Editing Apomixis (section)

== In flowering plants (angiosperms) ==

Agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms<ref name=Savidan/> and a simple [[hierarchy|hierarchical]] classification of the different types is not possible. Consequently, there are almost as many different usages of terminology for apomixis in [[angiosperm]]s as there are authors on the subject. For English speakers, Maheshwari 1950<ref name=Maheshwari>Maheshwari, P. 1950. An introduction to the embryology of the angiosperms. McGraw-Hill, New York.</ref> is very influential. German speakers might prefer to consult Rutishauser 1967.<ref name=Rutishauser>Rutishauser, A. 1969. Embryologie und Fortpflanzungsbiologie der Angiospermen: eine Einführung. Springer-Verlag, Wien.</ref> Some older text books<ref>Fitting, H., et al. 1930. Textbook of botany (Strasburger's textbook of botany, rewritten). Macmillan, London.</ref> 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 (see [[nucellar embryony]]).

=== Types in flowering plants ===
[[File:Agave angustifolia (Caribbean Agave) in Hyderabad W IMG 8660.jpg|thumb|right|[[Caribbean agave]] producing plantlets on the old flower stem.]]
Maheshwari<ref name=Maheshwari/> 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 [[megagametophyte]] is formed. The new embryo may then arise either from the egg (haploid parthenogenesis) or from some other cell of the gametophyte (haploid apogamy)." The haploid plants have half as many [[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 [[archesporium|archesporia]]l cell or from some other part of the [[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.<ref name=Nogler>Nogler, G.A. 1984. Gametophytic apomixis. In Embryology of angiosperms. Edited by B.M. Johri. Springer, Berlin, Germany. pp. 475–518.</ref> 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 (plant)|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.<ref>{{cite journal|author=Carman, J.G.|year=1997|title=Asynchronous expression of duplicate genes in angiosperms may cause apomixis, bispory, tetraspory, and polyembryony|journal=Biological Journal of the Linnean Society|volume=61|issue=1|pages=51–94|doi=10.1111/j.1095-8312.1997.tb01778.x|doi-access=free}}</ref><ref name=Nygren>{{cite book|author=Nygren, A. |year=1967 |title= Handbuch der Pflanzenphysiologie |chapter= Apomixis in the angiosperms |publisher=Springer-Verlag |location=Berlin|editor= W. Ruhland |pages= 551–596|volume=18}}</ref> Apomictic species or individual plants often have a hybrid origin, and are usually polyploid.<ref name=Nygren/>

In plants with both apomictic and meiotic embryology, the proportion of the different types can differ at different times of year,<ref name=Nogler/> and [[Photoperiodism|photoperiod]] can also change the proportion.<ref name=Nogler/> 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.<ref name=Nogler/>

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.<ref name=Koltunow>{{cite journal|author1=Koltunow, A.M. |author2=Johnson, S.D. |author3=Bicknell, R.A. |year=2000|title=Apomixis is not developmentally conserved in related, genetically characterized ''Hieracium'' plants of varying ploidy|journal=Sexual Plant Reproduction|volume=12|issue=5|pages=253–266|doi=10.1007/s004970050193|s2cid=23186733 }}</ref> 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.<ref name=Koltunow/>