Self Pollination

Jai Ganesh · 2024-09-29 16:00:04

Self Pollination

Gist

A flower is self-pollinated (a “selfer”) if pollen is transferred to it from any flower of the same plant and cross-pollinated (an “outcrosser” or “outbreeder”) if the pollen comes from a flower on a different plant.

It is the transfer of pollen from the anther of a flower to the stigma of the same flower or sometimes to that of a genetically identical flower (as of the same plant or clone)

Summary

Self-pollination is a form of pollination in which pollen arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperms) of the same plant. The term cross-pollination is used for the opposite case, where pollen from one plant moves to a different plant.

There are two types of self-pollination: in autogamy, pollen is transferred to the stigma of the same flower; in geitonogamy, pollen is transferred from the anther of one flower to the stigma of another flower on the same flowering plant, or from microsporangium to ovule within a single (monoecious) gymnosperm. Some plants have mechanisms that ensure autogamy, such as flowers that do not open (cleistogamy), or stamens that move to come into contact with the stigma.

The term selfing that is often used as a synonym is not limited to self-pollination, but also applies to other type of self-fertilization.

Occurrence

Few plants self-pollinate without the aid of pollen vectors (such as wind or insects). The mechanism is seen most often in some legumes such as peanuts. In another legume, soybeans, the flowers open and remain receptive to insect cross pollination during the day. If this is not accomplished, the flowers self-pollinate as they are closing. Among other plants that can self-pollinate are many kinds of orchids, peas, sunflowers and tridax. Most of the self-pollinating plants have small, relatively inconspicuous flowers that shed pollen directly onto the stigma, sometimes even before the bud opens. Self-pollinated plants expend less energy in the production of pollinator attractants and can grow in areas where the kinds of insects or other animals that might visit them are absent or very scarce—as in the Arctic or at high elevations.

Self-pollination limits the variety of progeny and may depress plant vigor. However, self-pollination can be advantageous, allowing plants to spread beyond the range of suitable pollinators or produce offspring in areas where pollinator populations have been greatly reduced or are naturally variable.

Pollination can also be accomplished by cross-pollination. Cross-pollination is the transfer of pollen, by wind or animals such as insects and birds, from the anther to the stigma of flowers on separate plants.

Types of self-pollinating flowers

Both hermaphrodite and monoecious species have the potential for self-pollination leading to self-fertilization unless there is a mechanism to avoid it. 80% of all flowering plants are hermaphroditic, meaning they contain both sexes in the same flower, while 5 percent of plant species are monoecious. The remaining 15% would therefore be dioecious (each plant unisexual). Plants that self-pollinate include several types of orchids, and sunflowers. Dandelions are capable of self-pollination as well as cross-pollination.

Advantages

There are several advantages for self-pollinating flowers. Firstly, if a given genotype is well-suited for an environment, self-pollination helps to keep this trait stable in the species. Not being dependent on pollinating agents allows self-pollination to occur when bees and wind are nowhere to be found. Self-pollination or cross pollination can be an advantage when the number of flowers is small or they are widely spaced. During self-pollination, the pollen grains are not transmitted from one flower to another. As a result, there is less wastage of pollen. Also, self-pollinating plants do not depend on external carriers. They also cannot make changes in their characters and so the features of a species can be maintained with purity. Self-pollination also helps to preserve parental characters as the gametes from the same flower are evolved. It is not necessary for flowers to produce nectar, scent, or to be colourful in order to attract pollinators.

Disadvantages

The disadvantages of self-pollination come from a lack of variation that allows no adaptation to the changing environment or potential pathogen attack. Self-pollination can lead to inbreeding depression caused by expression of deleterious recessive mutations, or to the reduced health of the species, due to the breeding of related specimens. This is why many flowers that could potentially self-pollinate have a built-in mechanism to avoid it, or make it second choice at best. Genetic defects in self-pollinating plants cannot be eliminated by genetic recombination and offspring can only avoid inheriting the deleterious attributes through a chance mutation arising in a gamete.

Mixed mating

About 42% of flowering plants exhibit a mixed mating system in nature. In the most common kind of system, individual plants produce a single flower type and fruits may contain self-pollinated, out-crossed or a mixture of progeny types. Another mixed mating system is referred to as dimorphic cleistogamy. In this system a single plant produces both open, potentially out-crossed and closed, obligately self-pollinated cleistogamous flowers.

Details

Pollination is transfer of pollen grains from the stamens (the flower parts that produce them) to the ovule-bearing organs or to the ovules (seed precursors) themselves. In gymnosperm plants such as conifers and cycads, in which the ovules are exposed, the pollen is simply caught in a drop of fluid secreted by the ovule. In flowering plants, however, the ovules are contained within a hollow organ called the pistil, and the pollen is deposited on the pistil’s receptive surface, the stigma. There the pollen germinates and gives rise to a pollen tube, which grows down through the pistil toward one of the ovules in its base. In an act of double fertilization, one of the two sperm cells within the pollen tube fuses with the egg cell of the ovule, making possible the development of an embryo, and the other cell combines with the two subsidiary sexual nuclei of the ovule, which initiates formation of a reserve food tissue, the endosperm. The growing ovule then transforms itself into a seed.

As a prerequisite for fertilization, pollination is essential to the perpetuation of the vast majority of the world’s wild plants as well as to the production of most fruit and seed crops. It also plays an important part in programs designed to improve plants by breeding. Furthermore, studies of pollination are invaluable for understanding the evolution of flowering plants and their distribution in the world today. As sedentary organisms, plants usually must enlist the services of external agents for pollen transport. In flowering plants, these are (roughly in order of diminishing importance) insects, wind, birds, mammals, and water.

