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. [...]
[...] To conclusively prove the presence of apomixis it is necessary to conduct a combination of genetic and cytological analyses, yet very few plants have been adequately examined at this level. In addition, most, if not all apomicts are also capable of some level of sexual reproduction. This ‘faculta-tiveness' can mask the presence of the trait, particularly when it is expressed only at a low level. It seems, therefore, that as our understanding of apomixes increases and our ability to discern and detect it improves many new apomicts will be discovered and some plants that were originally described as apomicts may need to be reclassified. [...]
[...] In dimorphic systems such as Antennaria, Limonium, Coprosma and Cortaderia, apomixis is readily detected by the predominance of a single form in a population when the normal ratio of forms is expected to be closer to 1:1. In other plants cellular characters are used as a diagnostic for apomixis. In Panicum and Pennisetum, apomixis proceeds through the mediation of a four-celled embryo sac. Sexuals in these plants form an eight-celled embryo sac so the prevalence of apomixis is measured through the quantification of these different cyto-types. [...]
[...] Interestingly, although all of the grasses mentioned above show considerable synteny with the rice genome, in each case the region associated with the inheritance of apomixis aligns with a different region of that genome. There is some evidence for non- recombinant sector(s) associated with apomixis genes in Erigeron, and gamete selection in Taraxacum, indicating that similar difficulties with cloning may also arise in these plants. The ‘Synthesis' of Apomixis The successful installation of apomixis into sexual crops will require the integration of several component processes. [...]
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