in biology, sexual process in which two lower organisms of the same species, such as bacteria, protozoans, and some algae and fungi, exchange nuclear material during a temporary union (e.g., ciliated protozoans), completely transfer one organism's contents to the other organism (bacteria and some algae), or fuse together to form one organism (most bacteria and fungi and some algae). Within a given population of organisms, the various forms that may engage in conjugation are known as mating types. The forms may or may not resemble each other in size, shape, or motility. They differ in some physiological or genetic characteristic and are distinguished by biologists through the use of symbols such as + and - or by letters of the alphabet if more than two forms occur.