A Mutant of Chlamydomonas reinhardii with Abnormal Cell Division

This paper describes a cytokinesis-deficient mutant (cyt I) of the green alga Chlamydomonas reinhardii, isolated after mutagenesis with N-methyl-N-nitroso-N-nitroguanidine. The mutant exhibits abnormal cell division, producing multinucleate cells with 1-7 nuclei instead of the normal single nucleus. Notably, the number of flagella pairs per cell correlates directly with nuclear number, whereas pyrenoid division lags behind nuclear division in some cells. Culture age, pH, and presence of other mutations modulate the expression of the cytokinesis deficiency. Genetic analysis, complicated by the presence of multinucleate gametes, demonstrates that the deficiency is inherited as a single gene mutation mapping close to the ac 29 locus on linkage group VI. Electron microscopy reveals incomplete cytokinetic furrow formation and defective chloroplast division. Approximately 50% of large multinucleate cells can eventually complete cytokinesis. The mutation provides insight into the genetic control of cell division and suggests that nuclear/flagellar division and organelle division may be regulated by separate control systems.

Key findings

• The cyt I mutation causes multinucleate cells (1-7 nuclei) with flagella pairs matching nuclear number, indicating coordinated nuclear-flagellar division uncoupled from cytokinesis
• Cytokinesis-deficient gene maps close to ac 29 on linkage group VI and is inherited as a single gene difference despite complications from multinucleate gametes
• Culture age and pH significantly modify mutation expression, with increased multinucleate cells during log phase that decrease during stationary phase
• Pyrenoid and chloroplast division lag behind nuclear division, suggesting separate genetic control systems for nuclear/flagellar versus organellar division
• Double mutants combining cyt I with paralysed flagella mutations (pf2, pf19B, pf20) show reduced multiflagellate cell proportions compared to cyt I alone