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Apomixis is a
form of asexual
fertilization. reproduction that occurs via
Apomixis is seeds in which embryos develop without
frequently in Compositae, known in some
300 species belonging to about 400amilies,
Rosaceae & Graminae. it occurs most
Ine .first discovery of this phenomenon
is credited to Leuwenhock as early as
seeds. 1719 in Citrus
Apomixis, derived
from two Greek word "ÁPo" (away
mixing or mingling). Apomixis from) and "mixis" (act of
has been described as a nightmare
because it allows fixation for taxonomists, mostly
of interspecific or even
eliminated if they were to have attempted intergeneric hybrids that would have been
reproducing sexually, because of
.different classifications of apomixis types their sterility. Many
and pathways of development
Over the years but the most widely have been proposed
accepted and easiest to use is that classification
Nogler (1984), proposed by
Facultative apomixis ismore common
in plants than obligate apomixis. Apomíxis occurs in
polyploidy species of Poa, Paspalum, Panicum,
'Pennisetum, Cenchrus, etc., in Citrus, Malus,
Bubus and Mangifera and in some wild relatives
of Zea mays and Beta vulgaris.
Among the major cereals,
maize, wheat and pearl millet have apomictic relatives. As a rule,
apomixis is more prevalent in the polyploid species. For
example, seven wild species of
Pennisetum are apomicticand all are polyploid. Tripsacum dactyloides, an
apomictic relative of
maize, is also polyploid. It is
also widespread in several forage grasses such as Cenchrus,
Eragrostis, Paspalum, Poa; Bothriochloa, Capillipedium and Dichanthium,
and most of them are
polyploids.
Types of apomixes:
1, Adventive Embryony
In this case, embryos develop directly from vegetative cells, such as, nucellus, integument, and
chalazae. Development of embryo does not involve production of embryo sac. It occurs in mango
(Alangifera indica), Citrus, etc.
(2.) Apospory
In apospory, unreduced embryo sacs arise from somatic ces in the ovule. The somatic cells of the ovule
from which embryo sacs develop belong to the nucellus. Several cells of the nucellus may start
aposporous development but usually only one of them gives rise to mature embryo sac. Apospory is
initiated after MMC differentiation. The megaspore degenerates and the aposporous embryo sac
occupies. Apospory occurs in some species of Hieracium, Panicum etc.
(3.) Diplospory
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In diplospory unreduced embryo sacs are
"develops parthenogenetically. formed from MMCS by circumvention
of meiosis. The embryo
from the. egg. and the endosperm develops
(without fertilization) from the unreducedunreduced autonomously
polar nuclei Thus, pollination in the diplosporous
not necessary. apomicts
(4:). Parthenogensis
The embryo develops from the egg cell.: Depending upon
whether the embryo sac is haploid or
diploid, parthenogenesis is
termed äs haploid or díploid parthenogenesis. Haploid
arthenogenesis occurs accidently and has been reported in
Nícotiana, Crepis. Diploid
parthenogenesis oCCúrs in many grasses, e.g Taraxacum.
In many species, e.g., Nicotiana, Datura,
rice etc., pollen grains may be induced in vitro to
produce haploidembryosor plantlets. For this, a
anthers or pollen grains are cultured on a
suitable tissue culture medium Thistechnigue offers several possibilities for crop improvement.
(5.) Apogamy
In apogamy, synergids or antipodal cells develop into an embryo. Like parthenogenesis,
apogamy may be haploid or diploid depending upon the haploid or diploid state of the embryo
sac. Diploid apogamy occurs,in Antennaria, Alchemilla, Allium and many other plant species.
