Anisogamy

Anisogamy (noun) (also called heterogamy) refers to a form of sexual reproduction involving the union or fusion of two dissimilar gametes (differing either in size alone or in size and form) — anisogamous, anisogamic, (adj.). The smaller gamete is considered to be male (sperm cell), whereas the larger gamete is regarded as female (egg cell).

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 (ovum) is fertilized by a small, motile sperm cell (spermatozoon). 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.

Evolution of anisogamy
Anisogamy is the phenomenon of fertilization of small gametes (sperm) and big gametes (eggs). Gamete size difference is the fundamental difference between males and females. Anisogamy first evolved in multicellular haploid species after the differentiation of different mating types had already been established.

Two main theories have been proposed for the evolution of anisogamy. One focuses on sperm competition and the selfishness of sperm; the other focuses on the cooperation of different mating types to deal with fertilization risks. Both theories assume that anisogamy originated through disruptive selection acting on an ancestral isogamous population, and that there is a trade-off between larger gamete number and smaller fitness of each gamete, because the total resource one individual can invest in reproduction is fixed.

The theory of sperm competition was the first theory to explain the evolution of anisogamy, which was proposed by Geoff Parker. He proposed that because the size of the zygote is determined by the sizes of both gametes and one gamete becoming smaller will decrease the zygote size to no less than half, so there is a large benefit to reduce gamete size in order to greatly increase gamete number. Then, the many smaller gametes can still "parasitize" the larger gametes to pass on their genes. Then, larger gametes have to increase their sizes to compensate the nutrient loss of the zygotes. So sperm competition, which was the selection force to greatly increase gamete number, was proposed to be the cause of size differentiation of different gametes, and eventually the evolution of sperm and eggs.

However, competition not only exists among sperm, but also exists among eggs. Parker provided no explanations for why it was sperm that became smaller but not eggs, or what caused proto-sperm and proto-eggs to be unequal. In fact, a recent mathematical model shows that sperm competition is neither necessary nor sufficient for the evolution of anisogamy. Sperm competition only caused sperm to become as small as possible (considering them still functional) after anisogamy and smaller sperm had already evolved. The real reason for the evolution of anisogamy is proposed to be fertilization risk. An important problem for the mating of multicellular organisms is the transport of gametes from place to place for fertilization, which should evolve for at least one mating type because gametes of different individuals usually do not have direct contact. However, the transport has a probability of failure and poses a fertilization risk. When the risk increases, gametes of one mating type need to become smaller to increase the number to overcome this risk, and gametes of the other mating type need to become larger to compensate for nutrient loss of the zygotes.

So it is fertilization risk and the cooperation of different mating types to deal with it that caused the evolution of anisogamy. Sperm evolve to increase fertilization events and eggs evolve to compensate the nutrient loss. Additionally, sperm can evolve even smaller when eggs become larger to ensure zygote nutrient, and eggs can evolve even larger when sperm become smaller to reduce fertilization risks. This cooperation may be the reason why many organisms have extremely small sperm and extremely large eggs.