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.

Social cooperation impacts the development and survival of species. In higher taxa, kin recognition occurs via visual, chemical, or tactile cues that dictate cooperative versus competitive interactions. In microbes, the outcome of cooperative versus competitive interactions is conferred by identity at allorecognition loci, so-called kind recognition. In syncytial filamentous fungi, the acquisition of multicellularity is associated with somatic cell fusion within and between colonies. However, such intraspecific cooperation entails risks, as fusion can transmit deleterious genotypes or infectious components that reduce fitness, or give rise to cheaters that can exploit communal goods without contributing to their production. Allorecognition mechanisms in syncytial fungi regulate somatic cell fusion by operating precontact during chemotropic interactions, during cell adherence, and postfusion by triggering programmed cell death reactions. Alleles at fungal allorecognition loci are highly polymorphic, fall into distinct haplogroups, and show evolutionary signatures of balancing selection, similar to allorecognition loci across the tree of life.

Organisms require the ability to differentiate themselves from organisms of different or even the same species. Allorecognition processes in filamentous fungi are essential to ensure identity of an interconnected syncytial colony to protect it from exploitation and disease. Neurospora crassa has three cell fusion checkpoints controlling formation of an interconnected mycelial network. The locus that controls the second checkpoint, which allows for cell wall dissolution and subsequent fusion between cells/hyphae, cwr (cell wall remodeling), encodes two linked genes, cwr-1 and cwr-2. Previously, it was shown that cwr-1 and cwr-2 show severe linkage disequilibrium with six different haplogroups present in N. crassa populations. Isolates from an identical cwr haplogroup show robust fusion, while somatic cell fusion between isolates of different haplogroups is significantly blocked in cell wall dissolution. The cwr-1 gene encodes a putative polysaccharide monooxygenase (PMO). Herein we confirm that CWR-1 is a C1-oxidizing chitin PMO. We show that the catalytic (PMO) domain of CWR-1 was sufficient for checkpoint function and cell fusion blockage; however, through analysis of active-site, histidine-brace mutants, the catalytic activity of CWR-1 was ruled out as a major factor for allorecognition. Swapping a portion of the PMO domain (V86 to T130) did not switch cwr haplogroup specificity, but rather cells containing this chimera exhibited a novel haplogroup specificity. Allorecognition to mediate cell fusion blockage is likely occurring through a protein-protein interaction between CWR-1 with CWR-2. These data highlight a moonlighting role in allorecognition of the CWR-1 PMO domain.