Many flowering plants prevent inbreeding and increase genetic diversity by a process called self-incompatibility, in which pollination fails to set seed if the pollen is identified as its own by the pistil. A research team, led by Teh-hui Kao, Professor of Biochemistry and Molecular Biology at Penn State, has announced, in a paper published in the May 20 issue of Nature, the discovery of a gene of petunias that controls pollen function in self-incompatibility.
This discovery completes a critical missing link in the understanding of how self-incompatibility works. Ten years ago, Kao announced, in another paper published in Nature, the identification of the gene, called the S-RNase gene (S for self-incompatibility), that controls pistil function in self-incompatibility. "This male component turned out to be much more elusive than the pistil component," says Kao. "Our team, as well as others, has worked for the past ten years to find it." The recently identified gene, named PiSLF (for Petunia inflata S-locus F-box), encodes a new member of a large family of F-box proteins that are known to mediate protein degradation in diverse organisms, including animals, plants and yeast. While a species may have as many as 50 or 60 different S-alleles, each plant has only two of them, one inherited from each parent. An allele is one of a number of possible variants of a particular gene; for example, two alleles exist for each of the three genes that determine eye color in humans. Pollen grains are haploid, meaning that they contain only a single set of chromosomes, and thus each pollen grain contains only one of the two S-alleles of the parent plant. The pistil, on the other hand, is diploid, meaning that it has two sets of chromosomes (one from each parent) and therefore has both S-alleles of the parent plant. During pollination, if the S-allele of the pollen does not match either of the two S-alleles in the pistil, the pollen will germinate on the surface of the pistil to produce pollen tubes, which will then grow through the pistil to the ovary to effect fertilization. However, if the S-allele of the pollen matches either of the two S-alleles in the pistil, growth of the pollen tube is stopped about one third of the way to the ovary, preventing fertilization. Triggering this self-incompatibility response requires an interaction between the product of an S-allele produced in pollen and the product of a genetic counterpart produced in the pistil. To identify the pollen component in self-incompatibility, the team examined the DNA sequence of a chromosomal region containing the S2-allele of the S-RNase gene (the previously identified pistil component for plants containing the specific S-locus allele that is labeled S2). "The gene controlling the pollen function must be very closely linked to the S-RNase gene to prevent recombination," says Kao. "Otherwise, recombination between these two genes would cause the breakdown of self-incompatibility, which has never been observed in nature" After identifying the PiSLF gene, located approximately 161 kb from the S-RNase gene, Kao's team had to demonstrate that the gene was indeed the pollen component of self-incompatibility. "Other labs have found similar genes in the vicinity of the S-RNase gene in various other species" he says. "But proximity alone is insufficient to show the relationship." They took advantage of a phenomenon known as competitive interaction to demonstrate the function of the PiSLF gene in self-incompatibility. It has been known for some time that if pollen has two different S-alleles (which could result when the chromosomal region containing the pollen S-allele is duplicated in a plant), the pollen fails to function in self-incompatibility and thus cannot be rejected by any plant pistil. However, pollen with two identical S-alleles (again resulting from duplication of the pollen S-allele) remains functional in self-incompatibility. The team carried out three sets of experiments. In one set, the S2-allele of PiSLF was introduced into plants of S1S1 genotype – plants containing two identical S-locus genes of a type labeled S1 - via standard plant transformation techniques. For each transgenic plant generated, half of the pollen produced contained the endogenous (originating from within the plant) pollen S1-allele plus the PiSLF2 transgene (a gene that is introduced from a source outside the plant), whereas the other half only contained the endogenous pollen S1-allele. If PiSLF is the pollen component, the pollen that contained PiSLF2 should contain two different pollen S-alleles, S1 from the endogenous gene and S2 from the transgene, and based on competitive interaction, should fail to function in self-incompatibility. However, the pollen that contained only the endogenous pollen S1-allele should function normally. Thus, the prediction was that the transgenic plants would set seeds upon self-pollination (i.e., becoming self-compatible) and that all the resulting progeny should inherit the PiSLF2 transgene. The results from this set of experiments, as well as from two other sets using different genotypes of plants as recipient of PiSLF2, were completely in agreement with the prediction based on competitive interaction and based on the assumption that PiSLF is the pollen component. The team that made this discovery consisted of five graduate students, Paja Sijacic, Xi Wang, Andrea L. Skirpan, Yan Wang and Peter E. Dowd, and a postdoctoral scholar, Andrew G. McCubbin. In addition, a research scientist, Shihshieh Huang, at Monsanto (a former graduate student of Kao's group) participated in the project as a collaborator. This discovery could have commercial application for hybrid seed production in crop plants, such as corn and soy bean, that have lost self-incompatibility. Raising hybrid seed has been one of the major goals of horticultural and agricultural practice, because hybrid plants are more productive (due to hybrid vigor) and more uniform in quality than plants derived from self-pollination or random pollination. To raise hybrid seed, self-pollination and sib-pollination (pollination by a plant of the same hybrid) must be circumvented. One method is hand emasculation of the line used as female parent, which is then naturally cross-pollinated by pollen from the line serving as male parent and planted in an adjacent row. However, this process is very labor intensive and invariably expensive. If the crop plants can be made self-incompatible by the introduction of the genes controlling self-incompatibility, then all seeds produced will be hybrids resulting from cross-pollination between two different lines. This would facilitate the production and increase the yield of hybrid seed and, at the same time, reduce the labor costs.
