Rapid loss of viability in the dark and resistance to UV radiation co-segregate in Chlamydomonas monoica

Karen VanWinkle-Swift and Aubrey Funke

Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, 86011-5640, USA

Although C. monoica is an obligate photoautotroph, cells remain viable for many days in the absence of light. We have obtained several mutants selected on the basis of increased resistance to ultraviolet (UV) radiation that undergo rapid loss of viability in darkness and accumulate large cytoplasmic granules. A more rapid loss of viability is also observed following prolonged maintenance of these strains under continuous light. UV resistance and "dark death" co-segregate in tetrads and are the consequence of single Mendelian gene mutations. The seven mutants under analysis define two unlinked gene loci that confer similar phenotypes. UV resistance is strongly enhanced relative to wildtype in both vegetative cells and zygospores of the mutant strains if cells are placed in darkness for 18-24 hours immediately following irradiation (a protocol we routinely use to block photoreactivation of UV-induced DNA damage by photolyase). Surprisingly, the mutants are more sensitive than wildtype to UV if placed under continuous illumination immediately after irradiation. Ultrastructural analysis is underway to identify the source of the cytoplasmic granules associated with loss of viability in the dark and to evaluate chloroplast structure. Models incorporating the known detrimental effects of UV radiation on photosynthesis will be proposed to accommodate the unusual responses of these mutants to ultraviolet and photosynthetically active radiation. (Supported by NIH grant 2 R15 GM071374-02).

e-mail address of presenting author: Karen.VanWinkle-Swift@nau.edu