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Different forms of anisogamy: A) anisogamy of motile cells, B) _(egg_cell">oogamy_(
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Anisogamy is a form of
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
that involves the union or fusion of two
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s that differ in size and/or form. The smaller gamete is
male
Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization.
A male organism cannot reproduce sexually without access to ...
, a
sperm cell
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
, whereas the larger gamete is
female
Female (Venus symbol, symbol: ♀) is the sex of an organism that produces the large non-motile ovum, ova (egg cells), the type of gamete (sex cell) that fuses with the Sperm, male gamete during sexual reproduction.
A female has larger gamet ...
, typically an [
gg cell. Anisogamy is predominant among multicellular organisms.
In both plants and animals gamete size difference is the fundamental difference between
female
Female (Venus symbol, symbol: ♀) is the sex of an organism that produces the large non-motile ovum, ova (egg cells), the type of gamete (sex cell) that fuses with the Sperm, male gamete during sexual reproduction.
A female has larger gamet ...
s and
male
Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization.
A male organism cannot reproduce sexually without access to ...
s.
Anisogamy most likely evolved from
isogamy
Isogamy is a form of sexual reproduction that involves gametes of the same morphology (indistinguishable in shape and size), found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as male or fe ...
.
Since the biological definition of male and female is based on gamete size, the evolution of anisogamy is viewed as the evolutionary origin of male and female
sex
Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes (spermatozoa, sperm, pollen), while females produce larger ones ( ova, of ...
es.
Anisogamy is the prerequisite to
sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
, and led the sexes to different primary and secondary sex characteristics including sex differences in behavior.
Geoff Parker
Professor Geoffrey Alan Parker FRS (born 24 May 1944) is an emeritus professor of biology at the University of Liverpool and the 2008 recipient of the Darwin Medal. Parker has been called “the professional’s professional”.
He has a par ...
,
Robin Baker, and Vic Smith were the first to provide a mathematical model for the evolution of anisogamy that was consistent with modern evolutionary theory.
Their theory was widely accepted but there are alternative hypotheses about the evolution of anisogamy.
Etymology
Anisogamy comes from the ancient Greek words 'aniso' meaning unequal and 'gamy' meaning marriage. The first known use of the term anisogamy was in the year 1891.
Definition
Anisogamy is the form of
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
that involves the union or fusion of two
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s, which differ in size and/or form. The smaller gamete is considered to be
male
Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization.
A male organism cannot reproduce sexually without access to ...
(a
sperm cell
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
), whereas the larger gamete is regarded as
female
Female (Venus symbol, symbol: ♀) is the sex of an organism that produces the large non-motile ovum, ova (egg cells), the type of gamete (sex cell) that fuses with the Sperm, male gamete during sexual reproduction.
A female has larger gamet ...
(typically an
gg_cell,_if_non-
gg_cell,_if_non-motile">
gg_cell,_if_non-motile).
There_are_several_types_of_anisogamy._Both_gametes_may_be_gg_cell,_if_non-motile">
gg_cell,_if_non-motile).
There_are_several_types_of_anisogamy._Both_gametes_may_be_flagellum">flagellated_
A_flagellum_(;_)_is_a_hairlike_appendage_that_protrudes_from_certain_plant_and_animal_sperm_cells,_and_from_a_wide_range_of_microorganisms_to_provide_motility._Many_protists_with_flagella_are_termed_as_flagellates.
A_microorganism_may_have_fro_...
_and_therefore_motility.html" "title="flagellum.html" "title="motile.html" ;"title="gg cell, if non-motile">gg cell, if non-motile).
There are several types of anisogamy. Both gametes may be flagellum">flagellated
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have fro ...
and therefore motility">motile
Motility is the ability of an organism to move independently, using metabolic energy.
Definitions
Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
. Alternatively, both of the gametes may be non-flagellated. The latter situation occurs in some algae and plants. In the red algae, red alga ''Polysiphonia'', non-motile eggs are fertilized by non-motile sperm. In flowering plants, the gametes are non-motile cells within gametophytes.
