In flowering plants .28angiosperms.29 Apomixis

1 in flowering plants (angiosperms)

1.1 types in flowering plants

1.1.1 types of gametophytic apomixis


1.2 incidence in flowering plants

in flowering plants (angiosperms)

agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms , simple hierarchical classification of different types not possible. consequently, there many different usages of terminology apomixis in angiosperms there authors on subject. english speakers, maheshwari 1950 influential. german speakers might prefer consult rutishauser 1967. older text books on basis of misinformation (that egg cell in meiotically unreduced gametophyte can never fertilized) attempted reform terminology match term parthenogenesis used in zoology, , continues cause confusion.

agamospermy occurs in 2 forms: in gametophytic apomixis, embryo arises unfertilized egg cell (i.e. parthenogenesis) in gametophyte produced cell did not complete meiosis. in adventitious embryony (sporophytic apomixis), embryo formed directly (not gametophyte) nucellus or integument tissue (see nucellar embryony).

types in flowering plants

caribbean agave producing plantlets on old flower stem.

maheshwari used following simple classification of types of apomixis in flowering plants:

nonrecurrent apomixis: in type megaspore mother cell undergoes usual meiotic divisions , haploid embryo sac [megagametophyte] formed. new embryo may arise either egg (haploid parthenogenesis) or other cell of gametophyte (haploid apogamy). haploid plants have half many chromosomes mother plant, , process not repeated 1 generation (which why called nonrecurrent). see parthenogenesis , apogamy below.
recurrent apomixis, more called gametophytic apomixis: in type, megagametophyte has same number of chromosomes mother plant because meiosis not completed. arises either archesporial cell or other part of nucellus.
adventive embryony, called sporophytic apomixis, sporophytic budding, or nucellar embryony: here there may megagametophyte in ovule, embryos not arise cells of gametophyte; arise cells of nucellus or integument. adventive embryony important in several species of citrus, in garcinia, euphorbia dulcis, mangifera indica etc.
vegetative apomixis: in type flowers replaced bulbils or other vegetative propagules germinate while still on plant . vegetative apomixis important in allium, fragaria, agave, , grasses, among others.

types of gametophytic apomixis

gametophytic apomixis in flowering plants develops in several different ways. megagametophyte develops egg cell within develops embryo through parthenogenesis. central cell of megagametophyte may require fertilization form endosperm, pseudogamous gametophytic apomixis, or in autonomous gametophytic apomixis fertilization not required.

in diplospory (also called generative apospory), megagametophyte arises cell of archesporium.
in apospory (also called somatic apospory), megagametophyte arises other nucellus cell.

considerable confusion has resulted because diplospory defined involve megaspore mother cell only, number of plant families have multicellular archesporium , megagametophyte originate archesporium cell.

diplospory further subdivided according how megagametophyte forms:

allium odorum–a. nutans type. chromosomes double (endomitosis) , meiosis proceeds in unusual way, chromosome copies pairing (rather original maternal , paternal copies pairing up).
taraxacum type: meiosis fails complete, meiosis ii creates 2 cells, 1 of degenerates; 3 mitotic divisions form megagametophyte.
ixeris type: meiosis fails complete; 3 rounds of nuclear division occur without cell-wall formation; wall formation occurs.
blumea–elymus types: mitotic division followed degeneration of 1 cell; 3 mitotic divisions form megagametophyte.
antennaria–hieracium types: 3 mitotic divisions form megagametophyte.
eragrostis–panicum types: 2 mitotic division give 4-nucleate megagametophyte, cell walls form either 3 or 4 cells.

incidence in flowering plants

apomixis occurs in @ least 33 families of flowering plants, , has evolved multiple times sexual relatives. apomictic species or individual plants have hybrid origin, , polyploid.

in plants both apomictic , meiotic embryology, proportion of different types can differ @ different times of year, , photoperiod can change proportion. appears unlikely there apomictic plants, low rates of sexual reproduction have been found in several species thought entirely apomictic.

the genetic control of apomixis can involve single genetic change affects major developmental components, formation of megagametophyte, parthenogenesis of egg cell, , endosperm development. however, timing of various developmental processes critical successful development of apomictic seed, , timing can affected multiple genetic factors.