Apomixis - Some Terms Related To Apomixis

Some Terms Related To Apomixis

• Parthenogenesis: Development of an embryo directly from an egg cell without fertilization is called parthenogenesis. It is of two types:
  • Haploid parthenogenesis: Parthenogenesis of a normal haploid egg (a meiotically reduced egg) into an embryo is termed haploid parthenogenesis. If the mother plant was diploid, then the haploid embryo that results is monoploid, and the plant that grows from the embryo is sterile. If they are not sterile, they are sometimes useful to plant breeders (especially in potato breeding, see dihaploidy). This type of apomixis has been recorded in Solanum nigram, Lilium spp., Orchis maculate, Nicotiana tabacum etc.
  • Diploid parthenogenesis: When the megagametophyte develops without completing meiosis, so that the megagametophyte and all cells within it are meiotically unreduced (aka diploid, but diploid is an ambiguous term), this is called diploid parthenogenesis, and the plant that develops from the embryo will have the same number of chromosomes as the mother plant. Diploid parthenogenesis is a component process of gametophytic apomixis (see above).
• Androgenesis and androclinesis are synonyms. These terms are used for two different processes that both have the effect of producing an embryo that has "male inheritance".

The first process is a natural one. It is may also be referred to as male apomixis or paternal apomixis It involves fusion of the male and female gametes and replacement of the female nucleus by the male nucleus. This has been noted as a rare phenomenon in many plants (e.g. Nicotiana and Crepis), and occurs as the regular reproductive method in the Saharan Cypress, Cupressus dupreziana.

The second process that is referred to as androgenesis or androclinesis involves (artificial) culture of haploid plants from anther tissue or microspores.

• Apogamy: Although this term was (before 1908) used for other types of apomixis, and then discarded as too confusing, it is still sometimes used when an embryo develops from a cell of the megagametophyte other than the egg cell. In flowering plants, the cells involved in apogamy would be synergids or antipodal cells.
• Addition hybrids, called B_III hybrids by Rutishauser: An embryo is formed after a meiotically unreduced egg cell is fertilized. The ploidy level of the embryo is therefore higher than that of the mother plant. This process occurs in some plants that are otherwise apomictic, and may play a significant role in producing tetraploid plants from triploid apomictic mother plants (if they receive pollen from diploids). Because fertilization is involved, this process does not fit the definition of apomixis.
• Pseudogamy refers to any reproductive process that requires pollination but does not involve male inheritance. It is sometimes used in a restrictive sense to refer to types of apomixis in which the endosperm is fertilized but the embryo is not. A better term for the restrictive sense is centrogamy.
• Agamospecies, the concept introduced by Göte Turesson: "an apomict population the constituents of which, for morphological, cytological or other reasons, are to be considered as having a common origin," i.e., basically synonymous with "microspecies.