Self-pollination

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Self-pollination is a form of pollination that can occur when a flower has both stamen and a carpel (pistil) in which the cultivar or species is self fertile and the stamens and the sticky stigma of the carpel contact each other in order to accomplish pollination. The term is inaccurately used in many cases where an outside pollinator is actually required; such plants are merely self-fertile, or self pollenizing.

Occurrence[edit]

Few plants actually self-pollinate. 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, either by wind or insect, from the anther to the stigma between different flowers.

Types of self pollination[edit]

There are two types of self-pollination:

In Type I self-pollination{autogamy} , pollen is transferred from the anther to the stigma of the same flower. Such flowers are hermaphrodites, which have both sexes.

In Type II pollen is transferred from the anther of one flower to the stigma of another flower from the same plant.

Types of flowers that self pollinate[edit]

Both hermaphrodite and monoecious species have the potential for self-pollination leading to self-fertilization unless there is a mechanism to avoid it. Eighty percent of all flowering plants are hermaphroditic, meaning they contain both sexes, while 5 percent of plant species are monoecious, or unisexual.

Advantages of self-pollination[edit]

There are several advantages for self-pollinating flowers. 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 can be an advantage when the number of flowers are small or widely spaced.

Disadvantages of self-pollination[edit]

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, or 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[edit]

About 42% of flowering plants exhibit a mixed mating system in nature.[1] 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.[2] Experiments with the herb Ruellia nudiflora that displays dimorphic cleistogamy have provided insight into the mechanisms underlying the stability of mixed mating systems.[2]

Self-pollinating species[edit]

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.[3] A few well-studied examples of self-pollinating species are described below.

Paphiopedilum parishii[edit]

Self-pollination in the slipper orchid Paphiopedilum parishii occurs when the anther changes form a solid to a liquid state and directly contacts the stigma surface without the aid of any pollinating agent or floral assembly.[4]

Holcoglossum amesianum[edit]

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, but at a time when insects are scarce.[5] 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.

Caulokaempferia coenobialis[edit]

In 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.[6] 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 rubella[edit]

Capsella rubella (Red Shepard’s purse) is studied as a model for understanding the evolution of self-fertilization.[7][8] C. rubella 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 thaliana[edit]

Arabidopsis thaliana is a predominantly self-pollinating plant with an out-crossing rate in the wild estimated at less than 0.3%.[9] A study of the genome-wide pattern of linkage disequilibrium suggested that self-pollination evolved roughly a million years ago or more.[10]

Long-term benefit of meiosis in self-pollinators[edit]

Meiosis followed by self-pollination produces little overall genetic variation, including beneficial variation. This raises the question of how meiosis in self-pollinating plants is adaptively maintained over extended periods (i. e. for at least 104 to 106 years) 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 during formation of germ cells.[11][12] This benefit can be realized at each generation (even when genetic variation is not produced).

See also[edit]

References[edit]

  1. ^ Goodwillie C, Kalisz S, Eckert CG (2005) The evolutionary enigma of mixed mating systems in plants: Occurrence, theoretical explanations, and empirical evidence. Annu. Rev. Ecol. Evol. Syst. 36: 47-79. doi:10.1146/annurev.ecolsys.36.091704. 175539
  2. ^ a b Munguía-Rosas MA, Campos-Navarrete MJ, Parra-Tabla V (2013). "The effect of pollen source vs. flower type on progeny performance and seed predation under contrasting light environments in a cleistogamous herb". PLoS ONE 8 (11): e80934. doi:10.1371/journal.pone.0080934. PMC 3829907. PMID 24260515. 
  3. ^ Wright SI, Kalisz S, Slotte T (June 2013). "Evolutionary consequences of self-fertilization in plants". Proc. Biol. Sci. 280 (1760): 20130133. doi:10.1098/rspb.2013.0133. PMC 3652455. PMID 23595268. 
  4. ^ Chen LJ, Liu KW, Xiao XJ, Tsai WC, Hsiao YY, Huang J, Liu ZJ (2012). "The anther steps onto the stigma for self-fertilization in a slipper orchid". PLoS ONE 7 (5): e37478. doi:10.1371/journal.pone.0037478. PMC 3359306. PMID 22649529. 
  5. ^ Liu KW, Liu ZJ, Huang L, Li LQ, Chen LJ, Tang GD (June 2006). "Pollination: self-fertilization strategy in an orchid". Nature 441 (7096): 945–6. doi:10.1038/441945a. PMID 16791185. 
  6. ^ Wang Y, Zhang D, Renner SS, Chen Z (September 2004). "Botany: a new self-pollination mechanism". Nature 431 (7004): 39–40. doi:10.1038/431039b. PMID 15343325. 
  7. ^ Brandvain Y, Slotte T, Hazzouri KM, Wright SI, Coop G (2013). "Genomic identification of founding haplotypes reveals the history of the selfing species Capsella rubella". PLoS Genet. 9 (9): e1003754. doi:10.1371/journal.pgen.1003754. PMC 3772084. PMID 24068948. 
  8. ^ Slotte T, Hazzouri KM, Ågren JA, Koenig D, Maumus F, Guo YL, Steige K, Platts AE, Escobar JS, Newman LK, Wang W, Mandáková T, Vello E, Smith LM, Henz SR, Steffen J, Takuno S, Brandvain Y, Coop G, Andolfatto P, Hu TT, Blanchette M, Clark RM, Quesneville H, Nordborg M, Gaut BS, Lysak MA, Jenkins J, Grimwood J, Chapman J, Prochnik S, Shu S, Rokhsar D, Schmutz J, Weigel D, Wright SI (July 2013). "The Capsella rubella genome and the genomic consequences of rapid mating system evolution". Nat. Genet. 45 (7): 831–5. doi:10.1038/ng.2669. PMID 23749190. 
  9. ^ Population genetic structure and outcrossing rate of Arabidopsis thaliana (L.) Heynh. Abbott RJ, Gomes MF. Heredity 1989 62:411-418
  10. ^ Tang C, Toomajian C, Sherman-Broyles S, Plagnol V, Guo YL, Hu TT, Clark RM, Nasrallah JB, Weigel D, Nordborg M (August 2007). "The evolution of selfing in Arabidopsis thaliana". Science 317 (5841): 1070–2. doi:10.1126/science.1143153. PMID 17656687. 
  11. ^ Bernstein H, Hopf FA, Michod RE. (1987). The molecular basis of the evolution of sex. Adv Genet 24:323-70. Review. PMID: 3324702
  12. ^ Harris Bernstein, Carol Bernstein and Richard E. Michod (2011). Meiosis as an Evolutionary Adaptation for DNA Repair. Chapter 19 in DNA Repair. Inna Kruman editor. InTech Open Publisher. DOI: 10.5772/25117 http://www.intechopen.com/books/dna-repair/meiosis-as-an-evolutionary-adaptation-for-dna-repair