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The molecular biology of apomixis

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  1. .Introduction
  2. What apomixis is and where it is found.
    1. The asexual structure of seeds.
    2. The distribution of the trait.
    3. Asexual reproduction in animals.
    4. The widespread distribution of apomixis amongst flowering plants.
  3. Apomixis in agriculture.
    1. Apomixis and the structure of large genetically uniform populations.
  4. The mechanisms of apomixis.
    1. Definition.
    2. Two main mechanisms.
    3. The structure of an endosperm.
  5. The detection and measurement of apomixis.
    1. Detection through the structure of 'maternal' progeny.
    2. The need to base any quantitative study of apomixis on a reliable and sensitive method of detecting the trait.
    3. Flow cytometry.
  6. The identification of 'apomixis genes'.
    1. The finding that gametophytic apomixis was often simply inherited.
  7. The 'Synthesis' of apomixis.
  8. Conclusion.

Apomixis is the subject of a number of recent reviews. Some authors have focused on the potential agronomic and economic benefits of apomixes. Others have discussed the developmental and genetic basis of apomixes or reviewed current theory regarding the evolutionary and ecological implications of the trait. This text provides a general overview of the field of apomixis research, with an emphasis on our current understanding of the genetic mechanism(s) that underlie its expression in flowering plants, and a sketch of the strategies being taken to introduce this characteristic into crop species. Apomixis is the asexual structure of seeds, avoiding the processes of meiotic decrease and recombination at fertilization, and leading to the formation of genetically uniform progeny. The term ?apomixis' was first coined by Winkler (1908) to describe ?substitution of sexual reproduction by an asexual multiplication process without nucleus or cell fusion'. This is a broad definition that can be interpreted to contain all forms of asexual reproduction.

[...] The detection and measurement of apomixis In most cases apomixis has been detected through the structure of ?maternal' progeny, which appears to be morphologically equal to the mother parent. This is a notoriously inaccurate approach for assessing genetic difference and, therefore, for inferring the action of a particular breeding system. Several mechanisms can give rise to apparently identical progeny, most commonly though self-fertilization. Conversely, the forms of some plants, such as the apomicts of Taraxacum officinale, are sufficiently plastic in different environments to tempt the conclusion of genetic variation, despite the clonal nature of these populations. [...]

[...] It seems likely that some form of genetic testing will be a routine part of most studies into apomixis in the near future. The identification of ?apomixis genes' With the finding that gametophytic apomixis was often simply inherited, several groups are attempting to identify the sequences involved in a number of plant species. Ozias-Akins and colleagues noted in Pennisetum that the inheritance of apospory was associated with the transfer of a genomic region located near the telomere of a P. [...]

[...] The widespread distribution of apomixis amongst flowering plants, its apparent polyphyletic origins and the occurrence of analogous phenomena in other organisms has led to the proposition that it has arisen many times from the variation of conserved functions, common to a wide variety of multicellular organisms. Specifically, the functions of meiosis, egg cell specification and zygote specification appear to be involved in this transition. Recently, molecular evidence has emerged to support this hypothesis. This is discussed below. As indicated above, apomicts are normally classified by genus and species in a manner related to other flowering plants. [...]

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