oogamy

Oogamy is a form of sexual reproduction in which the gametes are morphologically distinct: a large, non-motile female gamete (egg or ovum) and a smaller, typically motile male gamete (sperm). This asymmetry, the most specialized form of anisogamy, reflects division of reproductive labor between a nutrient-rich egg that provisions early development and a small, mobile sperm that seeks the egg to fertilize it. Oogamy is found across several major eukaryotic lineages, particularly animals and many plants and algae.

In animals, fertilization often occurs through the encounter of mobile sperm with a relatively immobile egg, typically within the female reproductive tract or in the external environment. In plants and some algae, the egg remains embedded in tissue, and male gametes reach it via a pollen tube or other delivery mechanism; in gymnosperms and angiosperms, non-motile sperm are delivered by pollen tubes.

Evolutionarily, oogamy is associated with specialization and differentiation of gametes, enabling resource provisioning in the zygote and efficient fertilization strategies. It is viewed as a key development in the evolution of conspicuous sexual dimorphism and complex life cycles, and it often coincides with centralized maternal investment in early embryonic stages.

Taxonomic distribution is broad among eukaryotes. In animals, oogamy is the standard pattern of fertilization. In many algae and land plants, oogamy has evolved independently in certain lineages, with obligate or facultative reliance on pollen tubes or other mechanisms to deliver sperm.