Sunday, July 26, 2009

CLEISTOGAMY - FLORAL BIOLOGY

Contents

  • Definition
  • Charasteristics of Cleistogamous flowers
  • Cleistogamous spikelets of Muhlenbergia microsperma
  • Dehiscence
  • Self pollination
  • Reproduction
  • Mapping and identification of cleistogamy gene
  • Advantages of Cleistogamy
  • Potential industrial application
  • Conclusion



  • Definition:

    Cleistogamy or automatic self-pollination describes the trait of certain plants to propagate by using non-opening, self-pollinating flowers. Especially in peanuts, peas, and beans, this behaviour is most widespread in legumes. The flowers are bisexual, very small, in conspicuous, colourless and do not secrete nectar or honey e.g. Oxalis, Groundnut, Commelina bengalensis.

    More common flowers " opposite of cleistogamy is called chasmogamy – meaning "opening flowers".

    Cleistogamous species are distributed worldwide: well-known examples include violets, rushes, wood sorrel, and some rice species. Practically all cleistogamous plants can also produce normal open or chasmogamous flowers. Cleistogamy appears to be related to environmental conditions: when the environment is harsh, plants are more likely to produce cleistogamous flowers. The classic example is jewel weed (Impatiens capensis), an annual species that in moist, high-light environments produces many large and beautiful chasmogamous flowers, which offer large amounts of nectar to pollinating bees. However, on unfavorable sites, the plants are stunted and produce only cleistogamous flowers.



    Characteristics of Cleistogamous flowers:


    The calyx, generally tipped with red at this time, has been cut apart to expose the flower at the left



    One of S. roemeriana's most interesting features is its hidden flower (cleistogamous) phase, shown below, which begins in late summer and lasts until frost. Since this bloom lacks a showy red corolla, the secret of an auto-pollinating stage is difficult to discover. The success of pollination being guaranteed, this stage also produces great amounts of seed (often the maximum of 4 seeds per bloom), even during extreme drought. In October 1999, after a period of 12 weeks without measurable rain, the fertile cleistogamous phase was in full swing in many areas.

    S. roemeriana produces up to 4 nutlets (far left) at the base of the flower cup. Originally white, they turn nearly black and fall out when mature, the calyx becomming orangish brown. Those shown here are from cleistogamous blooms, the source of the seeds at the left has been cut open.

    Cleistogamous spikelets in Muhlenbergia microsperma:

    Little seed muhly (Muhlenbergia microsperma). A basal leaf sheath has been split apart to expose two cleistogamous spikelets. These are fertile,self-pollinated spikelets that are completely contained within the tightly-rolled leaf sheath.


    Dehiscence:


    Some angiosperm plants produce cleistogamous flowers which do not dehisce and remain closed to all pollinating agents. The adaptive capacity of some flower to close in overcast or rainy weather is beneficial in limiting anther exposure to times when pollination is most likely to save. The number of cleistogamic florets increased in wheat under adverse conditions of rain &low or extremely high temperature.

    Self-pollination (autogamy):

    Pollination that occurs before the flower opens is always self-pollination. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are then pollinated. Cleistogamous flowers must of necessity be self-compatible or self-fertile plants. Other plants are self-incompatible. These are end points on a continuum, not absolute points.

    Pollen moves to the female part of the same flower or to another flower on the same individual plant. This is sometimes referred to as self-pollination, but this is not synonymous with autogamy. Cleistogamy: pollination that occurs before the flower opens is always self-pollination

    Reproduction:

    Cleistogamous flowers of each species bore anthers. The petals were shorter than the calyx and the flower remained closed until the young ovary forced the calyx lobes apart. F2 individuals from cleistogamous flowers of interspecific hybrids segregated for the traits observed. Seed from the cleistogamous flowers of a dwarf plant growing among normal plants produced only dwarfs, whereas 75 per cent of the seed from showy flowers yielded tall normal-appearing plants. The data indicated that the reproductive mechanism in the cleistogamous flowers was sexual and that self-fertilization prevailed in this flower type. There was no evidence for apomixis or a unisexually pistillate condition.

