The condition in which embryo sac
or the female gametophyte develops without nuclear fusion is known as apomixes.
Maheswari
(1950) recognized three types of apomixes –
1. Non Recurrent Apomixes:- In this
type the megaspore mother cell undergoes the usual meiotic divisions and a
haploid embryo sac is formed. Here the embryo arises either from the egg
(haploid parthenogenesis) or from other cell of the gametophyte (haploid
apogamy). The plants produced by this method are haploid and generally sterile
and do not reproduce sexually any more. This type of apomixes is found in
several species such as Solanum nigram,
Lilium spp., Bergenia, Erythraea
centaurium, Orchis manculata, Nicotiana tabacum, etc.
2. Recurrent Apomixis:- In this type
the embryo sac generally arises either from an archesporial cell (generative apospory) or from other part of
the nucellus (somatic apospory). Here all the nuclei of the embryo sac are
diploid, and there is no meiotic division. The embryo arises either from the
egg (diploid parthenogenesis) or from some other cell of the gametophyte
(diploid apogamy). Generative apospory has been observed in Eupatorium glandulosum, Parthenium argenatum, etc. Somatic
apospory has been observed in Hierarcium
excellens, H. flagellare and H. aurantiacum.
3. Adventive Embryony:- This type of
apomixes is also known as sporophytic budding. Here, the developed embryo sacs
may be haploid or diploid, but the embryos do not arise from the cells of the
gametophyte, and they arise only from the cells of the nucellus or the
integument. There is no alternation of generations, because the diploid tissue
of the present sporophyte directly gives rise to the new embryo. Adventive
embryony has been frequently reported in Citrus,
Euphorbia dulcis, Capparis frondosa, Mangifera indica and Hiptage
madablota.
In
another type the flowers are replaced by bulbils or other vegetative propagules
which generally germinate while still on the plant. However, this is only a
kind of vegetative reproduction. The bulbils have been reported in Globba bulbifera, Allium sativum, Agave, Dioscorea bulbifera, Oxalis, Pine-apple, etc.
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