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Re: apomictic?


On Tue, 23 Feb 1999 16:45:43 -0600, dave-poole@ilsham.demon.co.uk
wrote:

>relying upon literal translations from either the
>Greek or Latin is as unreliable as you can get, because these are
>invariably considerably distanced from the reality and are only
>descriptions in the broadest possible sense.

Ain't that the truth!

>Most of us are familiar with the idea of a placenta and umbilical cord 
>in animals (including us).  These are [...] tissues which remain part of 
>the parent but are
>essential in the continued development of the embryo.  Genetically
>they are identical to the host parent (mother) and any tissue culture
>taken from these, resulting in an offspring, would be clones of the
>mother.

Ummmm..... Don't mean to be contrary (and I'm certainly no expert on
plant reproduction) but I think that the above statement shows the
danger of drawing parallels between plant and animal sexual
reproduction.

I happen to be a human geneticist, so I know a fair amount about human
sexual reproduction and embryology. I know much less about plants. So
please, those of you who know, point out where I'm wrong below. This
is what I gather from my limited reading about flowering plant
reproduction.

The basic mechanism of meiosis is the same in plants and animals (at
least higher vertebrates like humans) but that's where the similarity
ends. 

Meiosis in humans ends in one haploid cell, the gamete. These cells do
not divide further. Two such cells combine to form a zygote. The
zygote is a diploid cell with nuclear genes derived from both parents
and mitochondrial genes, as well as most of the cytoplasm, derived
from the mother. The zygote divides and its cells form the embryo
(including the umbilical cord), the extra-embryonic membranes (chorion
and amnion) and the placenta. These are fetal, not maternal, tissues
in humans. They are diploid and are genetically identical to the
fetus.

In plants, meiosis ends in a haploid cell. This is where things begin
to differ from animals. The haploid cell which resulted from meiosis
then goes on to divide into a number of identical haploid cells. These
haploid cells specialize to form reproductive structures such as
pollen tubes and sperm cells in the male gametophyte and the egg,
polar nuclei, antipodal cells and synergids in the female gametophyte.
This division and specialization of haploid cells seems to me to be a
fundamental difference in the mechanism of fertilization in plants and
animals, although oddly enough, the final result of fertilization is
very similar: a diploid cell (zygote) with nuclear genes derived from
both parents and maternally derived plastid genes. I don't know about
the origin of the cytoplasm in the plant zygote. A second difference
is that plants also have a nutritive tissue, the endosperm, which is
the result of a separate fertilization event and is not usually
diploid, from what I gather. There is no such thing in humans.

Of course, the human embryo would not get very far without the mother
and her specialized reproductive structure, the uterus and its adnexa,
but these are permanent structures in the mother, not ones formed anew
with each reproductive cycle. Only the uterine lining is renewed every
cycle.

Maybe someone can explain it better than I - I'm certainly no expert
in plant reproduction. But I implore you not to draw too many
parallels between the process in plants and animals - we are pretty
different at a fairly fundamental level. It's really amazing that so
many parts of the scheme are so similar.

>The general consensus appears to be that apomixis is asexual
>reproduction involving (almost exclusively) those parts of the ovum
>not directly involved with fertilization.

Do you need fertilization to trigger this asexual reproduction or can
it happen by itself?

Also, do you mean "the ovule"? The ovum is the unfertilized female
gamete (even in plants, according to my botanical dictionary). The
ovule seems to be the reproductive structure derived from maternal
tissue.
-- 
Magda Plewinska                   mplewinska@mindspring.com
Miami, FL, USA





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