 Endosymbionts are microorganisms that live inside the cells of another organism, usually for mutual benefit. These symbiotic relationships between endosymbionts and their hosts have been observed in many different species, including insects. Sexual operations, such as male-female chimeras, are often caused by endosymbionts. Endosymbionts can manipulate the development of their hosts to produce offspring with different sex ratios, leading to population level changes in the sex ratio of the host species. This has important implications for the evolution of sexual reproduction and its role in the maintenance of genetic diversity.