self pollination vs cross pollination
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self pollination vs cross pollination
In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Pollination has been well studied since the time of Gregor Mendel.
Mendel successfully carried out self- as well as cross-pollination in garden peas while studying how characteristics were passed on from one generation to the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-day cultivars. A case in point is today’s corn, which is a result of years of breeding that started with its ancestor, teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds—vastly different from today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the genetic difference between them is miniscule.
Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.
Self-Pollination
Self-pollinating species can reproduce even if animal pollinators are not present. However, reproduction through self-pollination reduces genetic diversity.
The anther opens and the pollen lands on the stigma of the same flower.
To promote cross-pollination and increase genetic diversity, plants have evolved a wide variety of sexual strategies to attract pollinators and spread pollen from one flower to another of the same species.
Self pollination is one of the two types of pollination in which pollen grains transfer from an anther to the stigma of the same flower or from an anther of one flower to the stigma of another flower of the same plant. Hence, self pollination occurs between two structures of the same flower or the same plant. Annual plants such as barley, peas, vetch, and peanut show self pollination.
Moreover, flowers that show self pollination possess different adaptations to increase self pollination as well as to reduce self pollination. These adaptations enhance self pollination and prevent the chances of cross pollination. Some adaptations include bearing closed flowers, occurring of pollination before the flower opens, presence of anthers on the top of carpals of the flower, etc.
Cross-Pollination
The cross-pollination is defined as the deposition of pollen grains from a flower to the stigma of another flower. Commonly, the process is done by insects and wind. By insects, the process takes place in several plants like strawberries, grapes, raspberries, tulips, apples, plums, pears, daffodils, and more.
Advantages of cross-pollination
When compared with self-pollination (the transfer of pollen within a flower or between flowers on the same plant), cross-pollination clearly has certain evolutionary advantages. The seeds formed by outbreeding 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, the genetic variability within a cross-pollinated population may enable some individuals to be adapted to 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: 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.
Adaptations for Cross Pollination
Cross pollinating flowers show different adaptations. Normally, they possess a nice colour and a smell to attract pollinators. Some plants show special types of adaptations for cross pollination. One adaptation is unisexuality, which is the presence of different male and female plants. Dichogamy is another adaptation. That is, the maturity of the gynoecium and the androecium of the same flower takes place at two different times. Dimorphism is another adaptation. Here, some flowers possess short styles and stamens at the mouth of the corolla tube. Other flowers have long styles and anthers attached to the corolla tube below the mouth.
Wind pollinated flowers also show some adaptations to enhance cross pollination. The flowers are small, not coloured, not scented and have no nectar. Stigma is large and feathery. Pollen grains are small, light and produced in large numbers. They are dry with smooth extine. The flowers are simple. The anthers are versatile. Insect-pollinated flowers also show some adaptations to enhance cross pollination. They are large, brightly coloured, scented flowers with nectar. Stigma is small and sticky while anthers are not versatile. Pollen grains are large and heavy with a rough extine. These flowers show a complex structure.
Why do pollinators visit flowers?
While food is often a sufficient lure for pollinators, flowering plants also attract pollinators using a combination of petal shapes, scents, and colors. “Pollination syndromes” have been described to depict the attraction of certain types,
shapes, colors, and fragrances of flowers to a range of pollinators.
Plant materials and treatments
Two ten-year-old tea cultivars, namely, C. sinensis cv. Fudingdabai and C. sinensis cv. Yulv, cultivated in tea germplasm repository of Tea Research
Institute of Fujian Academy of Agricultural Sciences were used in this study. Flowers from both of the two tea cultivars have three petals and trifid stigmas. The stigmas of ‘Fudingdabai’ divided at the base but ‘Yulv’ at the upper part.
Flower buds of the two tea cultivars were harvested at 4:00 pm for pollens collection. Besides,
the remaining flower buds of two cultivars were emasculated and used
for artificial pollination next morning. A total of four pollination combinations were conducted: ‘Fudingdabai’
(♂) × ‘Fudingdabai’ (♀), ‘Yulv’ (♂) × ‘Yulv’ (♀), ‘Fudingdabai’ (♂) × ‘Yulv’ (♀), and Yulv’ (♂) × ‘Fudingdabai’ (♀), as shown in Fig. 2. The un-pollinated and pollinated pistils at 8, 24, 48 and 72 h were picked
from each combination and frozen quickly in liquid nitrogen and stored at − 80 °C for RNA extraction.
Difference Between Self-Pollination and Cross-Pollination
Self-Pollination | Cross-Pollination |
Transfer pollen grains from the anther to the stigma of the same flower. | Transfer pollen grains from the anther to the stigma of a different flower. |
This process can take place in the same flower or a different flower of the same plant. | This process can take place between two flowers present on different plants. |
It occurs in the flowers which are genetically identical. | It occurs between flowers which are genetically different. |
Few species that exhibit self-pollination – Paphiopedilum parishii, Arabidopsis thaliana | Few species that exhibit cross-pollination – apples, daffodils, pumpkins and grasses |
Causes homogenous conditions in progenies. | Causes heterozygous condition in progenies. |
Self-pollination increases genetic uniformity and decreases genetic variation. | Cross-pollination decreases genetic uniformity and increases genetic variation. |
Causes inbreeding. | Causes outbreeding. |
Reduces the gene pool. | Maintains the gene pool. |
Produces limited amounts of pollen grains. | Produces large amounts of pollen grains. |
In self-pollination, both the stigma and anther simultaneously mature | In cross-pollination, both the stigma and anther mature at different times. |
Transfers a limited number of pollens. | Transfers large numbers of pollen. |
This process is carried out even when the flowers are closed. | For cross-pollination to happen, the flower should be open. |
No need for pollinators to transfer pollen grains. | Require pollinators to transfer pollen grains. |
Pollen grains are transferred directly to a flower’s stigma. | Pollen grains are carried via wind, insects, animals, water, etc. |