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HYB: Chromosome Counts

From: Sharon McAllister <73372.1745@compuserve.com>

Christy Hensler wrote:

<       It's been suggested to me that if a seedling were to have an uneven
number of Cs that it would *have* to be sterile.        >

A non-hybridizer speaking, of course. <G>  There are many, many examples of
highly fertile aneuploids.  CAPITOLA immediately leaps to mind.

<       If two seedlings have the same number of Cs, even if the count is
uneven, wouldn't a cross between them simply produce more seedlings with
the same uneven number? >

Not necessarily.  The "leftover" chromosome has nothing to pair with on
meiosis.  To keep it simple, let's look at the diploid CORDELIA.  Parent of
the famous DOMINION, among others.  Randolph counted it as having 25
chromosomes, so it can produce n=12 and n=13 gametes.  If crossed with
another 25-chromosome diploid, the resultant seedlings could be 12:12, 12:
13, or 13:13.

<       Also, in some crosses, there must be other factors at work since
some plants seem to "recognize" close relatives even if the C counts aren't
quite the same.         >

The "recognize" homologous chromosomes, so the number of these is a measure
of affinity even between species of different ploidy.

<       One additional question........ When we talk about chromosomes, we
are talking about *pairs* of Cs aren't we...... as in 2n=whatever?      >

Sometimes.  It depends on the context.

<       I'm curious about what would most likely happen if a cross of two
cvs, would be likely to result in an uneven count.

The ones I'd use as an example are I.fulva (2n=42) X I.ensata (2n=24) or
I.fulva X crossed to a 28C Sib. (The counts are from Kohlein's "Iris".)

Would the "extra" be dropped or could a population with an uneven count be
used as a base for developing a new line?       >

As the diploid example I gave shows, an unbalanced F1 isn't a stable
breeder.  In the case of CORDELIA, there were enough homologous chromosomes
to produce fertility.  Often, however, dipoids from wide crosses don't
exhibit enough fertility to be used as a base for developing a new line.  

Sharon McAllister

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