Types: self-pollination and cross-pollination:

Cross-pollination

An egg cell in an ovule of a flower may be fertilized by a sperm cell derived from a pollen grain produced by that same flower or by another flower on the same plant, in either of which two cases fertilization is said to be due to self-pollination (autogamy); or, the sperm may be derived from pollen originating on a different plant individual, in which case the process is called cross-pollination (heterogamy). Both processes are common, but cross-pollination clearly has certain evolutionary advantages for the species: the seeds formed may combine the hereditary traits of both parents, and the resulting offspring generally are more varied than would be the case after self-pollination. In a changing environment, some of the individuals resulting from cross-pollination still may be found capable of coping with their new situation, ensuring survival of the species, whereas the individuals resulting from self-pollination might all be unable to adjust. Self-pollination, or selfing, although foolproof in a stable environment, thus is an evolutionary cul-de-sac. There also is a more direct, visible difference between selfing and outbreeding (cross-pollination): in those species where both methods work, cross-pollination usually produces more, and better quality, seeds. A dramatic demonstration of this effect is found with hybrid corn (maize), a superior product that results from cross-breeding of several especially bred lines.

Mechanisms that prevent self-pollination:

Structural

Not surprisingly, many species of plants have developed mechanisms that prevent self-pollination. Some—e.g., date palms (Phoenix dactylifera) and willows (Salix species)—have become dioecious; that is, some plants produce only “male” (staminate) flowers, with the rest producing only “female” (pistillate or ovule-producing) ones. In species in which staminate and pistillate flowers are found on the same individual (monoecious plants) and in those with hermaphroditic flowers (flowers possessing both stamens and pistils), a common way of preventing self-fertilization is to have the pollen shed either before or after the period during which the stigmas on the same plant are receptive, a situation known as dichogamy. The more usual form of dichogamy, which is found especially in such insect-pollinated flowers as fireweed (Epilobium angustifolium) and salvias (Salvia species), is protandry, in which the stamens ripen before the pistils. Protogyny, the situation in which the pistils mature first, occurs in arum lilies and many wind-pollinated plants, such as grasses—although several grasses are self-pollinated, including common varieties of wheat, barley, and oats. Avocado has both protogynous and protandrous varieties, and these often are grown together to encourage cross-fertilization.

A structural feature of flowers that discourages selfing is heterostyly, or variation in the length of the style (neck of the pistil). This occurs in the common primrose (Primula vulgaris) and species of wood sorrel (Oxalis) and flax. In most British primrose populations, for example, approximately half the individuals have so-called “pin” flowers, which possess short stamens and a long style, giving the stigma a position at the flower’s mouth, whereas the other half have “thrum” flowers, in which the style is short and the stamens are long, forming a “thrumhead” at the opening of the flower. Bees can hardly fail to deposit the pollen they receive from one type of flower onto the stigmas of the other type. The genetic system that regulates flower structure in these primroses is so constituted that cross-pollination automatically maintains a 50:50 ratio between pins and thrums. In the flowers of purple loosestrife (Lythrum salicaria), the stamens and styles are of three different lengths to limit self-fertilization.

Chemical

Chemical self-incompatibility is another device for preventing self-fertilization. In this phenomenon, which depends on chemical substances within the plant, the pollen may fail to grow on a stigma of the same flower that produced it or, after germination, the pollen tube may not grow normally down the style to effect fertilization. The process is controlled genetically; it need not be absolute and can change in degree during the flowering season. Not surprisingly, chemical incompatibility usually is not found in those plants that have strong structural or temporal barriers against self-pollination. Formation of one such mechanism during evolution apparently was enough for most plant species.

Mechanisms that permit self-pollination:

Self-pollination

In many instances, successful self-pollination takes place at the end of a flower’s life-span if cross-pollination has not occurred. Such self-pollination may be achieved by curving of stamens or style as occurs, for example, in fireweed. It can be an evolutionary advantage when animal pollinators are temporarily scarce or when the plants in a population are widely scattered. Under such circumstances, selfing may tide the species over until better circumstances for outbreeding arrive. For this reason, selfing is common among annual plants; these often must produce an abundance of seed for the rapid and massive colonization of any bare ground that becomes available. If, in a given year, an annual plant were to produce no seed at all, survival of the species might be endangered.

A persistent habit of self-pollination apparently has been adopted successfully by some plant species whose natural pollinators have died out. Continued selfing also is practiced by many food-crop plants. Some of these plants are cleistogamous, meaning that the flowers fail to open, an extreme way of ensuring self-pollination. A similar process is apomixis, the development of an ovule into a seed without fertilization. Apomixis is easily demonstrated in lawn dandelions, which produce seeds even when stamens and styles are cut off just before the flowers open. Consistent apomixis has the same pros and cons as continued selfing. The offspring show very little genetic variability, but there is good survival if the species is well adapted to its habitat and if the environment does not change.

Additional Information

Self pollination is a process where pollen from the same plant reaches the stigma of a flower or ovule. This can occur in both angiosperms (flowering plants) and gymnosperms (non-flowering plants like conifers). Self- pollination can be further divided into two types: Autogamy and Geitonogamy.

Autogamy

Autogamy is a form of self fertilization where fusion of two gametes from the same individual take place. It is most commonly observed in the form of self-pollination.

* Autogamy is generally avoided in nature as the seeds produced result in subsequent species with low genetic diversity.
* This type of pollination can be seen in wheat, rice, pea, and other crops.

Geitonogamy

Geitonogamy is a type of self-pollination that involves the transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.

* Functionally it is similar to cross-pollination, but genetically similar to autogamy.
* It requires pollinating agents, such as wind, insects,birds,etc. A pollinator can visit multiple flowers on the same plant to complete the process of pollination.
* Examples: Monoecious plants like maize.

Math Is Fun Forum

#1 2024-09-29 16:00:04

Self Pollination

Board footer