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Re: RE: OT: ID a Tree & rebloom news

  • Subject: Re: [iris-photos] RE: OT: ID a Tree & rebloom news
  • From: Bruce & Linda Silversity@comcast.net
  • Date: Tue, 11 Oct 2005 13:04:24 -0500

Title: Re: [iris-photos] RE: OT: ID a Tree & rebloom news
   You are wonderful.  Thank you so much for all the info!
Have a great day!

There are lots of good discussions about chromosomes and the results of crossing various classes and/or species with differing numbers.  I think the 'World of Iris' has some good discussions on the basics.  Again I am away from my resources, so I still can't look up 'Lady Emma', nor cite references.

Anyway, here is a bit of an "in a nut shell" summary, that I hope makes sense:

The IB class is mostly derived from crosses between 40 chromosome species or dwarf cultivars with 48 chromosome TB species or cultivars, yielding 44 chromosome (give or take one or two) IB offspring.  The old IB cultivars are mostly derived from I. lutescens crossed with 48 chromosome TB species or cultivars, and certain hybridizers such as Sass and others made bunches of these.  Newer ones tend to be from TB cultivars crossed with SDB cultivars in search of desirable character combinations from the two parental classes.  Of course they are all interfertile, so they are crossed with one another too.  

Some of the 40 chromosome dwarfs (I know the plural of dwarf is dwarves, but it sounds wrong for Iris somehow) are hybrids of 32 chromosome dwarf species or cultivars with 48 chromosome TB species or cultivars, and I. lutescens and other wild 40 chromosome species of bearded Iris also probably derived originally from 32 chromosome species crossed with 48 chromosome species.  This is overly simplified, as 18 and 24 chromsome species (diploids) may have been involved too - through unreduced gametes.

Anyway it sort of works out like this:

(32/2) + (48/2) = 40  This lead to wild species such as I. lutescens and most cultivated SDB's (which are fertile with species such as I. lutescens)

As for wild candidates there are a lot, but potential examples could be as follows.  It is likely that the parent species of the original wild 40 chromosome species were those that occur around or near the Mediterranean, but things to move around over time.  It seems that more notherly or inland small or smallish 24 and 48 chromosome species such as I. aphylla, I. croatica, I. junoniana, I. reichenbachii, I. suaveolens, and their kin. probably weren't involved.  Probably not I. variegata either, though perhaps.  Also, these (except for I. variegata, which was important) mostly weren't involved in early cultivars either, but they are more and more now:

I. attica (unreduced = 16) + I. pallida (unreduced = 24) = 40


I. pumila (normally reduced 16) + I. cypriana (normally reduced 24) = 40

These could be examples of how wild species such as I. lutescens came to be.  They are fertile, because they have two sets of 8 and two sets of 12, which act as if they are two sets of 20 (everything is ballanced).  They are technically tetraploids, but functionally they are diploids.  This is one way that chromosome numbers can be instantly and greatly increased through addition of unlike sets to form new sets.

These tend to be dwarf (mostly SDB), but not as dwarf as the parent species (mostly MDB) with a base set of 8 chromosomes (the 16 and 32 chromosome species).

When you cross the 40 chromosome plants with 48 chromosome plants, you get an unbalanced set of one 8 and three 12's (usually - sometimes it doesn't all sort out quite right and you get 43 or 45, or even occasionally numbers futher away from 44, all the way from 40 to 48, because sometimes things don't sort out quite as neatly as you might expect).  Anyway, these usually end up with 8 + 12 + 12 + 12 = 44, and they are usually intermediate to SDB and TB, yielding mostly IB plants.  The wild "species" I. germanica, I. albicans, I. florentina, etc. are of this sort of ancestry as well.  They tend to be infertile (they will make some seeds), and due to their most common ancestries they tend to be early flowering.

The MTB's tend to fall nearly all into two broad categories.  One is the diploids (nearly 100% of older cultivars) derived mostly from I. variegata and I. pallida cengialtii, with a bit of other smallish diploid species such as I. reichenbachii mixed in occasionally.  They tend to be late-flowering with more delicately proportioned stems and flowers than the 44 chromosome class of (mostly) IB's, and they also tend to have morphological traits dominated by I. variegata, while the 44 chromosome class mostly look morphologically most like I. germanica or I. albicans (ignore the flower colors).

The other mostly recent MTB group is very different and is derived from cultivars of 48 chromosome TB's and BB's (BB's are usually genetically TB's, but they just don't get as tall), crossed with small northern 48 chromosome species such as I. aphylla, or cultivars derived from them.  They tend to inherit an earlier bloom time from the small species and some flower almost as early as the 44 chromosome group, and often much earlier than most of the diploid MTB's.

All the AIS categories tend to follow basic genetic groupings, and so MTB's tend to be mostly 18 or 36 chromosome plants.  SDB's tend to be mostly 40 chromosome plants.  IB's mostly 44 chromosome plants of recuced fertility, and TB's tend to be 24 or 48 chromosome plants.  There are a number of dwarves that are 24 or 48 chromosome plants, but they tend to be mostly wild northern species or species crosses, and not to be too much involved in hybridization of cultivars (yet).

The MTB's are mostly 24 chromosome plants close to I. variegata, but now there is a second group of them that are 48 chromosome plants of a different ancestry.

TB's (and BB's) are mostly 48 chromosome plants of Mediterranean / Near Eastern species often with I. variegata and/or I. pallida ancestry.  But many old ones (usually smaller) are 24 chromosome plants, closely akin to the MTB diploids, but usually with as much or more I. pallida than I. variegata in their ancestry.

The problem with the AIS system is that it only uses measurements as a guide, and plants are inherently variable.  So, sometimes a 44 chromosome plant that would logically be an IB by genetics, will be a MTB or SDB by measurement.  So, they get classed with plants that are not similar in chromosome make up.  The same with 40 chromosome plants.  Even though they are genetically most like most SDB's, they can be larger or smaller than average, and become MDB or IB by AIS registration.

As the years go by, the ancestries get more and more mixed up, muddled, and genes blended together in new combinations.  However, it is very difficult to get the chromosomes of the base 8 species into plants with base 12 chromosome numbers and visa versa, because once they are mixed, they don't tend to cross back to either main parental group very easily.  The 40 chromosome intermediates are fertile, but in effect they are isolated from either parental group by their new chromosome make-up.  When they are crossed with the parental groups in either direction they tend to yield sterile plants with +/-44 chromosomes or +/-36 chromosomes.

The 36 chromosome class doesn't seem to be well-developed, but they should be mostly MDB plants, and I'm sure there are a lot of them out there that just haven't been recognized as such yet.  They may even be some of those "problem" parents that don't make fruit or pollen well, for "unexplained" reasons.  These would mostly come from a cross of (32/2 = 16) + (40/2 = 20) = 36.

The more backcrosses are tried, the more will be successful, and the more the chromosomes will get recombined in cultivars eventually.  Some day there may be MDB with 32 chromosomes, but with genes from TB's and visa-versa.  Also, the more new species are added to the mix, the more complicated, mixed up, and interesting the cultivars will become.

And this doesn't even consider any of the Psammiris, Regalias, Arils, etc., which can and are gradually being added to the mix too.


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