Date: Sat, 3 Jan 1998 12:21:19 From: SCHLAUER@chemie.uni-wuerzburg.de To: cp@opus.hpl.hp.com Message-Id: <aabcdefg22$foo@default> Subject: Re: Sexual Reproduction of cultivars
Dear Carl,
> (...) if
> you self a plant, like S. flava for instance, don't the seedlings of that S.
> flava x self have the same genetic makeup of the original plant?
The problem is the (usually; in order to prevent impertinent
comments) statistical distribution of allelic chromosomes during
meiosis, i.e. the paternal (pollen) and maternal (ovule) chromosomes
that make up the (nuclear) genome of the plant are not divided as
unchanged sets to the gametes but *any* chromosome of an allelic pair
may be grouped with those of the other pairs to form a haploid set.
This results in gametes that have a mixture of paternal and maternal
chromosomes. In selfing the plants, the mixed sets of pollen and
ovule are combined, and the resulting embryo may either become a +/-
exact copy of the parent plant or several allelic pairs may be
entirely paternal or entirely maternal (relative to the parent plant).
In the extreme case it is thus also possible to reconstruct diploid
clones of the maternal or paternal gametes (that formed the parent
plant).
Therefore, usually several alleles are lost by selfing (i.e. the
maternal or paternal allele is eliminated because both alleles are
from the same original parent: homozygous condition). So a genetic
degeneration is frequently encountered among the offspring. Because
repeated selfing eliminates almost all heterozygous alleles, this
strategy is utilized in breeding +/- genetically pure lines with
+/- uniform properties. This enables breeders to produce hybrids with
predictable characteristics.
Crossing over (the exchange of +/- homologous DNA *between* two
allelic chromosomes) is a mechanism to further complicate the
process of sexual reproduction (i.e. to increase the variability
among the offspring). Moreover, extranuclear inheritance, mutations
at all levels, transposition, and other factors permanently affect
the genetical makeup of most living beings. Thus, the idea of
identical clones, even if produced by vegetative division, is an
essentially theoretical one.
Multiplicate all the trouble mentioned above by the ploidy/2.
> If this is
> the case wouldn't any plant selfed reproduce an exact copy of itself?
This is not the case (see above), so the result is not as stated.
> I was
> always under the impression that if you created a nice cross you could self
> it to maintain its identity.
It would be better to forget this impression ASAP.
> If this is not the case, a red vft x a second
> of the same clone = a genetically new plant?
Yes.
Kind regards
Jan
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