re: Re:Auto-tetraploid versus amphidiploid was Hyb spots
iris@hort.net
  • Subject: re: Re:Auto-tetraploid versus amphidiploid was Hyb spots
  • From: &* W* <i*@telp.com>
  • Date: Fri, 1 Mar 2013 16:39:52 -0700
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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