Apomixis

Source: Wikipedia, the free encyclopedia.
This article is about plants. For similar processes in animals and Oomycetes, see Parthenogenesis.
Not to be confused with automixis or autogamy.
Vegetative apomixis in Poa bulbosa; bulbils form instead of flowers

In

bulbils
.

Apomictically produced offspring are genetically identical to the parent plant, except Nonrecurrent apomixis. Its etymology is Greek for "away from" + "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.[1]

Apogamy is a related term that has had various meanings over time. In plants with independent gametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis of gametophyte cells.

Male apomixis (paternal apomixis) involves replacement of the genetic material of an egg by the genetic material of the pollen.

Some authors included all forms of asexual reproduction within apomixis, but that generalization of the term has since died out.[1]

Evolution

Because apomictic plants are genetically identical from one generation to the next, each lineage has some of the characters of a true

Nardus stricta (matgrass), Hieracium (hawkweeds) and Taraxacum (dandelions). Apomixis is reported to occur in about 10% of globally extant ferns.[2] Among polystichoid ferns, apomixis evolved several times independently in three different clades.[2]

Although the evolutionary advantages of sexual reproduction are lost, apomixis can pass along traits fortuitous for evolutionary fitness. As Jens Clausen put it:[3]: 470 

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.

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;[4][5] 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

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.[6][7]

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

angiosperms as there are authors on the subject. For English speakers, Maheshwari 1950[8] is very influential. German speakers might prefer to consult Rutishauser 1967.[9] Some older text books[10] 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

nucellus or integument tissue (see nucellar embryony
).

Types in flowering plants

Caribbean agave
producing plantlets on the old flower stem.

Maheshwari[8] used the following simple classification of types of apomixis in flowering plants:

  • Nonrecurrent apomixis: In this type "the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac
    chromosomes
    as the mother plant, and "the process is not repeated from one generation to another" (which is why it is called nonrecurrent). See also parthenogenesis and apogamy below.
  • Recurrent apomixis, is now more often called gametophytic apomixis: In this type, the megagametophyte has the same number of chromosomes as the mother plant because meiosis was not completed. It generally arises either from an
    nucellus
    .
  • Adventive embryony, also called sporophytic apomixis, sporophytic budding, or nucellar embryony: Here there may be a megagametophyte in the ovule, but the embryos do not arise from the cells of the gametophyte; they arise from cells of nucellus or the integument. Adventive embryony is important in several species of Citrus, in Garcinia, Euphorbia dulcis, Mangifera indica etc.
  • Vegetative apomixis: In this type "the flowers are replaced by
    bulbils or other vegetative propagules which frequently germinate while still on the plant". Vegetative apomixis is important in Allium, Fragaria, Agave
    , and some grasses, among others.

Types of gametophytic apomixis

Gametophytic apomixis in flowering plants develops in several different ways.[11] 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.[12][13] Apomictic species or individual plants often have a hybrid origin, and are usually polyploid.[13]

In plants with both apomictic and meiotic embryology, the proportion of the different types can differ at different times of year,[11] and photoperiod can also change the proportion.[11] It appears unlikely that there are any truly completely apomictic plants, as low rates of sexual reproduction have been found in several species that were previously thought to be entirely apomictic.[11]

The genetic control of apomixis can involve a single genetic change that affects all the major developmental components, formation of the megagametophyte, parthenogenesis of the egg cell, and endosperm development.[14] 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.[14]

Related terms

The first process is a natural one. It 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.[15][16][17] Recently, the first example of natural androgenesis in a vertebrate, a fish, Squalius alburnoides was discovered.[18] 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.[19]
The second process that is referred to as androgenesis or androclinesis involves (artificial) culture of haploid plants from
microspores.[20] Androgenesis has also been artificially induced in fish.[21]

See also

  • Cytomixis – Migration of the nucleus from one plant cell to another, a process of nuclear fusion that occurs during pollen meiosis
  • Klepton – Species that requires input from another biological taxon to complete its reproductive cycle, a phenomenon known in zoology where mating with another taxon is required to complete reproduction
  • Meiosis – Cell division producing haploid gametes
  • Parthenocarpy – Production of seedless fruit without fertilisation, the production of seedless fruits
  • Parthenogenesis – Asexual reproduction without fertilization, the animal equivalent of apomixis
  • Plant reproductive morphology – Parts of plant enabling sexual reproduction

References

  1. ^
    PMID 15131250
    .
  2. ^
    ISSN 2090-0120
    .
  3. ^ Clausen, J. (1954). "Partial apomixis as an equilibrium system". Caryologia. 1954, Supplement: 469–479.
  4. ^
    ISBN 9780470650158
    .
  5. Wikidata Q115563996
    .
  6. S2CID 19209851
    .
  7. ISBN 9780226580838
    .
  8. ^ a b Maheshwari, P. 1950. An introduction to the embryology of the angiosperms. McGraw-Hill, New York.
  9. ^ a b Rutishauser, A. 1969. Embryologie und Fortpflanzungsbiologie der Angiospermen: eine Einführung. Springer-Verlag, Wien.
  10. ^ Fitting, H., et al. 1930. Textbook of botany (Strasburger's textbook of botany, rewritten). Macmillan, London.
  11. ^ a b c d Nogler, G.A. 1984. Gametophytic apomixis. In Embryology of angiosperms. Edited by B.M. Johri. Springer, Berlin, Germany. pp. 475–518.
  12. doi:10.1111/j.1095-8312.1997.tb01778.x
    .
  13. ^ a b Nygren, A. (1967). "Apomixis in the angiosperms". In W. Ruhland (ed.). Handbuch der Pflanzenphysiologie. Vol. 18. Berlin: Springer-Verlag. pp. 551–596.
  14. ^
    S2CID 23186733
    .
  15. PMID 18202380
    .
  16. doi:10.1051/forest:2000108
    .
  17. S2CID 39046191
    .
  18. PMID 28573029
    .
  19. PMID 22473310
    .
  20. ^ a b Solntzeva, M.P. (2003). "About some terms of apomixis: pseudogamy and androgenesis". Biologia. 58 (1): 1–7.
  21. S2CID 11750658
    .
  22. ISBN 1433102161, 2009, pp. 122, 194

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.

External links

Scholia has a profile for apomixis (Q528033).
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