cells Review Syncytia in Fungi Alexander P. Mela 1, Adriana M. Rico-Ramírez 1 and N. Louise Glass 1,2,* 1 Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA; [email protected] (A.P.M.); [email protected] (A.M.R.-R.) 2 Environmental Genomics and Systems Biology Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA * Correspondence: [email protected] Received: 31 August 2020; Accepted: 29 September 2020; Published: 8 October 2020 Abstract: Filamentous fungi typically grow as interconnected multinucleate syncytia that can be microscopic to many hectares in size. Mechanistic details and rules that govern the formation and function of these multinucleate syncytia are largely unexplored, including details on syncytial morphology and the regulatory controls of cellular and molecular processes. Recent discoveries have revealed various adaptations that enable fungal syncytia to accomplish coordinated behaviors, including cell growth, nuclear division, secretion, communication, and adaptation of the hyphal network for mixing nuclear and cytoplasmic organelles. In this review, we highlight recent studies using advanced technologies to define rules that govern organizing principles of hyphal and colony differentiation, including various aspects of nuclear and mitochondrial cooperation versus competition. We place these findings into context with previous foundational literature and present still unanswered questions on mechanistic aspects, function, and morphological diversity of fungal syncytia across the fungal kingdom. Keywords: syncytia; filamentous fungi; heterokaryon; nucleus; morphology 1. Syncytia Syncytia can be defined as multinucleated cells within a common cytoplasmic environment whose main purpose is to function as a single coordinated unit. There are many cases of syncytia in nature. In animals, for example, the placenta is a multinucleate organ that has an important role in protecting the fetus and transporting nutrients [1]. In muscle cells, multinucleated fibers are formed via the fusion of myoblasts [2], and osteoclasts are multinuclear bone-resorbing cells that are formed via cell fusion [3] (Figure1a). In the slime mold, Physarum polycephalum, individual amoebae fuse to form a large mass of protoplasm containing multiple nuclei [4] that undergo synchronous mitoses [5] (Figure1b). In plants, syncytia also occur, for example, the endosperm–placental syncytia [6] (Figure1c). In fungi, diversity of morphological characteristics that have evolved over time, and the syncytial lifestyle may have arisen in hyphal and non-hyphal organisms. For example, chytrids species belong to an ancient phylum of fungi (Chytridiomycota) that diverged at least 750 million years ago from other fungal lineages [7]. Chytrids do not form long multinucleate hyphae that search for nutrients. Instead, they have anucleate ‘rhizoids’ that are derived from a unicellular, multinucleate structure, or more complex rhizomycelia, which share morphological features with filamentous fungal syncytia and multinucleate hyphae. Cells 2020, 9, 2255; doi:10.3390/cells9102255 www.mdpi.com/journal/cells Cells 2020, 9, 2255 2 of 14 Cells 2020, 9, x 2 of 14 Figure 1.FigureExamples 1. Examples of syncytia of syncytia in nature. in nature. (a) Myoblasts (a) Myoblasts are fused are to fused form to multinucleated form multinucleated muscle muscle fibers. (bfibers.) Slime (b) mold SlimePhysarum mold Physarum polycephalum polycephalum, multinucleate, multinucleate protoplasm protoplasm formed formed by the fusionby the offusion of individualindividual amoebae. amoebae. (c) Endospermal–placental (c) Endospermal–placental syncytia syncytia in developmental in developmental stages of stagesUrticularia of Urticulariaseeds, seeds, the formationthe formation of syncytia of syncytia occurs occurs in the placenta.in the placenta. The syncytia The syncytia harbor harbor two populations two populations of nuclei, of nuclei, a a nucleusnucleus from the from nutritive the nutritive tissue, tissue, and a giantand a endosperm giant endosperm nucleus nucleus (modified (modified from [ 8from]). [8]). The hallmarkThe hallmark growth growth habit habi of filamentoust of filamentous fungi fungi is as is multinucleate as multinucleate syncytia syncytia (Figure (Figure2a,b). 