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Re: HYB: tetraploids, Diploids, Triploids etc

  • To: Multiple recipients of list <iris-l@rt66.com>
  • Subject: Re: HYB: tetraploids, Diploids, Triploids etc
  • From: Tom Tadfor Little <telp@rt66.com>
  • Date: Mon, 4 May 1998 09:34:42 -0600 (MDT)

>Sorry to be an asker of questions once more but....... Can someone give me
>a rundown of the differences between tetraploids, diploids, triploids....
>what the hell colchicine treatment is.... and what the benefits of the
>multi-ploids are?  I know some of the info, but I';d really like to see it
>all drawn together into one document for reference.

There have already been some replies, but some of them include some
misinformation about which of the bearded classes are tetraploid.

Diploids have two sets of chromosomes; triploids, three; and tetraploids,
four. Diploids and tetraploids tend to be fertile; triploids tend to be

Early tall bearded (TB) irises were diploids. However, during the period
1900-1940, tetraploids became increasingly common, finally replacing the
diploids entirely, so that essentially all new TBs introduced are now
tetraploids. Border beardeds (BBs) are tetraploid too, since they are
mostly just small segregates from TB breeding.

The miniature tall bearded (MTB) class is derived mostly from smaller
antique TBs, and so most of them are diploid. Recently, Ben Hager and
others have used the species I. aphylla to produce tetraploid MTBs. Most
MTBs today are still diploids, but the fraction of tetraploids is increasing.

The MDB, SDB, and IB classes are essentially all tetraploid, too, but with
two different kind of chromosome sets present, which means they do not
breed like regular tetraploids do.

SDBs have two chromosome sets from TB irises (each with 12 chromosomes)
plus two sets from the tiny dwarf species Iris pumila (each with 8
chromosomes). This "two sets of one kind, two sets of another kind"
arrangement is rather special, and a special term is used for irises of
this type: amphidiploid. All amphidiploids are tetraploids, but not all
tetraploids are amphidiploids. Amphidiploids tend to be fertile.

Most MDBs have 3 pumila sets and 1 TB set, making them "unbalanced
tetraploids", usually not very fertile. There are also quite a few MDBs
that are amphidiploids like the SDBs (Hager, again, is responsible for most
of these). There are also a few MDBs that have all four chromosome sets
from Iris pumila.

Most IBs, on the other hand, have 3 TB sets and 1 pumila set, so they are
also "unbalanced tetraploids" and often have little or no fertility.

So if we let the letter "T" stand for a set of 12 TB chromosomes, and the
letter "P" stand for a set of 8 Iris pumila chromosomes, the typical
breakdown for each class of bearded iris is as follows:

modern TB and BB:   TTTT (proper tetraploid)
most MTB:           TT   (diploid)
some MTB:           TTTT (proper tetraploid)
IB:                 TTTP (unbalanced tetraploid)
SDB:                TTPP (amphidiploid tetraploid)
most MDB:           TPPP (unbalanced tetraploid)
some MDB:           TTPP (amphidiploid tetraploid)

There are several advantages of tetraploids over diploids:

1. The possible gene combinations increases dramatically, so more variation
in color, pattern, and form becomes possible.

2. Tetraploid plants and flowers tend to be larger more durable than their
diploid counterparts (but note that many dwarf species are tetraploid, so a
tetraploid is not necessarily a giant)

3. Perhaps most importantly, crossing two different types of iris (TB and
Iris pumila, say, or TB and aril) can produce fertile (amphidiploid)
offspring if both parents are tetraploid, but will produce sterile
(unbalanced diploid) offspring if both parents are diploid. This means that
wide crosses using diploids are "dead ends", whereas wide crosses using
tetraploids can lead to a whole new family of irises.

For further reading, you might enjoy two essays on my web site (URL below):
"Big Ideas for Little Irises" (which is also in the October '97 AIS
Bulletin) and "Iris Chromosome Notation and Classification Thoughts", as
well as a posting titled "a little chromosome essay" in the list archives
for March 1996.

Tom Tadfor Little                       telp@Rt66.com
Iris-L list owner  *  USDA zone 5/6  *  AIS region 23
Santa Fe, New Mexico (USA)
Telperion Productions      http://www.rt66.com/~telp/
Iris-L Web Site  http://www.rt66.com/~telp/garden.htm

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