The form of anisogamy that occurs in
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s, including
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s, is
oogamy
Oogamy is an extreme form of anisogamy where the gametes differ in both size and form.
In oogamy the large female gamete (also known as ovum) is immobile, while the small male gamete (also known as sperm) is mobile. Oogamy is a common form of a ...
, where a large, non-motile egg (ovum) is fertilized by a small, motile sperm (
spermatozoon
A spermatozoon (; also spelled spermatozoön; ; ) 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. (A zygote is a single cell, with a complete set of chromosomes, t ...
). The egg is optimized for longevity, whereas the small sperm is optimized for motility and speed. The size and resources of the egg cell allow for the production of
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 like hormones outside the body of the secreting individual, to affect the behavio ...
s, which attract the swimming sperm cells.
Sexual dimorphism
Anisogamy is a core element of
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
that helps explain phenotypic differences between sexes.
Most species do not sexually reproduce, but most of those that do have male and female sexes, both of which are optimized for reproductive potential. Due to their differently sized and shaped gametes, both males and females have developed physiological and behavioral differences that optimize the individual's
fecundity
Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
.
Since most egg laying females typically must bear the offspring and have a more limited reproductive cycle, this typically makes females a
limiting factor
A limiting factor is a variable of a system that causes a noticeable change in output or another measure of a type of system. The limiting factor is in a pyramid shape of organisms going up from the producers to consumers and so on. A factor not l ...
in the reproductive success rate of males in a species. This process is also true for females selecting males, and assuming that males and females are selecting for different traits in partners, would result in phenotypic differences between the sexes over many generations. This hypothesis, known as the
Bateman's Principle
Bateman's principle, in evolutionary biology, is that in most species, variability in reproductive success (or reproductive variance) is greater in males than in females. It was first proposed by Angus John Bateman (1919–1996), an English geneti ...
, is used to understand the evolutionary pressures put on males and females due to anisogamy.
Although this assumption has criticism, it is a generally accepted model for sexual selection within anisogamous species. The selection for different traits depending on sex within the same species is known as sex-specific selection, and accounts for the differing phenotypes found between the sexes of the same species. This sex-specific selection between sexes over time also lead to the development of
secondary sex characteristic
Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a sp ...
s, which assist males and females in reproductive success.
In most species, both sexes choose mates based on the available phenotypes of potential mates.
These phenotypes are species specific, resulting in varying strategies for successful sexual reproduction. For example, large males are sexually selected for in elephant seals because their large size helps the male fight off other males, but small males are sexually selected for in spiders for they can mate with the female more quickly while avoiding
sexual cannibalism
Sexual cannibalism is when an animal, usually the female, cannibalizes its mate prior to, during, or after copulation.Polis, G.A. & Farley, R.D. Behavior and Ecology of Mating in the journal of Arachnology 33-46 (1979). It is a trait observed in m ...
. However, despite the large range of sexually selected phenotypes, most anisogamous species follow a set of predictable desirable traits and selective behaviors based on general reproductive success models.
Female phenotypes
For
internal fertilizers, female investment is high in reproduction since they typically expend more energy throughout a single reproductive event. This can be seen as early as
oogenesis
Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.
O ...
, for the female sacrifices gamete number for gamete size to better increase the survival chances of the potential zygote; a process more energetically demanding than
spermatogenesis
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubule ...
in males.
[Keyne Monro, Dustin J. Marshall Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm] Oogenesis occurs in the
ovary
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
, a female specific organ that also produces hormones to prepare other female-specific organs for the changes necessary in the reproductive organs to facilitate egg delivery in external fertilizers, and zygote development in internal fertilizers. The egg cell produced is not only large, but sometimes even immobile, requiring contact with the more mobile sperm to instigate fertilization.
Since this process is very energy-demanding and time-consuming for the female,
mate choice
Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
is often integrated into the female's behavior.
