteins with a cyan-P are phosphorylated as part of a MAPK cascade, and proteins with a yellow-P with a green outline are phosphorylated in a MAK-2-dependent manner.