Don't forget about independent assortment at meiosis. This would make new
mixes each time meiosis occurred and that new mix would join another new
mix in the egg by the same process. Granted, no new genes would be
introduced, but new combinations would. If there were 10 kinds of
chromosomes and each had one gene that was heterozygous, there would be 2
to the 10th power possible combinations when they pair up and separate,
thus the same number of different kinds of gametophyte genotypes possible.
In bryophytes, evidence is beginning to suggest that somatic mutations may
have been evolutionarily important because of the potential for each cell
to dedifferentiate and develop a new individual when it has been removed
from the parent plant. Mutations that affect some of the pollen grains or
eggs could have the same result. Just how often should we expect a new
generation to arrive with no cross-overs or mutations? Granted, they may
be the exception, and they may be lethal, but some will occur and will be
different from other gamete cells on the same plant.
Hopefully I'm not so sleepy that I got it all wrong!
Janice at 1 am
On Fri, 26 May 2006, Jensen, Douglas wrote:
> Actually, because of the change in heterozygosity by selfing, I consider
> it as very different from cloning. Heterozygosity decreases by 50% with
> each self pollination. If we buy the evolutionary argument that
> heterozygosity is generally good within a population, then when
> outcrossing is not possible, it is probably better to clone oneself than
> it is to self-pollinate. Talk about the "incest taboo"... If anyone
> knows of empirical studies that have tested this, I would like to hear
> about them.
>> Here are two other interesting caviats that students don't always get.
> (1) Geitenogamy (pollination from one flower to another on the same
> plant) is genetically the same as self-pollination within a flower.
> (2) Selfing within a gametophyte leads to 100% homozygosity. Amazing!
>> My students wonder why I find plants to be so fascinating. I wonder how
> they can NOT find them to be so fascinating!
>> ________________________________
> Douglas P. Jensen
> Assistant Professor and Chair of Biology
> Converse College
> Spartanburg, South Carolina, 29302
>douglas.jensen at converse.edu> (864)596-9123
>> -----Original Message-----
> From: plant-ed-bounces at oat.bio.indiana.edu> [mailto:plant-ed-bounces at oat.bio.indiana.edu] On Behalf Of David R.
> Hershey
> Sent: Friday, May 26, 2006 4:33 PM
> To: bionet-plants-education at moderators.isc.org> Subject: [Plant-education] Re: Introductory Botany
>> I would have no problem rewording my Principle 14, perhaps as follows:
>> 14. Plants often clone themselves vegetatively. Many plant species are
> naturally self-pollinating. They come close to cloning themselves via
> self-pollination because they are highly homozygous. For example, the
> pea cultivars (cultivated varieties) Gregor Mendel started with were
> almost the equivalent of seed-propagated clones.
>> One important concept that is missing from many botany/biology texts is
> that sexual seeds can be used to produce genetically uniform crops.
> While it not technically cloning, it comes close enough for practical
> purposes. Seed-propagated cultivars are often either highly homozygous
> inbred lines or F1 hybrids created by crossing two inbred lines. Inbred
> lines are created by repeated self-pollinations. Homozygosity increases
> rapidly with repeated self-pollinations.
>> I limit the definition of plant or Plantae to embryophytes (bryophytes
> and vascular plants). Some authorities also include some algae in the
> Plantae. The obsolete two-kingdom Plantae is still often used. ASPB
> Principle 1 states that "plants are unique in that they have the
> ability to use energy from sunlight along with other chemical elements
> for growth." That is only true with the two-kingdom Plantae.
>> ASPB Principle 7 states that "Plants exhibit diversity in size and
> shape ranging from single cells to gigantic trees." If Plantae is
> limited to embryophytes, the smallest plant would be multicellular
> duckweed (Wolffia sp.).
>> Your Principle 2 provided details missing from ASPB Principle 4 which
> stated, "Reproduction in flowering plants takes place sexually,
> resulting in the production of a seed." That excluded all the
> nonflowering plants, which can also sexually reproduce.
>> It would be probably also be worthwhile to have a companion list that
> provides specific examples and exceptions for each of the plant
> principles. For example, your Principle 1 is correct that most plants
> are stationary, but certain plants parts (spores, seeds, fruits,
> pollen) can travel great distances via wind, water or animals. Some
> seeds or spores can also travel significant distances via explosive
> discharge. Some plants can also spread considerable distance by
> growing. The 'Pando' quaking aspen probably holds the record as it
> spread by root suckers to cover about 43 hectares with 47,000 trunks.
>>> David R. Hershey
>http://www.angelfire.com/ab6/hershey/bio.htm>> _______________________________________________
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