Non-random distribution of vacuoles inSchizosaccharomyces pombe.

A central question in eukaryotic cell biology asks, during cell division, how is the growth and distribution of organelles regulated to ensure each daughter cell receives an appropriate amount. For vacuoles in budding yeast, there are well described organelle-to-cell size scaling trends as well as inheritance mechanisms involving highly coordinated movements. It is unclear whether such regulation is necessary in the symmetrically dividing fission yeast, Schizosaccharomyces pombe, in which random partitioning may be utilized to distribute vacuoles to daughter cells. To test this idea, we utilized machine learning to develop automated methods for segmenting fission yeast cells and locating vacuoles in live fission yeast cells from brightfield and fluorescence micrographs. We have found that the scaling trends of vacuole number, volume and surface area-to-cell size is consistent across a variety of strains, and that vacuole proliferation is a key mechanism for vacuole growth. We also show that vacuoles are not distributed evenly throughout the cell with respect to available cytoplasm. Rather, vacuoles show distinct peaks in distribution close to the nucleus, and this clustering was confirmed in mutants in which nucleus position is perturbed. Future work will establish the molecular mechanism for this clustering.