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HIST: 26-chromosome Aneuploids

From: Sharon McAllister <73372.1745@compuserve.com>

Message text written by Linda Mann:
Well....where did these 26 chromosome guys come from? 

From 24-chromosome diploids with incompletely homologous chromosomes. 
Mitra, for example, found considerable variation in I. variegata plants
collected from different regions.  All were identified as I. variegata on
the basis of source and appearance, so I'd expect hybridizers to
inter-cross them without question.   Without an in-depth morphological
study -- NOT just a chromosome count -- there'd be no way of knowing that
their chromosome complements were not completely homologous.  

For the sake of this example, let's take just the specimen from Yogoslavia
that was analyzed as having two heteromorphic chromosomes.  To simplify
notation, I'll just call these H1 and H2.  On meiosis, the 11 homologous
sets would pair and split up into daughter cells.  Most gametes would have
either H1 or H2, but because they don't actually pair sometimes both will
go to the same daugher cell.  That means the plant produces four types of
gametes:  11-chromosomes [with neither H1 nor H2],12-chromosomes [with H1],
12-chromosomes [with H2]; and 13-chromosomes[with both H1 & H2].  

I'm not suggesting that functional 26-chromosome diploids would be as
common as the 24-chromosome diploids in the population as a whole, merely
that there is a recognized mechanism for their production.  And when
hybridizers get involved, it only takes ONE. 

Linda continued:
> From the HIPS
page, hardly any are 26.  Two out of 682, to be exact, & one of those is

But bear in mind that these are NOT representative of the population as a
whole.  Tens of thousands have been introduced and only those suspected of
unusal breeding behavior in favor of fertility were selected for counting. 
Aneuploids would normally be screened out by those criteria, but  BLACK
PRINCE / LIBERIA was of interest as a parent of AMBASSADEUR.  The 25s can
also lead to 49s & 50s -- but I didn't get into that because it's somewhat
more complex.

Next winter (is that this weekend? brrr), I'm going to wade thru the
pedigrees of all of this latter bunch and see if they seem to fall in
groups & then try to get more of them to torture here.  So far, I have
at least one 50 - ROSY WINGS (now y'all can see another reason I picked
that one to experiment with), and if I follow Dave Holm's suggestion, I
may wind up with a LOT of ROSY WINGS :)  .

O.K.  I'll pause to give you time to reflect.

Sharon McAllister

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