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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: dkramb@tso.cin.ix.net (Dennis Kramb)
  • Date: Sat, 2 May 1998 10:02:50 -0600 (MDT)

>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.

I'm not a botanist, a geneticist, and most of all I'm not an iris expert,
but I've done a lot of reading...so I'll give you my understanding of these

DIPLOID: having two sets of chromosomes...when two diploid plants are crossed
the resulting offspring each get one half of each parent's chromosome pair,
and are therefore diploids too.

TETRAPLOID:  having four sets of chromosomes (a double diploid if you will)...
when two tetraploids are crossed the resulting offspring again each get half
of each parent's chromosome set, and therefore are tetraploids too.

In the beginning most irises collected in the wild were naturally diploid.
There are apparently some naturally occurring tetraploid regelia (aril)
species though.  I recently learned that all the bearded irises on the
market today (with the exception of some oncocyclus x regelia hybrids) are
all tetraploids.

Tetraploids are "better" because the plants tend to be more vigorous growers,
and produce bigger flowers.  Tetraploid hybrids are also more often fertile
than wide diploid crosses.

TRIPLOID:  having three sets of chromosomes...when a diploid and a tetraploid
are crossed, the offspring still get half of each parent's chromosome set,
but in this case they only get one from the diploid, and two from the tetra-
ploid, resulting in a triploid plant.

I don't think many triploids are on the market.  They tend to be infertile.
Hybridizers occasionally luck out and get fully fertile triploids, which
they can continue using in their hybridization program.

I presume that colchicine is a chemical of some sort, and that if you treat
newly sprouted seedlings with it, it somehow manages to double up the
chromosomes if you're lucky.  I think there is a pretty low success rate
with this treatment though.  Some seedlings die.  Some seedlings are not
converted.  Some seedlings are converted, but revert back to diploids over
time.  Beyond that vague bit of information, I know nothing about it or how
it works.  Hybridizers use this to create tetraploids (amphidiploids?) from
diploid crosses.

I've read a lot about this in the Aril Society yearbooks.  Since the oncos
are all diploids in the wild, and all the TB's on the market are tetraploid,
they use colchicine to raise the oncos to tetraploid levels and then cross
with the TB's to get fertile tetraploid arilbreds.

The other way to get fertile tetraploid arilbreds is to start with a diploid
onco crossed with a tetraploid TB to produce a triploid arilbred.  Crossing
the triploid arilbred with a tetraploid (either a TB or arilbred) will then
produce offspring that will be a mixture of tetraploids and triploids.  You
will know which are the tets by their growth habits and their level of
fertility.  This method doesn't require colchicine, but it does require
finding that lucky fertile triploid!

To confuse matters...apparently you can apply colchicine to triploid seedlings
to develop hexaploid plants...which I would guess tend to be more vigorous,
and fertile than the triploids.  Harald Mathes in Germany is doing this with
some arilbreds, but I think he only just started in the last few years, so
this is a pretty new field and I'm not aware of any hexaploids availble

In the last few days I've learned from other L-listers that (except for the
really old historics) all the TB, MTB, IB, BB, SDB, and MDB irises on the
market today are tetraploids.  Most of the arilbreds are tetraploids.  Most
of the arils are NOT tetraploids.  Most of the Lousianas are NOT tetraploids.
Most of the species are NOT tetraploids.

I have no idea about PCN, spuria, siberians, and the rest.  My guess is they
are mostly not tetraploids on the market.

Now...before I start believing I know what I'm talking about...can all you
hybridizers and botanists out there critique my little essay here?  Thanks!
It's hard to try to learn and understand this stuff from just reading
it and not actually experimenting with it yourself.

In conclusion, I would like to compliment and thank Sharon McAllister.  It
was her extremely well written article in the January 1998 American Iris
Society bulletin that taught me a lot of this stuff.  Prior to this, I had
tried to read other books and articles on the subject, but my brain turned
to goo when I couldn't even pronounce some of the words I was reading!!

Dennis Kramb; dkramb@tso.cin.ix.net
Cincinnati, Ohio USA; USDA Zone 6; AIS Region 6
Member of AIS, ASI, HIPS, RIS, SIGNA, & Miami Valley Iris Society
Primary Interests: Hybridizing Arilbreds, Raising Native Ohio Species Irises

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