[In some species the embryo is not produced as a result of meiosis and
fertilization, but from a cell in the embryo sac or surrounding nucellus,
which does not undergo meiosis but develops to form a zygote of the same
genetic makeup as the female parent. (Nonrecurrent apomixis is an
exception). The occurrence of this asexual reproductive process -- in
place of sexual reproductive processes of reduction, division, and
fertilization to produce an embryo -- is known as apomixis. Seedling
plants produced in this manner are known as apomicts. Plants that produce
only apomictic embryos are known as obligate apomicts; those that produce
both apomictic and sexual embryos are facultative apomicts.
Types of Apomixis
RECURRENT APOMIXIS
An embryo sac (female gametophyte) develops from the egg mother cell (or
from some adjoining cell, the egg mother cell disintegrating), but
complete meiosis does not occur. Consequently, the egg has the normal
diploid number of chromosomes, the same as the mother plant. The embryo
subsequently develops directly from the egg nucleus without
fertilization.
This series of events is known to occur in some species of Crepis,
Taraxacum (dandelion), Poa (bluegrass), and Allium (onion) without the
stimulus of pollination; in others, e.g., species of Parthenium
(guayule), Rubus (rasberry), Malus (apple), some Poa species, and
Rudbeckia, pollination appears to be necessary, either to stimulate
embryo development or to produce a viable endosperm.
ADVENTITIOUS EMBRYONY
Also known as nucellar embryony. In this type of apomixis the embryos
rise from a cell or group of cells either in the nucellus (usually) or in
the integuments, as shown in Fig. 3-9. It differs from recurrent apomixis
in that such embryos develop outside of the embryo sac and in addition to
the regular embryo. In some plants. Citrus for instance, fertilization
takes place in the usual manner, and a sexual plus several apomictic
embryos may develop. In others, e.g., some species of Opuntia, apomictic
embryos develop spontaneously, no pollination or fertilization apparently
being needed.
NONRECURRENT APOMIXIS
In nonrecurrent apomixis an embryo arises from the egg nucleus without
fertilization. Since the egg is haploid, the resulting embryo will also
be haploid. This case is rare and primarily of genetic interest. It does
not consistently occur in any particular kind of plant as do recurrent
apomixis and adventitious embryony.
VEGETATIVE APOMIXIS
In some cases vegetative buds or bulbils are produced in the
inflorescence in place of flowers. This occurs in Poa bulbosa and some
Allium, Agave, and grass species. {{Pinellia??}}
POLYEMBRYONY
The phenomenon in which two or more embryos are present within a single
seed is called polyembryony.
It may result from one of several causes. Nucellar embryony, as described
in Citrus, is one cause. The occasional development of more than one
nucleus within the embryo sac (in addition to the egg nucleus) is
another. Cleavage of the proembryo during the early stages of
development, a common occurrence in conifers, leads to multiple embryos.
Apomixis occurs in nature in many different plant families. Usually
polyploids or complex hybrids are involved. Apomixis occurs in some kinds
of plant cultivars and can be produced by breeding.
The ability to reproduce plant cultivars apomictically has significant
uses in horticulture. Because the seedling plants produced result from an
asexual process, the procedure is a means of producing genetically
uniform seedling populations. Such apomictic seedling plants follow the
same seedling cycle as shown in sexually produced seedlings, including a
juvenile-to-mature transition, except that both the upper and lower
cycles are asexual.
...........
Many Citrus species and cultivars produce apomictic seedlings that are
used as rootstocks. These invariably are uniform and VIGOROUS in contrast
to the occasional WEAKER, variable hybrid seedlings that may also occur.
.......]
They go on to suggest it's usefulness in apple and other plants.
There you have it. More than you ever wanted to know about apomixis, but
were afraid to ask.
They don't address the philosophical question of why the sexually
produced seedling is weaker
than the apomictic seedling. Perhaps, like with the human appendix, there
is no logical reason.
Please forgive my poor spelling.
Ray
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