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Stewart Robert Hinsley · 12-08-2009, 12:24 AM posted to uk.rec.gardening

In message ,
writes
In article ,
K wrote:

Sorry, I don't follow. Yes, the plants will be cross pollinated - but
how does this affect the fruit? The fruit is formed vegetatively from
the parent plant, as I understand it, it's the seed itself which
contains genetic material from both parents, so that any plant growing
from the seed will be the result of cross pollination.

Nope. It ain't that simple. Nowhere NEAR that simple. When trying
to understand an explanation of Ipomoea indica/learii's complicated
infertility, I found out a little of this. My attempts to find out
a a coherent description of the fertilisation process in vascular
plants was a dismal failure :-(

It differs between different groups of plants. Flowering plants has
double fertilisation. Gnetophytes (if I've remembered the group
correctly) have a different form of double fertilisation. Other
gymnosperms don't. We don't necessarily know much about seed ferns, but
I'd guess that they were similar to gymnosperms. Other vascular plants
have macroscopic gametophytes, which produce free(ish) gametes which
unite to form sporophytes.

Among flowering plants, the details vary.

It seems that the pollen grains germinate on their own, and some
haploid cells then move towards and merge with the ovary. Depending
on the plant, seeds can have components that are derived from the
combination, the ovary alone and the pollen grain alone, or even
all three. Maybe. Or maybe not. Trying to reverse engineer an
explanation of what is happening in a process that complicated from
incidental remarks in scientific papers in a field that one is not
an expert in is, shall we say, inclined to lead to misunderstandings.

In the majority of flowering plants the embryo sac (female gametophyte)
ends up with 7 cells contained 8 haploid nuclei. The pollen grain (male
gametophyte) has 3 haploid cells - the vegetative cell, and two sperm
cells. When the pollen grain germinates the vegetative cell grows down
the style to reach the embryo sac, carrying the two sperm cells.

One sperm cell unites with one cell from the embryo sac. The resulting
diploid cell develops into the embryo. The other sperm cell unites with
the embryo sac cell with the extra nuclei. This develops into a triploid
tissue - the endosperm - which is many plants provides nourishment for
the embryo after seed germination. (In many other plants the endosperm
is vestigial. There might be complications about syncytia as well - I
don't recall.)

I don't know what happens to the other cells of the embryo sac, but I'd
guess that they degenerate. The remaining tissues of the seed (e.g. the
seed coat) develop from the ovule, and have the maternal genotype. In
true fruits the fruit is derived from the ovary, and has the maternal
genotype. In false fruits parts of the fruit are derived from other
parts of the flower, but still have the maternal genotype.

There's one species of androgenetic cypress in which the seed genotype
is from the parental plant only, but otherwise I expect that the
vegetative cell of the pollen grain doesn't contribute to the seed, and
that seeds have no components derived from the pollen grain alone.

So what I'm asking is whether there is a mechanism by which the genetic
material from pollen can affect the development of the fruit (as opposed
to any seeds that fruit contains), and, if so, what that mechanism is?

Yes, there is. Definitely. Whether the mechanism is unknown even to
the experts, or merely known only to such experts, I don't know.
Given my record of asking such questions, it's about as likely either
way.

Naively one might expect that embryo and endosperm properties are
controlled by both maternal and parental genotypes (so cross-pollination
does matter), and the remainder of the seed and the fruit to be
controlled by the maternal genotype.

So, cross pollination would be significant in maize, and insignificant
in sweet peppers.

But, embryo and endosperm development start with a cell loaded with
proteins specified by the maternal genotype, including regulatory
proteins. Even in the absence of genetic imprinting (as occurs in
mammals), it would take time for the parental genotype to exert an
influence.

On the other hand, traits in tissues of the maternal genotype might be
influenced by cellular messages produced in the embryo or endosperm, so
it's not impossible that the genetic material in the pollen might effect
the development of the fruit. Like Nick I don't know whether any
specific such mechanism are known.

Regards,
Nick Maclaren.

--
Stewart Robert Hinsley