Story Source: Materials provided by Penn State. Note: Content may be edited for style and length. Self-pollination is a form of pollination in which pollen from the same plant arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperms). 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 types of self-fertilization.
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.[1] 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. 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 also capable of self-pollination as well as cross-pollination. 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. 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,[2] 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. About 42% of flowering plants exhibit a mixed mating system in nature.[3] 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.[4] The evolutionary shift from outcrossing to self-fertilization is one of the most common evolutionary transitions in plants. About 10-15% of flowering plants are predominantly self-fertilizing.[5] A few well-studied examples of self-pollinating species are described below. OrchidsSelf-pollination in the slipper orchid Paphiopedilum parishii occurs when the anther changes from a solid to a liquid state and directly contacts the stigma surface without the aid of any pollinating agent.[6] The tree-living orchid Holcoglossum amesianum has a type of self-pollination mechanism in which the bisexual flower turns its anther against gravity through 360° in order to insert pollen into its own stigma cavity—without the aid of any pollinating agent or medium. This type of self-pollination appears to be an adaptation to the windless, drought conditions that are present when flowering occurs, at a time when insects are scarce.[7] Without pollinators for outcrossing, the necessity of ensuring reproductive success appears to outweigh potential adverse effects of inbreeding. Such an adaptation may be widespread among species in similar environments. Self-pollination in the Madagascan orchid Bulbophyllum bicoloratum occurs by virtue of a rostellum that may have regained its stigmatic function as part of the distal median stigmatic lobe.[8] Caulokaempferia coenobialisIn the Chinese herb Caulokaempferia coenobialis a film of pollen is transported from the anther (pollen sacs) by an oily emulsion that slides sideways along the flower’s style and into the individual’s own stigma.[9] The lateral flow of the film of pollen along the style appears to be due solely to the spreading properties of the oily emulsion and not to gravity. This strategy may have evolved to cope with a scarcity of pollinators in the extremely shady and humid habitats of C. coenobialis. Capsella rubellaCapsella rubella (Red Shepard’s purse)[10][11] is a self-pollinating species that became self-compatible 50,000 to 100,000 years ago, indicating that self-pollination is an evolutionary adaptation that can persist over many generations. Its out-crossing progenitor was identified as Capsella grandiflora. Arabidopsis thalianaArabidopsis thaliana is a predominantly self-pollinating plant with an out-crossing rate in the wild estimated at less than 0.3%.[12] A study suggested that self-pollination evolved roughly a million years ago or more.[13] Meiosis followed by self-pollination produces little overall genetic variation. This raises the question of how meiosis in self-pollinating plants is adaptively maintained over extended periods (i.e. for roughly a million years or more, as in the case of A. thaliana)[13] in preference to a less complicated and less costly asexual ameiotic process for producing progeny. An adaptive benefit of meiosis that may explain its long-term maintenance in self-pollinating plants is efficient recombinational repair of DNA damage.[14] This benefit can be realized at each generation (even when genetic variation is not produced).
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Which method was developed by plants to prevent self-pollination
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