Females will often be very selective of the males they choose to reproduce with, for the phenotype of the male can be indicative of the male's physical health and heritable traits. Females employ mate choice to pressure males into displaying their desirable traits to females through
courtship
Courtship is the period wherein some couples get to know each other prior to a possible marriage. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marriage. A courtship may be an informal and private m ...
, and if successful, the male gets to reproduce. This encourages males and females of specific species to invest in courtship behaviors as well as traits that can display physical health to a potential mate. This process, known as
sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
,
results in the development of traits to ease reproductive success rather than individual survival, such as the inflated size of a termite queen. It is also important for females to select against potential mates that may have a
sexually transmitted infection
Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are Transmission (medicine), spread by Human sexual activity, sexual activity, especi ...
, for the disease could not only hurt the female's reproductive ability, but also damage the resulting offspring.
[Davies, N. B., Krebs, J. R., & West, S. A. (2012). ''An introduction to behavioural ecology''. Oxford: Wiley-Blackwell.]
Although not uncommon in males, females are more associated with
parental care
Parental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal k ...
. Since females are on a more limited reproductive schedule than males, a female often invests more in protecting the offspring to sexual maturity than the male. Like mate choice, the level of parental care varies greatly between species, and is often dependent on the number of offspring produced per sexual encounter.
In most species such as ''Drosophila melanogaster,'' females can utilize
sperm storage,
a process by which the female can store excess sperm from a mate, and fertilize her eggs long after the reproductive event if mating opportunities drop or quality of mates decreases. By being able to save sperm from more desirable mates, the female gains more control over its own reproductive success, thus allowing for the female to be more selective of males as well as making the timing of fertilization potentially more frequent if males are scarce.
Male phenotypes
For males of all species, the sperm cells they produce are optimized for ensuring
fertilization
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
of the female egg. These sperm cells are created through spermatogenesis, a form of
gametogenesis
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic d ...
that focuses on developing the most possible gametes per sexual encounter.
Spermatogenesis occurs in the
testes
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoster ...
, a male specific organ that is also produces hormones that trigger the development of secondary sex characteristics. Since the male's gametes are energetically cheap and abundant in every ejaculation, a male can greatly increase his sexual success by mating far more frequently than the female.
Sperm, unlike egg cells, are also mobile, allowing for the sperm to swim towards the egg through the female's sexual organs.
Sperm competition
Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and ...
is also a major factor in the development of sperm cells. Only one sperm can fertilize an egg, and since females can potentially reproduce with more than one male before fertilization occurs, producing sperm cells that are faster, more abundant, and more viable than that produced by other males can give a male reproductive advantage.
Since females are often the limiting factor in a species reproductive success, males are often expected by the females to search and compete for the female, known as
intraspecific competition
Intraspecific competition is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to r ...
.
This can be seen in organisms such as bean beetles, as the male that searches for females more frequently is often more successful at finding mates and reproducing. In species undergoing this form of selection, a fit male would be one that is fast, has more refined sensory organs, and spatial awareness.
Some secondary sex characteristics are not only meant for attracting mates, but also for competing with other males for copulation opportunities. Some structures, such as antlers in deer, can provide benefits to the male's reproductive success by providing a weapon to prevent rival males from achieving reproductive success.
However, other structures such as the large colorful tail feathers found in male peacocks, are a result of
Fisherian runaway
Fisherian runaway or runaway selection is a sexual selection mechanism proposed by the mathematical biologist Ronald Fisher in the early 20th century, to account for the evolution of ostentatious male ornamentation by persistent, directional fe ...
as well as several more species specific factors. Due to females selecting for specific traits in males, over time, these traits are exaggerated to the point where they could hinder the male's survivability.
However, since these traits greatly benefit sexual selection, their usefulness in providing more mating opportunities overrides the possibility that the trait could lead to a shortening of its lifespan through predation or starvation. These desirable traits extend beyond physical body parts, and often extend into courtship behavior and
nuptial gift
A nuptial gift is a nutritional gift given by one partner in some animals' sexual reproduction practices.
