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.). 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.
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. 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. 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.
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. However, in Ascomycetes, anisogamy evolved from isogamy before mating types.
The three main theories for the evolution of anisogamy are gamete competition, gamete limitation, and intracellular conflicts, but the last of these three is not well supported by current evidence. Both gamete competition and gamete limitation assume that anisogamy originated through disruptive selection acting on an ancestral isogamous population with external fertilization, due to a trade-off between larger gamete number and gamete size (which in turn affects zygote survival), because the total resource one individual can invest in reproduction is assumed to be fixed.
The first formal, mathematical theory proposed to explain the evolution of anisogamy was based on gamete limitation: this model assumed that natural selection would lead to gamete sizes that result in the largest population-wide number of successful fertilizations. If it is assumed that a certain amount of resources provided by the gametes are needed for the survival of the resulting zygote, and that there is a trade-off between the size and number of gametes, then this optimum was shown to be one where both small (male) and large (female) gametes are produced. However, these early models assume that natural selection acts mainly at the population level, something that is today known to be a very problematic assumption.
The first mathematical model to explain the evolution of anisogamy via individual level selection, and one that became widely accepted was the theory of gamete or sperm competition. Here, selection happens at the individual level: those individuals that produce more (but smaller) gametes also gain a larger proportion of fertilizations simply because they produce a larger number of gametes that ‘seek out’ those of the larger type. However, because zygotes formed from larger gametes have better survival prospects, this process can again lead to the divergence of gametes sizes into large and small (female and male) gametes. The end result is one where it seems that the numerous, small gametes compete for the large gametes that are tasked with providing maximal resources for the offspring.
Some recent theoretical work has challenged the gamete competition theory, by showing that gamete limitation by itself can lead to the divergence of gamete sizes even under selection at the individual level. While this is possible, it has also been shown that gamete competition and gamete limitation are the ends of a continuum of selective pressures, and they can act separately or together depending on the conditions. These selection pressures also act in the same direction (to increase gamete numbers at the expense of size) and at the same level (individual selection). Theory also suggests that gamete limitation could only have been the dominant force of selection for the evolutionary origin of the sexes under quite limited circumstances, and the presence on average of just one competitor can makes the ‘selfish’ evolutionary force of gamete competition stronger than the ‘cooperative’ force of gamete limitation even if gamete limitation is very acute (approaching 100% of eggs remaining unfertilized).
There is then a relatively sound theory base for understanding this fundamental transition from isogamy to anisogamy in the evolution of reproduction, which is predicted to be associated with the transition to multicellularity. Some comparative empirical evidence for the gamete competition theories exists, although it is difficult to use this evidence to fully tease apart the competition and limitation theories because their testable predictions are similar. It has also been claimed that some of the organisms used in such comparative studies do not fit the theoretical assumptions well.
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