oogamy

thumb|Oogamy in animals: small, motile sperm on the surface of an ovum

Oogamy is a type of sexual reproduction where the gametes differ greatly in both size and form. In oogamy in animals the large female gamete (also known as the ovum) is immotile, while the small male gamete (also known as a sperm) is mobile. Most sexually reproducing species – animals, land plants and some algae, are oogamous. It is generally accepted that isogamy is the ancestral state, from which oogamy evolved at least twenty times via anisogamy. Once oogamy evolves, males and females typically differ in various aspects. Internal fertilization may have originated from oogamy, although some studies suggest certain species may have evolved before the transition from external to internal fertilization. In streptophytes, oogamy occurred before the split from green algae.