thumb|right|Vegetative apomixis in [[Poa bulbosa; bulbils form instead of flowers]]
In botany, apomixis is asexual development of seed or embryo without fertilization.
Apomictically produced offspring are genetically identical to the parent plant, except in nonrecurrent apomixis. Its etymology is Greek for ἀπο- (apo-) "away from" and μίξις (míxis) "mixing".
Normal asexual reproduction of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis. In contrast to parthenocarpy, which involves seedless fruit formation without fertilization, apomictic fruits have viable seeds containing a proper embryo, with asexual origin.
In flowering plants, the term "apomixis" is used in a restricted sense to mean agamospermy, i.e. clonal reproduction through seeds. Although agamospermy could theoretically occur in gymnosperms, it appears to be absent in that group. This process is described mainly in lower plants, especially ferns, but shares certain features with apomixis in angiosperms.
Evolution
Because apomictic plants are genetically identical from one generation to the next, each lineage has some of the characters of a true species, maintaining distinctions from other apomictic lineages within the same genus, while having much smaller differences than is normal between species of most genera. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as species aggregates, typically listed in floras with the convention "Genus species agg." (such as the bramble, Rubus fruticosus agg.). In some plant families, genera with apomixis are quite common, for example in Asteraceae, Poaceae, and Rosaceae. Examples of apomixis can be found in the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Poa (meadow grasses), Nardus stricta (matgrass), Hieracium (hawkweeds) and Taraxacum (dandelions). Apomixis is reported to occur in about 10% of globally extant ferns. Among polystichoid ferns, apomixis evolved several times independently in three different clades.<blockquote>The apomicts actually have discovered the effectiveness of mass production long before Mr. Henry Ford applied it to the production of the automobile. ... Facultative apomixis ... does not prevent variation; rather, it multiplies certain varietal products.</blockquote>
Facultative apomixis means that apomixis does not always occur, i.e., sexual reproduction can also happen. It appears likely that all apomixis in plants is facultative; in other words, that "obligate apomixis" is an artifact of insufficient observation (missing uncommon sexual reproduction).
Apogamy and apospory in non-flowering plants
The gametophytes of bryophytes, and less commonly ferns and lycopods can develop a group of cells that grow to look like a sporophyte of the species but with the ploidy level of the gametophyte, a phenomenon known as apogamy. The sporophytes of plants of these groups may also have the ability to form a plant that looks like a gametophyte but with the ploidy level of the sporophyte, a phenomenon known as apospory.
See also androgenesis and androclinesis described below, a type of male apomixis that occurs in a conifer, Cupressus dupreziana.
In flowering plants (angiosperms)
Agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms is very influential. German speakers might prefer to consult Rutishauser 1967. Some older text books on the basis of misinformation (that the egg cell in a meiotically unreduced gametophyte can never be fertilized) attempted to reform the terminology to match the term parthenogenesis as it is used in zoology, and this continues to cause much confusion.
Agamospermy occurs mainly in two forms: In gametophytic apomixis, the embryo arises from an unfertilized egg cell (i.e. by parthenogenesis) in a gametophyte that was produced from a cell that did not complete meiosis. In adventitious embryony (sporophytic apomixis), an embryo is formed directly (not from a gametophyte) from nucellus or integument tissue
Types in flowering plants
thumb|right|[[Caribbean agave producing plantlets on the old flower stem.]]
Maheshwari A megagametophyte develops with an egg cell within it that develops into an embryo through parthenogenesis. The central cell of the megagametophyte may require fertilization to form the endosperm, pseudogamous gametophytic apomixis, or in autonomous gametophytic apomixis endosperm fertilization is not required.
- In diplospory (also called generative apospory), the megagametophyte arises from a cell of the archesporium.
- In apospory (also called somatic apospory), the megagametophyte arises from some other (somatic) cell of the nucellus.
Considerable confusion has resulted because diplospory is often defined to involve the megaspore mother cell only, but a number of plant families have a multicellular archesporium and the megagametophyte could originate from another archesporium cell.
Diplospory is further subdivided according to how the megagametophyte forms:
- Allium odorum–A. nutans type. The chromosomes double (endomitosis) and then meiosis proceeds in an unusual way, with the chromosome copies pairing up (rather than the original maternal and paternal copies pairing up).
- Taraxacum type: Meiosis I fails to complete, meiosis II creates two cells, one of which degenerates; three mitotic divisions form the megagametophyte.
- Ixeris type: Meiosis I fails to complete; three rounds of nuclear division occur without cell-wall formation; wall formation then occurs.
- Blumea–Elymus types: A mitotic division is followed by degeneration of one cell; three mitotic divisions form the megagametophyte.
- Antennaria–Hieracium types: three mitotic divisions form the megagametophyte.
- Eragrostis–Panicum types: Two mitotic division give a 4-nucleate megagametophyte, with cell walls to form either three or four cells.
Incidence in flowering plants
Apomixis occurs in at least 33 families of flowering plants, and has evolved multiple times from sexual relatives. Apomictic species or individual plants often have a hybrid origin, and are usually polyploid. However, the timing of the various developmental processes is critical to successful development of an apomictic seed, and the timing can be affected by multiple genetic factors. Recently, the first example of natural androgenesis in a vertebrate, a fish, Squalius alburnoides was discovered. It is also known in invertebrates, particularly clams in the genus Corbicula, and these asexually reproducing males are noted to have a wider range than their noninvasive non-hermaphroditic cousins, more similar to hermaphroditic invasive species in the genus, indicating that this does sometimes have evolutionary benefits.
:The second process that is referred to as androgenesis or androclinesis involves (artificial) culture of haploid plants from anther tissue or microspores. Androgenesis has also been artificially induced in fish.
- Apogamy: In modern usage, apogamy refers primarily to the development of a sporophyte directly from somatic cells of the gametophyte without fertilization, a process most commonly described in ferns and other plants with independent gametophyte generations.
See also
- , a process of nuclear fusion that occurs during pollen meiosis
- , a phenomenon known in zoology where mating with another taxon is required to complete reproduction
- , the production of seedless fruits
- , the animal equivalent of apomixis
References
Further reading
- Gvaladze G.E. (1976). Forms of Apomixis in the genus Allium L. In: S.S. Khokhlov (Ed.): Apomixis and Breeding, Amarind Pub., New Delhi-Bombay-Calcutta-New York pp. 160–165
- Bhojwani S.S.& Bhatnagar S.P. (1988). The Embryology of angiosperms. Vikas Publishing house Pvt.Ltd. New Delhi.
- Heslop-Harrison, J. (1972) "Sexuality in Angiosperms,"pp. 133–289, In Steward,F.C. (ed.) Plant Physiology, Vol. 6C, Academic Press New York.