Anisogamy
Anisogamy refers to a form of sexual reproduction involving the union or fusion of two dissimilar gametes —
anisogamous,
anisogamic,. The smaller gamete is considered to be male, whereas the larger gamete is regarded as female. There are several types of anisogamy. Both gametes may be flagellated and thus motile. Alternatively, neither of the gametes may be flagellated. This situation occurs for example in some algae and plants. In the red alga
Polysiphonia, large non-motile egg cells are fertilized by small, non-motile spermatia. In flowering plants, the gametes are non-motile cells within gametophytes. The form of heterogamy that occurs in animals is
oogamy. In oogamy, a large, non-motile egg cell is fertilized by a small, motile sperm cell. The large egg cell is optimized for longevity, whereas the small sperm cell is optimized for motility and speed. The size and resources of the egg cell allow for the production of pheromones, which attract the swimming sperm cells.