title: Cell length growth patterns in fission yeast reveal a novel size control mechanism operating in late G2 phase.
abstract: BACKGROUND
  Because cylindrically shaped fission yeast cells grow exclusively at their tips, cell volume is proportional to length and can be easily monitored by time-lapse microscopy. Here, we analysed the growth pattern of individual cells from several fission yeast strains to determine the growth function that describes them most adequately and to perform size control studies.
  
  
  RESULTS
  The growth pattern of most cells during their growth period is best described by a bilinear function (i.e., two linear segments of different growth rates separated by a rate-change point). Linear growth patterns were also observed in several cases, but exponential ones only rarely. Since the bilinear patterns are separated into two segments by a breakpoint, we examined the existence of size control by regression analyses of the appropriate growth parameters in both segments. This confirmed the existence of known size controls in late G1, mid-G2 and late G2 during the fission yeast cycle. The present analyses also revealed that, contrary to the commonly accepted current view, late G2 size control is a general characteristic third event in the cycle. The level of the critical late G2 size that needs to be reached in an individual fission yeast cell is influenced by the growth rate of the cell in a manner similar to budding yeast, suggesting an evolutionary conserved mechanism.
  
  
  CONCLUSIONS
  The present study of individual cell growth patterns in wild-type and several cell cycle mutant fission yeast strains confirmed that, for most cells, growth is best described by a bilinear function. Three different size control mechanisms were found to operate in the different strains, and, as a novel observation, cell size was always found to be monitored before mitotic onset, irrespective of the existence of any earlier size checkpoints.
  
  
  CONCLUSIONS
  Studying the pattern of growth and the mechanism of size control helps to clarify the connections between cell growth and division, since their coordination must work properly to maintain size homeostasis. In this study, we argue that most individual fission yeast cells grow following a bilinear pattern, and we confirm the existence of three different size control mechanisms.
authors.affiliation: Department of Applied Biotechnology and Food Science, Budapest University of Technology and Economics, Budapest, Hungary.
authors.forename: Anna
authors.initials: A
authors.lastname: Horváth
authors.affiliation: Department of Applied Biotechnology and Food Science, Budapest University of Technology and Economics, Budapest, Hungary.
authors.forename: Anna
authors.initials: A
authors.lastname: Rácz-Mónus
authors.affiliation: Department of Molecular and Cellular Pharmacology, Miller School of Medicine, University of Miami, Miami, FL, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Buchwald
authors.affiliation: Department of Applied Biotechnology and Food Science, Budapest University of Technology and Economics, Budapest, Hungary. asveiczer@mail.bme.hu.
authors.forename: Ákos
authors.initials: Á
authors.lastname: Sveiczer
mesh_terms: D003198:Computer Simulation; D016195:G2 Phase; D008954:Models, Biological; D009154:Mutation; D012568:Schizosaccharomyces / Q000166:cytology* / Q000235:genetics / Q000254:growth & development*
keywords: Bilinear growth pattern; Cell growth; Fission yeast; Size control; Statistical criterion