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The evolution of multicellularity is one of the major evolutionary transitions that has shaped the diversity of life on Earth. Multicellular organisms have arisen numerous times across the tree of life, but explaining why some lineages are multicellular while others are unicellular remains puzzling. In my thesis, we tested how the ecological variables may differentially favour multicellular and unicellular green algae (N=82). Specifically, we examined the fitness consequences of being unicellular, facultatively
multicellularity and obligately multicellular across fluctuating environments (Paper I, II, III). We identified the physiological responses that were associated with facultative and obligate multicellularity (Paper II, III). We also modelled the ecological differentiation between unicellular and multicellular species in nature
(Paper I, III, IV). We found that multicellularity in unicellular green algae is inducible by a range of environmental disturbances including predation, turbulence, nutrient imbalance and temperature variations. Importantly, we show that it is more of the severity of environmental stress rather than the type of the environment that triggers multicellularity in green algae. We further establish the link between multicellular group formation to unicellular stress response physiology, indicated by increased reactive oxygen species and starch contents. In Paper III, we found that multicellular algae have different life-history traits than unicellular algae, typically growing slower but have better survival over deteriorating environment. Such difference may have been one key cause to the niche differentiation between multicellular and unicellular green algae in the wild, shown in Paper IV.