Septins and cytokinesis in the polymorphic fungus Aureobasidium pullulans.

During cytokinesis of animal and fungal cells, a contractile actomyosin ring (CAR) assembles at target locations and constricts to drive cell separation. In animal cells, the position of the CAR is determined by the mitotic spindle so that the cleavage plane is perpendicular to the mitotic axis. However, in budding yeasts, the location of CAR assembly is specified by a cortical septin cytoskeleton that recruits CAR components to the neck. In the polymorphic fungus Aureobasidium pullulans, we show that septins assemble at mother-bud necks and predict the sites of CAR assembly. Cells lacking septins stochastically failed to assemble CARs at a subset of bud necks. However, even cells lacking all four core septins were able to assemble CARs at 75% of bud necks. Our findings suggest the existence of a novel pathway that enables CAR positioning, assembly, and constriction with no requirement for septin scaffolds or nuclear/spindle cues.