Re: Re:Auto-tetraploid versus amphidiploid was Hyb spots
iris@hort.net
  • Subject: Re: Re:Auto-tetraploid versus amphidiploid was Hyb spots
  • From: B* C* <b*@ymail.com>
  • Date: Fri, 1 Mar 2013 23:06:27 -0800 (PST)
Thanks again Tom for the research.  Can I ask now if I am interpreting this
correctly?

If I breed a small tetraploid MTB (or an small aphylla clone) to a
TB, I could hope to develop an iris someday that has the height of the small
MTB parent with the ruffles and lace that the TB had. I am assuming that
selecting and crossing the F1s for several further generations would be
required.

For those of you who think I am crazy for having such a goal,
you'll have to get in line. LOL

Bill Chaney
________________________________
 From: Tom Waters <irises@telp.com>
To:
iris@hort.net 
Sent: Friday, March 1, 2013 3:39 PM
Subject: re: [iris]
Re:Auto-tetraploid versus amphidiploid was Hyb spots
 
>From _The World of
Irises_ (p.392):
"Heinig and Randolph (1963) studied the meiotic behavior of
tetraploid iris 
species and tall bearded varieties. Their observations
indicate that for 
many and possibly most chromosomes tetrasomic pairing can
occur, even among 
the technically allotetraploid cultivars. However, in any
one variety not 
all chromosomes showed such pairing and the number that
formed 
quadrivalents varied among the cultivars studied. Nevertheless, enough
homology exists among the n=12 genomes to allow occasional allosynapsis 
(the
pairing of chromosomes from different species). Thus, in tall bearded
hybridizing it is possible that any allele can eventually be recovered as a
tetraploid homozygote with four doses."


  

Tom Waters  


Telperion Oasis ~
www.telp.com/irises    


Cuyamungue, New Mexico, USA (zone 6) 
----------------------------------------
From: "Tom Waters" <irises@telp.com>
Sent: Friday, March 01, 2013 4:22 PM
To: iris@hort.net
Subject: re: [iris]
Re:Auto-tetraploid versus amphidiploid was Hyb spots 

Chuck wrote:
"So
fertile 4x 48 iris are should basically be considered 
amphidiploids. The
infertile ones are probably autotetraploids."
I believe I understand the point
you are making, but I think "amphidiploid" 

is a very poor choice of
terminology in this case. We know that TBs are not 

autotetraploids, and it
makes sense, as you suggest, that two chromosomes 
that are more closely
homologous than either is with the other 
near-homologs would pair and form a
bivalent, rather than a tetravalent 
with the other two. (Although I have no
idea if there are any observations 

that confirm or refute this conjecture.)
However, in a true amphidiploid, chromosomes from the "unlike" sets do not
pair at all, or at best rarely and with difficulties. It is clear that 
there
are no such two "unlike" components in the TB chromosome sets. If 
there were,
the amphidiploid SDBs and CGW arilbreds could not be fertile, 
because the two
TB sets they received from their supposed "amphidiploid TB" 

parent would not
be able to pair.
It's very confusing to refer to the tetraploid TBs as
amphidiploids when 
nearly 100 years of breeding demonstrates clearly that
they do not function 

as such in outcrosses to other types. Amphidiploids
function as diploids 
(hence the name), and so cannot produce fertile
offspring when crossed with 

other types.
I make essentially the same point
in my post about tet MTB parentages. If 
you routinely get fertile seedlings
when backcrossing to either of the 
parent types (for as many generations as
you like), you have a functional 
tetraploid, not a functional amphidiploid.
Regards, Tom

Tom Waters  

Telperion Oasis ~ www.telp.com/irises   
Cuyamungue, New Mexico, USA (zone 6) 
----------------------------------------
From: "Chuck Chapman" 
Sent:
Thursday, February 28, 2013 9:51 AM
To: iris@hort.net
Subject: [iris]
Re:Auto-tetraploid versus amphidiploid was Hyb spots 

We had been discussing 
aphylla  X TB hybrids and fertility.

Actually, it turns out that full
tetraploids, (autotetraploids) with  
four identical  genes, have much reduced
fertility compared to 
amphidiploids, or straight diploids.

This is because 
during meiosis (forming of  gametes, eggs and pollen)  
four sets of identical
genes often form tetravalents. And during first 
stage of meiosis,  either all
of these four genes go into  one cell, or 
the other, leaving no copies in the
other cell. When there is  small 
differences in the  four sets of genes, of a
particular chromosome, you 
get two  bivalents and  so proper  division during
meiosis

Now remember,  most tetraploid iris, at least the TB iris , have
multiple  species involved  in their creation, so probably a number of  
genes
from different species, with enough of a difference, and enough 
of a
similarity, to pair up as fertile bivalents , rather then 
tetravalents
during  meiosis.

Also almost all ( I'd say all, but there probably an
exception or 
several out there somewhere) fertile tetraploids found in
nature, are a 
result of an inter  species hybrids, and thus are 
amphidiploids, 
rather then autotetraploids.

So fertile 4x 48  iris are
should basically be considered 
amphidiploids. The infertile ones are probably
autotetraploids.

Look up "fertility in autotetraploids"

Chuck Chapman
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