HAPLOID SPORE FORMATION FOLLOWING ARRESTED CELL FUSION IN CHLAMYDOMOXAS (CHLOROPJIYTA) ¹

Abstract

Sexual fusion of haploid Chlamydomonas gametes produces a diploid zygote which undergoes sporulation (maturation). We have used a combination of genetic and cellular approaches to evaluate the role(s) of gametic cell and nuclear fusion in the progression of sporulation. A fusion‐arrested strain, zym‐26–3. was obtained following ultraviolet irradiation of vegetative haploid cells of the homothallic species Chlamydomonas monoica Strehlow. Using the DNA‐specific fluorochrome, DAPI, we determined that diploidy was rarely achieved although nuclear migration to the base of the cytoplasmic bridge connecting the gametes and attempted transit through the tubule could be easily documented. Unusual cytoplasmic‘buds’which developed adjacent to the cytoplasmic bridge in sporulating haploids were usually found to contain a migrant nucleus. Using transmission electron microscopy, we determined that ultrastructural changes typically associated with sporulation of a diploid zygote (e.g. spore wall formation; plastid dedifferentiation and associated lipid accumulation; nuclear migration and heterochromatization) could occur following arrested cell fusion despite the absence of nuclear fusion. Genetic analysis of the zym‐26–3 strain revealed two unlinked mutations: cf‐1 responsible for the failure to complete cell fusion; and ger‐8, a mutant allele not affecting cell fusion, but interfering with late stages of spore maturation and germination.‘Cytoplasmic budding’was observed in strains carrying each of these mutations singly and may be a common secondary consequence of disturbances in the relative timing of interrelated processes required for spare wall assembly.

THE ZYGOSPORE WALL OF CHLAMYDOMONAS MONOICA (CHLOROPHYCEAE): MORPHOGENESIS AND EVIDENCE FOR THE PRESENCE OF SPOROPOLLENIN1

“…Earlier studies (VanWinkle-Swift and Bauer 1982, VanWinkle-Swift and Burrascano 1983, VanWinkle-Swift et al 1987 on zygosporulation in C. monoica have focused on the identification of gene loci expressed specifically in zygotes and required for morphogenesis of the zygote into a zygospore. A classical genetic analysis of spore morphogenesis requires the isolation of appropriate mutant strains, namely, those that are unable to produce a normal zygcspore.…”

ISOLATION AND CHARACTERIZATION OF A CELL WALL‐DEFECTIVE MUTANT OF CHLAMYDOMONAS MONOICA (CHLOROPHYTA)1

“…Our work has focused on the homothallic species C. monoica because of the species' unique advantages for the study of sexual development, especially zygospore formation. Among the large collection of sexual cycle mutants obtained in C. monoica are those showing altered mating (VanWinkle‐Swift and Bauer 1982, VanWinkle‐Swift and Hahn 1986, VanWinkle‐Swift and Thuerauf 1991), gamete fusion (VanWinkle‐Swift et al 1987, Shi 1995), or zygospore formation (VanWinkle‐Swift et al 1998) as well as those carrying mutations that affect the uniparental inheritance of chloroplast genes (VanWinkle‐Swift and Salinger 1988, VanWinkle‐Swift et al 1994). These unique developmental mutants could be used to understand the pro‐cesses contributing to zygospore morphogenesis and organelle inheritance, if cloning of the genes by trans‐formation and complementation of mutant strains were possible.…”