Formally, a nuptial gift is a material presentation to a recipient by a donor during or in relation to sexual intercourse that is not simpl ...
s as well.
Although some behaviors in males are meant to work within the parameters of cryptic female choice, some male traits work against it. Strong enough males, in some cases, can force themselves upon a female, forcing fertilization and overriding female choice.
Since this can often be dangerous for the female, an
evolutionary arms race
In evolutionary biology, an evolutionary arms race is an ongoing struggle between competing sets of co-evolving genes, phenotypic and behavioral traits that develop escalating adaptations and counter-adaptations against each other, resembling an a ...
between the sexes is often an outcome.
History
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
wrote that anisogamy had an impact on the evolution of
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
. He also argued that anisogamy had an impact on sexual behavior. Anisogamy first became a major topic in the biological sciences when Charles Darwin wrote about
sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
.
Mathematical models seeking to account for the evolution of anisogamy were published as early as 1932, but the first model consistent with evolutionary theory was that published by
Geoff Parker
Professor Geoffrey Alan Parker FRS (born 24 May 1944) is an emeritus professor of biology at the University of Liverpool and the 2008 recipient of the Darwin Medal. Parker has been called “the professional’s professional”.
He has a par ...
,
Robin Baker and Vic Smith in 1972.
Evolution
Although its evolution has left no fossil records,
it is generally accepted that anisogamy evolved from
isogamy
Isogamy is a form of sexual reproduction that involves gametes of the same morphology (indistinguishable in shape and size), found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as male or fe ...
and that it has evolved independently in several groups of eukaryotes including protists, algae, plants and animals.
According to
John Avise
John Charles Avise (born 1948) is an American evolutionary geneticist, conservationist, ecologist and natural historian. He is a Distinguished Professor of Ecology & Evolution, University of California, Irvine, and was previously a Distinguished Pr ...
anisogamy probably originated around the same time sexual reproduction and
multicellularity
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
occurred, over 1 billion years ago.
Anisogamy first evolved in multicellular
haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
species after different
mating type
Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct Sex, sexes. They also occur in macro-organisms such as fungi.
Definition
Mating types are the microorganism equivalent ...
s had become established.
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.
[Lessells C.M., Snook R.R., Hosken D.J. 2009 The evolutionary origin and maintenance of sperm: selection for a small, motile gamete mating type. In Sperm biology: An evolutionary perspective (eds. Birkhead T.R., Hosken D.J., Pitnick S.), pp. 43-67. London, Academic press.] 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. In fact, Hanschen et al. (2018) demonstrate that anisogamy evolved from isogamous multicellular ancestors and that anisogamy would subsequently drive secondary sexual dimorphism.
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.
A valuable model system to the study of the evolution of anisogamy is the volvocine algae, which group of
chlorophytes is quite unique for its extant species exhibit a diversity of mating systems (
isogamy
Isogamy is a form of sexual reproduction that involves gametes of the same morphology (indistinguishable in shape and size), found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as male or fe ...
and anisogamy) in addition to its extremes in both
unicellularity and
multicellularity
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
with a diversity of forms in species of intermediate ranges of sizes. Marine algae have been closely studied to understand the trajectories of such diversified reproductive systems,
evolution of sex and mating types, as well as the adaptiveness and stability of anisogamy.
See also
*
Bateman's principle
Bateman's principle, in evolutionary biology, is that in most species, variability in reproductive success (or reproductive variance) is greater in males than in females. It was first proposed by Angus John Bateman (1919–1996), an English geneti ...
*
Evolution of sex
Sexual reproduction is an adaptive feature which is common to almost all multicellular organisms and various unicellular organisms, with some organisms being incapable of asexual reproduction. Currently the adaptive advantage of sexual repro ...
*
Meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
*
Sex
Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes (spermatozoa, sperm, pollen), while females produce larger ones ( ova, of ...
References
{{Authority control
Reproduction
Germ cells
Asymmetry