Anisogamy
Encyclopedia
Heterogamy
Heterogamy has a number of biological definitions:*In reproductive biology, heterogamy is the alternation of differently organized generations, applied to the alternation between parthenogenetic and a sexual generation...
) refers to a form of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
involving the union or fusion of two dissimilar gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s (differing either in size alone or in size and form) — anisogamous, anisogamic, (adj). The smaller gamete is considered to be male (sperm cell
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
), whereas the larger gamete is regarded as female (egg cell
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
).
There are several types of anisogamy. Both gametes may be flagellated
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
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 sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
atia. In flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s, the gametes are non-motile cells within gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
s.
The form of heterogamy that occurs in animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s is oogamy
Oogamy
Oogamy is the familiar form of sexual reproduction. It is a form of anisogamy in which the female gamete is significantly larger than the male gamete and is non-motile. The male gametes are typically highly motile spermatozoa competing for the fertilization of the immotile egg.By contrast to...
. In oogamy, a large, non-motile egg cell (ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
) is fertilized by a small, motile sperm cell (spermatozoon
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
). 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 pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s, which attract the swimming sperm cells.
Evolution of anisogamy
A popular theory has proposed that anisogamy originated through disruptive selectionDisruptive selection
Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups...
acting on an ancestral isogamous
Isogamy
Isogamy is a form of sexual reproduction that involves gametes of similar morphology , differing only in allele expression in one or more mating-type regions...
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.
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
Sigmoid function
Many natural processes, including those of complex system learning curves, exhibit a progression from small beginnings that accelerates and approaches a climax over time. When a detailed description is lacking, a sigmoid function is often used. A sigmoid curve is produced by a mathematical...
or concave
Concave function
In mathematics, a concave function is the negative of a convex function. A concave function is also synonymously called concave downwards, concave down, convex upwards, convex cap or upper convex.-Definition:...
, 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.
A more recent theory emphasizes the importance of physical constraints on the processes by which two gametes get together and demonstrates that this alone can provide sufficiently strong disruptive selection. This theory has the advantage that it is more thoroughly based on established relationships.
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.