oogamous

Oogamy is a mode of sexual reproduction in which gametes are of markedly different sizes and motilities. Specifically, the large female gamete (the egg) is non-motile or minimally motile, while the small male gamete (the sperm) is motile and mobile. This asymmetry makes oogamy a form of anisogamy, distinguishing it from isogamy, in which gametes are similar in size and mobility.

In animals, fertilization typically involves a large egg that provides nutrients and a small, flagellated or otherwise motile sperm that competes to reach and fertilize the egg. In many algae, the same pattern occurs; motile sperm swim to a non-motile egg. In land plants, the situation is slightly different: the male gamete is non-motile and is delivered to the ovule by a pollen tube, while the egg remains large and non-motile.

Ecological and evolutionary significance: The division of labor allows large eggs to invest in zygote survival, while small sperm maximize number and genetic variety. Oogamy is associated with the evolution of internal fertilization and more complex life cycles in animals and many plants.

See also: anisogamy, isogamy, fertilization.