Allogamy or cross-fertilization is the fertilization of an ovum from one individual with the spermatozoa of another. By contrast, autogamy 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 gametes fused in fertilization come from the same individual.
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This mechanism favours cross pollination (allogamy or xenogamy) in these plants.Cocucci, A. A. 1989. "El mecanismo floral de Schizanthus." Kurtziana 20:113–132.
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An organism produced by sexual cross-fertilization (allogamy) has at least two ancestors (its immediate parents), but a gene always has one ancestor per generation.
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Each segment is 4-angeled and 1-seeded. The seeds are little, brown, beanlike and about long. The dominant reproductive system is an allogamy (cross-pollinating) system. The reproductive age of M. scabrella is reached after around 3 years.
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Monocots have mechanisms to promote or suppress cross-fertilization (allogamy) and self- fertilization (autogamy or geitonogamy). The pollination syndromes of monocots can be quite distinct; they include having flower parts in multiples of three, adaptations to pollination by water (hydrogamy), and pollination by sexual deception in orchids.
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Allogamy, which is also known as cross-fertilisation, refers to the fertilisation of an egg cell from one individual with the male gamete of another. Autogamy which is also known as self-fertilisation, occurs in such hermaphroditic organisms as plants and flatworms; therein, two gametes from one individual fuse.
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Napa cabbage is an annual plant that reaches the generative period in the first year. It must be consumed in its vegetative period, so there is a challenge in cultivation not to reach the stadium of flowering. The stadium of flowering can be initiated by cold temperatures or the length of the day. Napa cabbage reproduces mainly by allogamy.
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Allogamy ordinarily involves cross-fertilization between unrelated individuals leading to the masking of deleterious recessive alleles in progeny.Michod, R.E. (1994). "Eros and Evolution: A Natural Philosophy of Sex" Addison-Wesley Publishing Company, Reading, Massachusetts. 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).
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Outcrossing, cross-fertilization or allogamy, in which offspring are formed by the fusion of the gametes of two different plants, is the most common mode of reproduction among higher plants. About 55% of higher plant species reproduce in this way. An additional 7% are partially cross-fertilizing and partially self-fertilizing (autogamy). About 15% produce gametes but are principally self-fertilizing with significant out-crossing lacking.
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Dioecy (Greek: διοικία "two households"; adjective form: dioecious) is a characteristic of a species, meaning that it has distinct male and female individual organisms. Dioecious reproduction is biparental reproduction. Dioecy is one method that excludes self-fertilization and promotes allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. Flowering plants have several other methods of excluding self-fertilization, called self-incompatibility.
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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.
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Various aspects of floral morphology promote allogamy. In plants with bisexual flowers, the anthers and carpels may mature at different times, plants being protandrous (with the anthers maturing first) or protogynous (with the carpels mature first). Monoecious species, with unisexual flowers on the same plant, may produce male and female flowers at different times. Dioecy, the condition of having unisexual flowers on different plants, necessarily results in outcrossing, and might thus be thought to have evolved for this purpose.
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The pendant flowers prevent pollination by Lepidoptera. In N. albimarginatus there may be either a long stigma with short and mid length anthers or a short stigma and long anthers (dimorphism). In N. triandrus there are three patterns of sexual organs (trimophism) but all have long upper anthers but vary in stigma position and the length of the lower anthers. Allogamy (outcrossing) on the whole is enforced through a late-acting (ovarian) self-incompatibility system, but some species such as N. dubius and N. longispathus are self-compatible producing mixtures of selfed and outcrossed seeds.
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This is common in plants (see Sexual reproduction in plants) and certain protozoans. In plants, allogamy is used specifically to mean the use of pollen from one plant to fertilize the flower of another plant and usually synonymous with the term "cross-fertilization" or "cross-pollination" (outcrossing), though 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. Parasites 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.
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Self-incompatibility (SI) is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage outcross and allogamy. It should not be confused with genetically controlled physical or temporal mechanisms that prevent self-pollination, such as heterostyly and sequential hermaphroditism (dichogamy). In plants with SI, when a pollen grain produced in a plant reaches a stigma of the same plant or another plant with a matching allele or genotype, the process of pollen germination, pollen-tube growth, ovule fertilization and embryo development is halted at one of its stages and consequently no seeds are produced. SI is one of the most important means of preventing inbreeding and promoting the generation of new genotypes in plants, and it is considered as one of the causes for the spread and success of angiosperms on the earth.
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