re: Re:Auto-tetraploid versus amphidiploid was Hyb spots
iris@hort.net
  • Subject: re: Re:Auto-tetraploid versus amphidiploid was Hyb spots
  • From: C* C* <i*@aim.com>
  • Date: Sat, 2 Mar 2013 14:42:49 -0500 (EST)
There is a lot of material to cover here, so I'll do it one step at a time.
There has been an incredible amount of new discoveries in molecular genetics and cellular biology in the past 50 years, since the article "Heinig and Randolph (1963). Randolph was a great scientists. He even published a joint article with Barbara McClintock ( to genetics something like Stephen Hawkins to physics). He did a lot of work with polyploids. but polyploid study was basically in its infancy at that time.
So his article was based on his observations, his understanding of polyploidy, and his interpretation based on the knowledge of the time.
The article referenced was published in the AIS bulletin #170, July 1963. I suggest that anyone interested in this discussion look it up. It is downloadable from AIS e-membeship site.
It is written in a "Journal" format, probably written for and submitted to a journal, but never published in a journal.
In early study of polyploidy, specifically tetraploids, it was thought that tetrasomy, or tetrads, were a good thing., and suggested viability of gametes. Since then there has developed a lot of new techniques to enable study of how meiosis works.
So I'm including information from the book "Principles of Genetics 6th edition" by Snustad and Simmons, 2012. At U. of Guelph this is considered the bible on teaching genetics. It is used as textbook in four undergraduate courses.
"In spite of their robust appearance, many polyploid species are sterile. Extra sets of chromosomes segregate irregularly in meiosis, leading to grossly unbalanced (that is, aneuploid) gametes. If such gametes unite in fertilization, the resulting zygotes almost always die. This inviability among the zygotes explains why many polyploid species are sterile. 
.............
.....
The meiotic uncertainties that occur in triploids also occur in tetraploids with four identical chromosome sets. Such tetraploids are therefore also sterile. However, some tetraploids are able to produce viable progeny. Close examination shows that these species contain two distinct sets of chromosomes and that each set has been duplicated. Thus, fertile tetraploids seem to have arisen by chromosome duplication in a hybrid that was produced by a cross of two different, but related , diploid species; most often thee species have the same or very similar chromosome numbers." 

Chuck Chapman



-----Original Message-----
From: Tom Waters <irises@telp.com>
To: iris <iris@hort.net>
Sent: Fri, Mar 1, 2013 6:45 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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