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 20:00:14 -0500 (EST)
Autotetraploids are always less fertile then the the corresponding diploids they were created from. The loss of fertility ranges from complete sterility, to a high of about 87% . In all cases , with a reversion back to diploidy (it happens) even sterile hybrids become fully fertile. Randolph did a study on fertility of tetraploid corn ( a field he was heavily into) over a ten year period, involving si x sib crosses, and selecting for increased fertility. What he found was that the number of tetravalents decreased as the fertility increased. A direct negative correlation between tetravalents and fertility.
Studies of the genes involved in a plant undergoing mitotic, and meiotic division has now identified a number of the genes and what they do, at the molecular level. There is a gene that that produces an enzyme that digests the centromere bond between bivalents. This allows the individual genes to be separated, with one going into one cell, and the other of the bivalent to go int the other cell, in mitosis and in second division of meiosis. But this gene is turned off in first stage of meiosis. Thus during cell division the sister chromosomes , that are joined at centromere,, are thus separated as pairs.
If there is a trivalent, or tetravalent, that is joined at centomere, it will be pulled all into one of the cells or the other. So that one cell has two copies of the three, or four chromosomes, and the other cell has no copies. Sometimes they can separate, but at other times not. One of the reasons for reduced fertility. Another event that can happen with tetravalents, is crossing of chromosome arms, in an unregular fashion, just because of the geometry. If this is a minor change, then we can, in next generation have two bivalents, instead of a tetravalent. But more often it is irregular enough to cause further problem. Usually unviable gametes don't produce viable seeds, so there is a force working on reducing similarity of genes in autotetraploids.
Just one more thing for this post. The article by Hening and Randolph found that usually ther was 1 or 2 tetravalents in a plant. In one cultivar, on one occasion, they found three. And for trivalents, the most they found in any species or hybrid was two. 

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)

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