    The Cleistogamous Breeding System: A Review of Its Frequency, Evolution, and Ecology in Angiosperms

    Cleistogamy, a breeding system in which permanently closed, self-pollinated flowers are produced, has received increasing attention in recent years, but the last comprehensive review of this system was over 20 years ago. The goal of this paper is to clarify the different types of cleistogamy, quantify the number of families, genera, and species in which cleistogamy occurs, and estimate the number of times and potential reasons why cleistogamy has evolved within angiosperms. Cleistogamous species were identified through a literature survey using 13 online databases with references dating back to 1914; only those species well-supported by floral descriptions or empirical data were included in the data set. On the basis of this survey, we suggest the use of three different categories of cleistogamy in future studies: dimorphic, complete, and induced. Based on these categories, cleistogamy in general is present in 693 angiosperm species, distributed over 228 genera and 50 families. When analyzed on a family level across the angiosperms, the breeding system has evolved approximately 34 to 41 times. Theoretical investigations indicate that the evolution of cleistogamy in taxa may be influenced by the presence of heterogeneous environments, inbreeding depression and geitonogamy, and differential seed dispersal, as well as by various ecological factors and plant size. Cleistogamy will undoubtedly be discovered in additional species as the reproductive biology of more taxa is examined in the future. Such information will be invaluable for understanding the selective pressures and factors favoring the evolution of cleistogamy as well as the evolutionary loss of this breeding system, a subject that has received little attention to date.

    Fine Mapping and Identification of Cleistogamy Gene:


    In Barley Cleistogamy in barley is controlled by two tightly linked genes or a multiple allelic system with three or more alleles, as evidenced by genetic studies, and has been mapped to the chromosome 2HL. Co linearity of Chr.2HL of barley with rice Chr.4 has been exploited in this study for marker enrichment as well as fine mapping of cly1, employing barley ESTs orthologous to rice Chr.4. A high level of conservation of gene order was observed between the two chromosomal regions, in spite of a large inversion in barley genome corresponding to approximately 1.86 Mb in rice, as evidenced in three mapping populations comprising of 2,652 F2 plants of Azumamugi × Kanto Nakate Gold, 1,328 F2 plants of OUH602 × Kanto Nakate Gold and 1,500 BC7F2 plants of Misato Golden × Satsuki Nijo. cly1 was mapped within this inversion between two barley ESTs placed at an interval of 0.7 cM, equivalent to a physical distance of 80 Kb in rice Chr.4. There were seven candidate genes within this region in rice and cly1 cosegregated with one of the candidate genes coding for a transcription factor. Mapping of markers developed from the Barley cv. Morex BAC clones selected based on this gene-based marker revealed three close recombination events involving cly1. This delineated the cly1 window to a physical distance equivalent to 7 Kb in Morex, which had a single ORF coding for the transcription factor cosegrgating with cly1, and hence was identified as the putative cly1 gene

    Advantage of cleistogamy:


    The main advantage of cleistogamy is that it is a cheap method. Producing and maintaining large, nectar-rich open flowers is biologically expensive. The cost of producing a seed through cleistogamy is only about two-thirds of that for one formed through chasmogamy. Another interesting point is that well-developed jewel weed plants that would normally form a number of chasmogamous flowers will make only cleistogamous ones after they have been grazed by deer or when the ends of the branches have been cut off--a quick response to an emergency situation.

    Potential industrial application:


    The cleistogamous trait is controlled by a single gene, called clgl, and can be transferred in populations by breeding methods, haploidisation or back-crosses. It can be utilized for autogamous reproduction (fixation and pure lines breeding, seeds production, canola production for industrial utilization) or in the field of crucifers GMO programmes in order to reduce risks of cross-pollinations.

    Conclusion:


    For genetically modified (GM) rapeseed, researchers hoping to minimise the admixture of GM and non-GM crops are attempting to use cleistogamy to prevent gene flow. However, preliminary results from Co-Extra, a current project within the EU research programme, show that although cleistogamy reduces gene flow, it is not at the moment a consistently reliable tool for biocontainment: due to a certain instability of the cleistogamous trait, some flowers may open and release genetically modified pollen.






    Hi5, Xanga, Tagged, Blogger Comments @ 123glitter.com

    No comments:

    Post a Comment

    Post a Comment