Allogamy or cross-fertilization is the

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

of an

ovum The egg cell or ovum (: ova) is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is not capa ...

from one individual with the

spermatozoa A spermatozoon (; also spelled spermatozoön; : spermatozoa; ) is a motile sperm cell (biology), cell produced by male animals relying on internal fertilization. A spermatozoon is a moving form of the ploidy, haploid cell (biology), cell that is ...

of another. By contrast,

autogamy Autogamy or self-fertilization refers to the Cell fusion, fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a Reproduction, reproductive mechanism employed by many flowering pl ...

is the term used for self-fertilization. In humans, the fertilization event is an instance of allogamy. Self-fertilization occurs in hermaphroditic organisms where the two

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

s fused in fertilization come from the same individual. This is common in

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

s (see Sexual reproduction in plants) and certain

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

ns. In plants, allogamy is used specifically to mean the use of

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

from one plant to fertilize the

flower Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...

of another plant and usually synonymous with the term "cross-fertilization" or "cross-pollination" (

outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing in animals Out ...

). The latter term can be used more specifically to mean pollen exchange between different plant strains or even different plant species (where the term ''cross-hybridization'' can be used) rather than simply between different individuals. Allogamy is achieved through the use of external pollinating factors. The process of allogamy involves two types of external pollinating agents, known as abiotic agents and biotic agents. The abiotic agents are water and wind. The biotic agents are insects and animals, which include bees, butterflies, snails, and birds. Wind pollination is referred to as anemophily, and water pollination is referred to as hydrophilly. Insect pollination is referred to as entomophily, bird pollination is referred to as omithophily, and snail pollination is referred to as malacophily. Allogamy can lead to homozygosity. After reaching homozygosity, the species develop homozygous balance and fail to exhibit inbreeding depression. Mechanisms that promote self-pollination include homogamy, bisexuality, cleistogamy, the position of anthers, and chasmogamy. Allogamy promotes genetic diversity and reduces the risk of inbreeding depression. The persistent prevalence of allogamy throughout different species implies that this strategy provides selective advantages concerning adaptation to changing environments and sustaining fitness.

Parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s having complex life cycles can pass through alternate stages of allogamous and autogamous reproduction, and the description of a hitherto unknown allogamous stage can be a significant finding with implications for human disease.

Avoidance of inbreeding depression

Allogamy ordinarily involves cross-fertilization between unrelated individuals leading to the masking of deleterious recessive

allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...

s in progeny. By contrast, close inbreeding, including self-fertilization in plants and automictic parthenogenesis in hymenoptera, tends to lead to the harmful expression of deleterious recessive alleles (

inbreeding depression Inbreeding depression is the reduced biological fitness caused by loss of genetic diversity as a consequence of inbreeding, the breeding of individuals closely related genetically. This loss of genetic diversity results from small population siz ...

). In dioecious plants, the stigma may receive pollen from several different potential donors. As multiple pollen tubes from the different donors grow through the stigma to reach the ovary, the receiving maternal plant may carry out pollen selection favoring pollen from less related donor plants. Thus post-pollination selection may occur in order to promote allogamy and avoid inbreeding depression. Also, seeds may be aborted selectively depending on donor–recipient relatedness.

References

Reproduction {{biology-stub

Avoidance of inbreeding depression

See also

References