| Summary: | 碩士 === 國立彰化師範大學 === 生物學系 === 105 === Phenotypic plasticity is the ability of a single genotype of organism to display variable phenotypes in response to fluctuating environments. The ability to alter their physiology, morphology and/or behavior in response to a change in the environmental conditions plays a crucial role for the evolutionary success of organisms. Virtually any abiotic or biotic factor can serve to induce plasticity, and resulting changes vary from harmful susceptibilities to highly integrated and adaptive alternative phenotypes. Organisms belonging to the freshwater green algae Desmodesmus and Scenedesmus (Scenedesmaceae, Chlorophyta) are characterized by a high degree of flexibility and allow them to adapt fluctuating environments. In natural waters, the importance of physical associations and biochemical interactions between microalgae and microorganisms is generally well appreciated, but the significance of these interactions to algal phenotypic plasticity have not been investigated. Previous studies have shown that many microorganisms associated with microalgae can produce indole-3-acetic acid (IAA). In addition, several studies have shown that IAA is a signaling molecule in microorganisms because it affects gene expression in several microorganisms. In our study, we found that the IAA can exert stimulatory and inhibitory effects on Desmodesmus. The morphology of Desmodesmus monocultures changed drastically under exposure to IAA compared with the control. The proportion of Desmodesmus unicells increased by increasing the concentration of environmental IAA, and these unicells experience a lower risk of sedimentation than large cells. Furthermore, we found that the accumulation of lipid droplets in algal cells grown with high concentration of IAA is specifically induced in response to the environmental change. The photosynthetic efficiency of algal cells was affected by its responses, suggesting algae can receive the phytohormone IAA signal in the environment and elicit physiological changes. Results also demonstrated that the presence of a competitor Pectinodesmus pectinatus further stimulate the inducible colony formation of Desmodesmus population. The proportion of induced colonies in cultures was dependent on the relative abundance of competitors. These results demonstrate that phenotypic plasticity allows microalgae to undergo controllable phenotypic changes to reduce fitness costs by optimizing the benefit-to-cost ratios when responding to environmental challenge.
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