Oswald Theodore Avery studied strains of pneumococcus of the genus Streptococcus in the US in the first half of the twentieth century. This bacterium causes pneumonia, a common cause of death at the turn of the twentieth century. In a 1944 paper, Avery demonstrated with colleagues Colin Munro MacLeod and Maclyn McCarty that deoxyribonucleic acid, or DNA, instead of protein, formed the material of heritable transformation in bacteria. Avery helped untangle some of the relationships between genes and developmental processes.

Bacteria of the genus Wolbachia are bacteria that live within the cells of their hosts. They infect a wide range of arthropods (insects, arachnids, and crustaceans) and some nematodes (parasitic roundworms). Scientists estimate that Wolbachia exist in between seventeen percent and seventy-six percent of arthropods and nematodes. The frequency of the bacteria makes them one of the most widespread parasites. In general, they are divided into five groups, from A to E, depending of the species of their host. They cause diverse reproductive and developmental changes on their numerous invertebrate hosts. Several mechanisms, like the feminization of the embryo's sexual characters, are involved in those processes. To reproduce, Wolbachia often exploit their hosts' reproductive processes. Additionally, they are symbiotic in that they are necessary for the normal development of organisms in some species