Not strictly true. Apomixis is the normally the reproduction from an
unfertilised egg cell or the somatic (nucellar in part) cells
associated with the egg cell. Asexual reproduction occurs in many
ways in plants - adventitious budding etc., but that is not apomixis.
In certain species, seeds are formed as a result of normal sexual
reproduction, but these can be polyembryonic. Only one embryo will be
the result of fertilisation of the flower and this can exhibit
uncertain characteristics as a result of hybrid variation caused by
two sets of genes coming into play. The remaining two, three or four
embryos will be 'nucellar' or apogamic seedlings, vegetatively formed
from the nucellus of the seed. They are identical to the mother plant
and are true clones.
In this way, it is perfectly possibly to get a true cultivar of a
dessert orange for instance, from seed by selecting out and retaining
the nucellar seedlings. The difference in appearance is invariably
quite marked and 'normal' seedling can be easily identified since it
is usually considerably less vigorous. These apomictic embryos are as
it were, adventitious buds formed on the nucellus in a similar way to
the adventitious buds produced on the leaves of certain Kalanchoe
(Bryophyllum) species and the rachis of Polystichum ferns etc.
Pollination of the flowers has to take place in order for the seed to
form in the first place. Fruits can be formed parthenocarpically
(without pollination), but no fully formed, true seeds are present.
> Is this carried in the genes of the apomictic plant so
>that pollination is no longer necessary?
No, pollination is necessary for seed formation, subsequent apomixis
where it occurs, follows on as described above.
Apomixis has been demonstrated in many plants where the ovary has been
cultured in vitro and can be initiated from somatic and embryonic sac
cells. When apomixis occurs naturally, it is most frequently the
result of embryogenesis from the nucellus.
David Poole
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