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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 alone or in size and form) — anisogamous, anisogamic, (adj).[1] 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
A popular theory has proposed that anisogamy originated through disruptive selection acting on an ancestral isogamous population. Starting with an isogamous population with two mating types and taking into account the functional relationship, g(m), between the fitness of a gamete and its size, m, as well as the relationship, f(S), between the fitness of a zygote and its size, S.[2]
Evolutionary game theory has been used to determine the existence and continuous stability of isogamous and anisogamous strategies for the two mating types under various models for the two functions g(m) and f(S). In the ancestral unicellular state, these two functions are likely to have been similar; this leads to isogamy whether they are sigmoidal or concave, though in the latter case allowance must be made for a minimal gamete size. The development of multicellularity may leave the functional relationship between the fitness of a gamete and its size relatively unchanged; while the relationship between the fitness of a zygote and its size moves to the right, leading to the evolution of anisogamy.
Therefore, the disruptive selection theory provides a powerful explanation of the origin of anisogamy, though other selective forces may have been involved in the subsequent specialization of micro and macrogametes.
References
- ^ http://www.thefreedictionary.com/anisogamy
- ^ Bulmer, M.G. and Parker, G.A. (2002). Proc Biol Sci. 269(1507): 2381–2388.
See also
The evolution of anisogamy can be shown as disruptive selection, acting over 1000's of generations to produce such a vast difference in morphology of human gametes. Thus selection has acted producing optimal gametes that are succesful at fusing, thus increasing the fitness of human populations as a whole.
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