2a,b). The The interconnectednessinterconnectedness of fungalof fungal syncytia syncyt occursia occurs via via fusion fusion of germinatedof germinated asexual asexual spores spores (germlings) (germlings) via via so-calledso-called conidial conidial anastomosis anastomosis tubes tubes (CATs) (CATs) [9] or [9] between or between hyphae hyphae within within a single a colonysingle colony or between or between coloniescolonies (anastomosis) (anastomosis) [10] (Figure [10]2a–d). (Figure Fusion 2a–d). results Fusion in the results formation in the of anformation interconnected of an networkinterconnected (mycelium)network [11,12 (mycelium)], which is the[11,12], foundation which for is colonythe founda establishmenttion for andcolony growth. establishment In Neurospora and crassa growth., In anastomosisNeurospora is associated crassa, anastomosis with cell-to-cell is associated communication with cell-to-cell processes thatcommunication regulate chemotropic processes growth that regulate of germlingschemotropic and hyphae growth before of germlings cell contact and [ 10hyphae]. Germlings before cell/hyphae contact send [10]. and Germlings/hyphae receive signals that send and guide chemotropicreceive signals interactions, that guide culminating chemotropic in cell–cellinteractions adhesion,, culminating cell wall in dissolution, cell–cell adhesion, membrane cell wall merger,dissolution, and cytoplasmic membrane mixing merger, [13–16 and]. Hyphal cytoplasmic anastomosis mixing primarily [13–16]. occursHyphal in anastomosis the interior primarily of a colonyoccurs either in through the interior hyphal of a branches colony either or by throug contacth betweenhyphal branches adjacent or hyphae, by contact resulting between in the adjacent formationhyphae, of a fusion resulting bridge in the [10 ,formation11]. Hyphal of fusiona fusion can bridge also occur [10,11]. between Hyphal colonies fusion withcan also the sameoccur orbetween non-identicalcolonies genotypes with the [same17,18 ].or non-identical genotypes [17,18]. The formationThe formation of an interconnected of an interconnected syncytium cansyncytium be beneficial can for be colony beneficial development, for colony facilitating development, the exchangefacilitating of genetic the exchange material, of nutrient genetic transport,material, nutrie improvingnt transport, colony improving establishment, colony and establishment, increasing and colonyincreasing size [17,19 colony–21]. Uponsize [17,19–21]. hyphal fusion, Upon hyphal cytoplasmic fusion,flow cytoplasmic can show flow dramatic can show changes dramatic in changes directionalityin directionality [10,22]. Germlings [10,22]. Germlings or young colonies or young can colonies share genetic can share resources genetic or nutrients resources through or nutrients cell fusion,through but thecell process fusion, can but be the restricted process as coloniescan be agerestricted and undergo as colonies hyphal age differentiation and undergo [17 ].hyphal The advantagesdifferentiation of interconnected [17]. The advantages syncytial of hyphae interconnected lie in their syncytial ability to hyphae transport lie nutrientsin their ability through to transport a continuousnutrients cytoplasm, through allowing a continuous the efficient cytoplasm, use of the nutrientsallowing once the aefficient local source use has of beenthe nutrients exhausted once [23]. a local Withinsource a has syncytium, been exhausted heterokaryons [23]. can arise due to a spontaneous mutation within a single colony (FigureWithin3a). In a asyncytium, laboratory strainheterokaryons of N. crassa can, around arise du 2–3%e to ofa thespontaneous nuclear population mutation harboredwithin a single mutationscolony that (Figure modified 3a). the In myceliuma laboratory phenotype, strain of N. showing crassa, thearound diffi culties2–3% of of the maintaining nuclear population a mycelium harbored with a uniformmutations genetic that backgroundmodified the [24 ].mycelium Heterokaryons phenotyp cane, also showing form via the germling difficulties or hyphal of maintaining fusion a betweenmycelium genetically with diff aerent uniform cells /colonies,genetic background resulting in [24] the. coexistence Heterokaryons of genetically can also diformfferent via nuclei germling or hyphal fusion between genetically different cells/colonies, resulting in the coexistence of genetically Cells 2020, 9, 2255 3 of 14 Cells 2020, 9, x 3 of 14 differentin a common nuclei cytoplasm in a common (Figure cytoplasm2e,f). Heterokaryon (Figure 2e,f). formation Heterokaryon is anessential formation element is an essential of many element fungal oflife many cycles fungal and maylife cycles be useful and tomay complement be useful to mutations. complement There mutations. are also There beneficial are also aspects beneficial of the parasexualaspects of the cycle, parasexual such as functionalcycle, such diploidy as function andal mitoticdiploidy recombination and mitotic recombination [25] (Figure3d). [25] However, (Figure 3d).heterokaryon However, formationheterokaryon can formation transmit geneticcan transmit infections genetic that infections negatively that impact negatively fitness, impact such fitness, as the suchtransmission