Extensive remodeling during <i>Chlamydomonas reinhardtii</i> zygote maturation leads to highly resistant zygospores

The unicellular soil alga Chlamydomonas reinhardtii forms diploid zygotes during its sexual cycle. The process of a zygote maturing into a highly resistant zygospore remains poorly understood despite its importance for survival under adverse environmental conditions. Here we describe the detailed timeline of morphological and physiological changes during zygote maturation in darkness on ammonium-free Tris-acetate-phosphate agar plates. The formation of a multilayered cell wall is primarily responsible for the increase in cell size in the first few days after zygote formation. Desiccation and freezing tolerance also develop in the period 3-7 days. Photosynthetic and respiratory activity decrease to reach minimal levels after 7-10 days, accompanied by a partial dedifferentiation of the chloroplast that includes chlorophyll degradation followed by the possible disappearance of the pyrenoid. In contrast to the decreasing concentrations of most carotenoids in the first few days after zygote formation, ketocarotenoids can first be detected after 3 days and their accumulation is completed after 10 days. Furthermore, the zygote degrades a large proportion of its starch and enriches oligosaccharides that may serve as osmoprotectants. The storage lipid triacylglycerol is accumulated at the expense of thylakoid membrane lipids, which mirrors the conversion of a metabolically active cell into a dormant spore on the metabolic level. Taken together, zygote maturation is a multifaceted process that yields mature zygospores after ~ 3 weeks. This work sheds light on the complete time course of the remodeling of a photosynthetically active eukaryotic cell into a dormant, highly resistant spore.