Conjugation is a process of genetic recombination that occurs between two organisms (such as bacteria) in addition to asexual reproduction. Conjugation only occurs between cells of different mating types. In bacteria, cells designated F+ and F-lie close together, and a narrow bridge of cytoplasm forms between them. F+ cells contain a plasmid or reproductive factor that is made of DNA, which replicates within the bacterial cell. A copy is transferred from a donor F+ cell to a recipient F-. Spirogyra, a freshwater filamentous alga, also exhibits conjugation, where two nearby filaments develop extensions that contact each other. The walls between the connecting channels disintegrate, and one cell moves through the conjugation tube into the other. The cells fuse to form a diploid zygote, the only diploid stage in the life of Spirogyra. The black bread mold, Rhizopus, reproduces asexually by spores and sexually by conjugation. During conjugation, the tips of short hyphae act as gametes, and fuse. The resulting zygote develops a protective wall and becomes dormant. Finally, meiosis occurs, and a haploid bread mold germinates and grows spore-producing sporangia.