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HYB: Aneuploid TBs

From: Sharon McAllister <73372.1745@compuserve.com>

Message text written by Linda Mann:
one of the things
I've been wondering about for some time is the 50 chromosome crosses
that were so important in starting the tetraploid TBs.  These were
(perhaps) crosses of tetraploid "tender" 12-12-12-12 TB's with
?unreduced diploid "tough" 12-12's.  I assume this is written with 12 as
the largest common denominator.  But maybe for purposes of this wild
idea, we could consider each 12 to be 3 sets of 4 [i.e., 3(4)] or even 6
sets of 2 [i.e., 6(2)].  
50 to 52 chromosomes seems to have been a common result in early
development of tet TBs (see HIPS page) which implies to me that some of
these sets don't match up right. 

BINGO!  It's not just the number of chromosomes that matters, but whether
or not they are completely homologous -- and a fairly wide range of
variation has been found even within individual species.  But let's stick
to 12 as the base number.  There's no evidence for breaking it down further
and plenty of evidence in support of it.  I'll show you how easy it is to
produce 50- and 52-chromosome aneuploids from 24-chromosome diploids and
48-chromosome tetraploids.  Just let your imagination take you to the days
of the early TB hybridizers, who were  working before much was known of
chromosome morphology.....

Let's assume you've assembled a nice collection of diploids and have been
merrily spreading pollen around, selecting the best seedlings for further
use -- oblivious to what's going on at the level of chromosome conjugation,
but providing conditions conducive to the production and preservation of
aneuploids.  Unmatched chromosomes left after the pairing process appear to
be distributed at random to the daughter cells, and the 25- or
26-chromosome aneuploids seem to have a survival advantage over the 22- or
23-chromosome aneuploids -- so you could easily have selected some 25- or
26-chromosome  hybrids as breeding stock....

Over in the tetraploid patch, the story is much the same.  You may have
collected several of the 48-chromosome tetraploids species -- but their
basic sets are not completely homologous.  So the  tetraploid breeding
stock you've produced could well contain 49- or 50-chromosome aneuploids.  

Continue mixing this volatile brew, and all it takes is one unreduced
gamete from a 26-chromosome aneuploid in a cross with a 50-chromosome
aneuploid to produce a 52-chromosome aneuploid.  

Now the stage is set for some serious speculation.  Are the four "extra"
chromosomes random leftovers?  Was the 26-chromosome aneuploid from an
intraspecies cross in which mates were picked up for both of the mismatched
chromosomes?  Was the 50-chromosome aneuploid from a cross that produced an
extra matched pair?  Could 52-chromosome aneuploids be line-bred to produce
a new race of 52-chromosome tetraploids?

I've found no indication that anyone tried this, but many of these weren't
even identified until they were past peak as breeders.  For the most part,
the early aneuploids were simply crossed back to the 48-chromosome
tetraploids and those that exhibited enough fertility to be useful
contributed their homologous chromosomes to the modern gene pool through a
process that again eliminated the unmatched ones.

Linda continued:

>Since different 50 chromosome 'tets' seem to have resulted in very
different weather tolerances amongst descendants, my wild thought is
that  ==========
which of those extra sets gets dropped might have a strong influence on
growth characteristics.

It's not an extra set that gets dropped, but an extra unmatched chromosome.
  In theory, it should be the extra one that made the plant an aneuploid
rather than a stable polyploid -- but in the mixed brew of multispecies
hybrids, what was originally an 'extra' chromosome can sometimes replace a
nearly-homologous one of a regular set.  For example, see the saga of Paul

This is what makes interspecies crosses such fun....

Sharon McAllister

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