Holobiont refers to a host organism and its microbial community and they together form one selection unit. Holobiont is formed via symbiogenesis and further evolves as a unit. Microbiome differences could be an important factor in reproductive isolation and speciation. Well-known examples are human holobiont, corals and hydra, and herein the interest focuses on the examples of corals, plants and mosquitoes. There is a connection between endosymbiosis and circadian rhythm. Stress is a reaction of an organism to any harmful agent from the environment (stressor) which affects its structure or function. Since plants have limited movement possibilities, they develop multiple stress-response mechanisms including hormone ethylene synthesis, which displays pleiotropy in plant phenotype. Faster adaptation also leads to an evolutionary advantage. Further studies on holobiont could bring us new conclusions on biology and ecology of particular organisms of interest and that could also bring a practical use, e.g. this concept is used in mosquito population biological control and tropical infectious diseases prevention.