Subject: Re: Hermaphroditic Plants???

From: Jan Wesenberg <[email-masked]>
Date: Mon, 09 Dec 1996 11:54:29 +0200

[email-masked] wrote:
> 
> I was wondering if a plant that was hermaphroditic pollenated itself,
> would there be any genetic variation or would the plant's seeds create an
> almost exact copy of itself?  Would the plants seeds produce
> hermaphroditic offspring.  I'm trying to settle a dispute with my roomate.
>  Any help would be great.  E-mail me or post in newsgroup
> 
> [email-masked]

The answer is, that depends.

This is not a hypothetical question. It is well known that most plant
species are hermaphrodites. Although many (most?) of them have different
kinds of mechanisms ensuring or promoting cross-pollination, there is
still a substantial number of plant species which regularly
self-pollinate. Some are exclusive selfers. This is typical for many
plants in arctic or alpine environments, where pollinating insects may
be scarce and the season is short. It is also typical for many weeds or
other short-lived plants of arid or frequently disturbed habitats, which
often statistically have very little time for producing ripe seed after
their own germination, before they are killed by drought or by the next
disturbance of the soil.

Whether the offspring produced by selfing will be an exact copy of the
parent plant, depends on the heterozygosity of the parent. Any one locus
in the genome of a diploid plant can be either homozygous or
heterozygous. The offspring will of course be exactly as the parent at
the homozygous loci. But at the heterozygous loci there wil be produced
offspring that is homozygous for one of the alleles, heterozygous, or
homozygous for the second allele - in the proportion 1:2:1. When a locus
which was heterozygous in the parent turns out homozygous in an
offspring individual, the locus is said to have been fixed. As there is
an enormous number of loci in the genome, different loci will be fixed
in different daughter plants, and a parent plant which is not completely
homozygous will therefore produce offspring plants which differ
genetically one from another. But, whereas the total offspring of a
cross-pollinating pair of parents can have as a maximum four alleles
present for any one locus, there will only be at most two alleles
present among the offspring at each locus when there is only one parent.
There is a theoretical possibility that a daughter plant could be
produced that is genetically identical with its self-pollinating parent,
but as long as the number of heterozygous loci is high, the chance is
very small (0.5 ** the number of heterozygous loci in the parent). 

The average heterozygosity of any daughter plant produced by selfing
will be half of that of the mother. Therefore, if you take a plant from
a cross-pollinating population and force it to self-pollinate (supposed
it has no self-incompatibility system preventing it), for each
generation the heterozygosity of the offspring population will be
halfed, i.e. the heterozygosity of the individuals will decline
exponentially. That means, the offspring will become more and more like
each other, and the parent, after each self-pollination.

Species that are normally self-pollinated, on the other hand, usually
have undergone innumerable generations of selfing, and the individuals
therefore tend to be completely homozygous, that means all their loci
are fixed. A completely homozygous plant individual will, when
self-pollinating, produce offspring that is identical to the parent. If
such species are genetically variable at all, then the genetical
variation tends to be between different populations or between different
parts of the geographical range of the species, and usually not within
any one population. This is because given enough time, the chance is
high that all individuals present in a population stem from a common
ancestor plant somewhen in the past, and if it was completely
homozygous, then all the individuals will be genetically identical. Of
course, this is not always the case - the species or population may be
to young or have a too complicated prehistory for this process to have
reached this stage.

There are at least three phenomena which modify this picture: 

- Many selfing plant species are not exclusive selfers. That means,
there is some amount of cross-pollination. If there also is some amount
af migration between populations, or at least a good mixing of
individuals within the population, the population can manage to maintain
some amount of heterozygosity over time.

- Many regularly selfing plants are polyploids, even if they act as
diploids in the meiosis. They may behave as completely homozygous in
that they produce offspring that is identical to the parent, but each
individual will contain several copies of each locus, and therefore may
have different alleles fixed at each of the parallel loci. This is
called fixed heterozygosity - that is, heterozygosity not within any
locus, but between different copies of a locus.

- Of course, in a selfing species, just as in a cross-pollinating
species, there will arise fresh mutations. There is therefore a constant
supply of new alleles, the fate of which, as always, is controlled by
the processes of genetic drift and natural selection. The difference
between a selfing mutant and a outcrossing mutant is that in the selfing
one the road to fixation of the mutant allele is short, and therefore
selection plays a more important role compared to genetic drift than in
a cross-pollinating species. In a selfing species, the amount of
heterozygosity produced by mutation is rather marginal compared to the
heterozygosity which can be maintained in a cross-pollinating species.

So, to summarize - if you take a plant from a species which is "used to"
cross-polinate, but which is capable of self-pollinating, and force it
to self-pollinate, the offspring will typically still be variable and
different from the parent, although within narrower limits than if the
offspring was produced by cross-pollination. If, on the other hand, you
look at the offspring of an individual of a species which is normally
self-pollinating, the offspring will typically be identical with each
other and the parent.

So, it seems that both of you won?

---

From: "brian cain" <[email-masked]>
Date: Tue, 10 Dec 1996 10:17:00 -0800

[email-masked] wrote in article
<[email-masked]>...
> almost exact copy of itself? 
> [email-masked]
> 

My understanding is that most flowering plants have self incompatability
sytems which mainly screen out pollen from the same plant when it lands on
the stigma and prevents it from developing any further to fertilise the
egg. I believe that some species do undergo self pollination but would not
think that they would necessarily produce hermaphroditic offspring. The
offspring would be genetically identical to the parent plant but might not
be exactly the same because its development would be dependent on its
enviroment - shade, moisture, and all the other enviromental factors which
can affect a plants development.

---

From: [email-masked]
Date: 9 Dec 1996 06:17:40 GMT

[email-masked]