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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 and/or form) — anisogamous, anisogamic, (''adj.'').〔http://www.thefreedictionary.com/anisogamy〕 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, including humans, 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. ==Anisogamy and Sexual Dimorphism== The emergence of only two sexes in species that reproduce sexually leads to the question of how this system was even implemented in the first place. In many sexually reproductive species, females are seen to invest high energy and resources to offspring and tend to be more discriminating when it comes to mating partners. Males, on the other hand, compete against one another for the limited number of mating opportunities and have evolved traits that aid them in the competition for the limiting sex. On a wide scale, this phenomenon is seen in organisms such as the peacock, where males have beautiful but physically hindering plumage, entirely adapted to attract the dull-colored females for mating opportunities. But this difference in sex is seen even in the cellular level, where females produce a low number of well-provisioned gametes versus males who produce many cheap gametes. One of the current hypotheses for the defined division between the sexes and the sizes of their respective gametes involves disruptive selection. Females produce large gametes known as "proto-ovum", which are energetically expensive to produce but have the advantage of allowing for early embryogenesis. Males, on the other hand, produce several tiny gametes ("protosperm") which are energetically inexpensive and can be produced in high frequencies. They thus invest minimum amounts of energy to create viable gametes that are able to fertilize female gametes. Intermediary gametes offer none of the advantages that the opposite ends of the spectrum offer; they require too much energy to be able to be created in high numbers, yet their size prevents embryogenesis. This disruptive selection has therefore essentially created two sexes with highly divergent gamete sizes, where males invest little in the production of offspring, thus allowing for more potential reproductive opportunities while females invest high amounts of energy in the production of offspring, causing females to be the limiting resource for males. Some theories even state that mating types, i.e. males and females, evolved as a consequence of the effect of anisogamy.〔Charlesworth, Brian (07/1978). "The population genetics of anisogamy". Journal of theoretical biology (0022-5193), 73 (2), p. 347.〕 Several other factors also play into the determination of size of gametes. Through numerical and analytical techniques, it has been confirmed that gamete size is linked with gamete surviability.〔Bell, G. (1978). "The Evolution of Anisogamy". Journal of Theoretical Biology 73: 247-270.〕 Due to the fact that females produce only a small amount of gametes, it is imperative that they remain long enough to be able to be fertilized. Additionally, female gametes harbor the zygote once it is fertilized by a sperm, thus putting even more importance on the survival of the egg, which in turn points toward the large size seen in female gametes. Male gametes, on the other hand, have the sole purpose of fertilizing the egg; thus their small, compact structure is based more on motility and energetic efficiency rather than survival. Other emerging theories also play their part in determining gamete size, one of which has to do with reducing parasite diversity for the benefit of the host cells.〔Hurst, L. (1990). "Parasite Diversity and the Evolution of Diploidy, Multicellularity and Anisogamy". Journal of Theoretical Biology 144: 429-443.〕 The small size of the sperm provides an advantage in that it is densely packed and space efficient, creating cytoplasmically sparse gametes that prevents wasting precious energy and resources in creating needless space as seen in ''Bryopsis hypnoides''. When fertilizing the less densely packed female gamete, which can be host to bacterial inclusions, the tight packing of the cyoplasm seen in the sperm could potentially be a mechanism to restrict parasites by not giving them any room, thus preventing parasite mixing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「anisogamy」の詳細全文を読む スポンサード リンク
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