Calvert et al. reveal that large fungal cells have more myosin II associated with their contractile rings than do smaller cells, which allows them to constrict the ring faster during cytokinesis.
A recent study of C. elegans embryos showed that the time required for the contractile ring to close during cytokinesis was about the same no matter the size of the cell, which means the ring must tighten faster in larger cells. Scientists have been keen to test whether girth affects constriction rate in other species. The obstacle has been finding organisms whose dividing cells differ enough in size.
Calvert et al. recognized a candidate for such a study—the fungus Neurospora crassa. Its filaments, or hyphae, can vary up to fourfold in diameter and undergo a process called septation that's somewhat akin to mitosis.
As in nematodes, the ring closed faster in larger hyphae. But N. crassa differed from nematodes in a couple of ways. In N. crassa, the contractile rings of larger cells started out with more myosin II than the rings of smaller cells, which could drive faster constriction. Indeed, reducing the amount of ring-associated myosin II slowed the rate of constriction. In addition, the amount of myosin II in the ring remained constant as the ring tightened, instead of declining as in nematodes. Therefore, in N. crassa, the design and mechanics of the contractile ring vary with size. Future studies will investigate whether the same relationship holds in other species.