Fungal syncytia

Eukaryotes are distinguished from prokaryotes by the presence of a nucleus. However, at some or all stages in their life cycle many fungal cells constitutively contain at least two nuclei in the same cytoplasm, resulting in a multinucleate 'coenocyte' (from the Greek for 'common' and 'cell') or a 'syncytium' (from the Greek for 'together' and 'cell'). Such organization is ancient and has evolved repeatedly. Crucially, multinucleation presents challenges and opportunities for gene expression programs, cytoplasm patterning, environmental sensing and evolution. Here, we focus on multinucleation in fungi where it is a ubiquitous strategy for cell organization (Figure 1). In this primer, we first give an overview of the evolutionary origins, prevalence and diversity of fungal multinucleation. We next describe the two distinct mechanisms that give rise to multinucleation in fungi. The first involves a specialized cell division called endomitosis, in which repeated nuclear divisions occur without cytokinesis or laying down septal cross walls. The other mechanism is cell-cell fusion, whereby two distinct fungal cells recognize and grow towards each other, fuse and allow their respective nuclei to intermingle within the same cytoplasm. Finally, we touch on some aspects of how fungal multinucleation impacts cytoplasmic organization, self/non-self recognition, genetic individuality, and perhaps even cross-species interactions in these organisms.