Big Ideas for Little Irises
[by Tom Tadfor Little]

Like many iris enthusiasts, I have hybridizing plans. Unlike many others, though, I don't have visions of those plans leading to commercial success and awards. I'm more interested in doing something that will be a learning experience and possibly expand breeding opportunities for others in the future. My vision is a simple one: to fill out the fertile families of tetraploid bearded and aril irises using species that were largely bypassed in their breeding to date. 

First, I'll explain a little of the theory of the project, and then describe what I think needs to be done to realize it.
 
Fertile Families of Bearded and Aril Irises

The bearded iris species of Europe and the eastern Mediterranean have given rise to modern tall bearded, median, and dwarf irises. They are the most widely grown and most extensively hybridized members of the genus. The bearded species fall into two broad categories with respect to their chromosome make-up: some have chromosomes in sets of 12, and others in sets of 8. In both categories, there are some diploid species (2 chromosome sets) and some tetraploid species (4 sets). Here's a quick breakdown, not meant to be botanically thorough:
 
8-chromosome category, diploids:
I. attica (dwarf)
I. pseudopumila (dwarf)

8-chromosome category, tetraploid: [=+/-32 chr]
I. pumila (dwarf)

12-chromosome category, diploids:
I. suaveolens (mellita) (dwarf)
I. reichenbachii (dwarf)
I. variegata (median)
I. imbricata (median)
I. pallida (tall)
most hybrids in the MTB class

12-chromosome category, tetraploids: 
I. reichenbachii (balkana) (dwarf)
I. reichenbachii (dwarf)
I. aphylla (median)
I. croatica (tall)
I. cypriana (tall)
I. kashmiriana (tall)
I. mesopotamica (tall)
I. trojana (tall)
virtually all modern hybrids in the TB and BB classes, and an increasing number of hybrid MTBs. 

Aril Irises

To complete the picture, let's add in the aril irises, with chromosomes that come in sets of 10 (oncocyclus) or 11 (regelia). I'll call them the 10-chromosome category, because regelias and oncocycli can be crossed, and the advanced-generation plants usually have chromosomes in sets of 10.
 
10-chromosome category, diploids:
I. korolkowii (regelia)
all oncocyclus species
most pure aril hybrids (regeliocycli and oncogelias)

10-chromosome category, tetraploids: 
I. stolonifera (regelia)
I. hoogiana (regelia)
a few pure aril hybrids, most notably those derived from diploid arils converted to tetraploidy by the use of colchicines

Each of these six groups constitutes a fertile family. That is, one can expect members within each group to produce fertile offspring when crossed. In general, crosses between members of different fertile families will not produce reliably fertile offspring. This means there is a "fertility barrier" to the program of combining the characteristics of different families of irises in a single line of breeding. The first generation can often be produced, but then progress is slowed or halted by the limited fertility of the hybrids.

There is one important exception to the rule that hybrids between different families are sterile. If two tetraploid families are used, the hybrids will be fertile amphidiploids. (An amphidiploid is a kind of tetraploid with two sets of one type and two sets of a different type.) Such amphidiploids also constitute a fertile family: they can be expected to produce fertile hybrids when crossed with each other, but not when crossed with members of other families.

Since there are three basic tetraploid families, there are three possible amphidiploid families (8-8-12-12, 8-8-10-10, and 12-12-10-10). Two of these have been developed by hybridizers, and are remarkable success stories in the history of iris breeding. These are the SDBs (8-8-12-12, from crossing I. pumila with TBs) and the amphidiploid arilbreds (12-12-10-10, mostly developed by C. G. White from crossing TBs with arils). The SDBs were relatively easy to produce, since tetraploid parents of both types were available for use. 

The amphidiploid arilbreds were harder to obtain, since White did not have tetraploid arils of oncocyclus ancestry to work with, but had to use diploid arils to bring the oncocyclus characteristics into his breeding lines. Luckily, the fertility barrier in working with diploids is not an absolute law; but getting past it requires Herculean persistence.

The third amphidiploid family comes from crossing arils with I. pumila. It has not yet received much attention from hybridizers, for several reasons: tetraploid arils with oncocyclus ancestry are a relatively recent development, I. pumila is not easy to grow in most climates where arils thrive, and there is not a great commercial demand for dwarf arilbreds.
 
Assessing the Tetraploid Fertile Families
Development of the tetraploid and amphidiploid fertile families through breeding has not involved a thorough effort to exploit the characteristics of many different species. Rather, the fertile families have all been established from a relatively small number of original parents. Subsequent breeding has thus merely exploited the latent potential of these pioneering crosses. With each successive generation of selective breeding, flower quality improves. This is good, of course, but it has the undesirable consequence of making hybridizers more reluctant to return to the original species to expand the genetic base of the family. What follows is a brief assessment of what has been done in each family, and what might be gained by filling it out using species that were bypassed originally.
 
The 48-Chromosome Tetraploids
This family includes the modern tall bearded irises, certainly the most hybridized family in the entire genus. The TBs derive primarily from the tetraploid tall bearded species (all of which are rather similar), I. pallida (a diploid TB), and I. variegata (a diploid median). A few of the other species (such as I. aphylla) have also contributed, but to a very limited degree.
Recently, I. aphylla has been used by a small number of dedicated breeders to expand the median classes (most notably the MTBs). These new tetraploid medians are fully fertile with the TBs and BBs, and provide a channel for bringing the characteristics of the highly developed modern TBs into smaller plants.  

There are two other avenues for expanding this family that should not be overlooked: 

1. The dwarf species I. reichenbachii (in some of its forms) is already a member of this family, and can be bred into it without much difficulty. The long-term result would be to extend the family into the SDB and MDB classes, without losing fertility with the TBs. Currently, almost all SDBs are members of a different fertile family, and tend to produce sterile offspring when crossed with anything else. 48-chromosome SDBs would not experience this fertility barrier when crossed with TBs or 48-chromosome dwarfs and medians, but would of course lack the delightfully varied I. pumila genes. 

2. The diploid species (dwarf, median, and tall) could be converted to tetraploidy by use of colchicine, or their genes worked in to the group through persistent hybridizing effort.
 
The 32-Chromosome Tetraploids
This family consists of the single species, I. pumila. Fortunately, this species exhibits remarkable variability in color, form, and plant habit. There are white, yellow, purple, blue, and blended forms, with or without contrasting spot pattern or beard. A good number of garden hybrids have been named and registered as MDBs.

The diploids I. attica and I. pseudopumila probably do not have that much to contribute to this group of plants. It is likely that I. pumila itself is a natural hybrid originating from these two diploid species. Breeders can work on producing garden pumilas that are more adaptable to climates with mild winters, but otherwise there is little to add to this single-species family.
 
The 40-Chromosome Tetraploids
These are the tetraploid arils. Originally the family consisted only of the two regelia species I. hoogiana and I. stolonifera, plus just a few aril hybrids that were tetraploid by fluke of nature. Now, the use of colchicine on oncocyclus species and pure aril hybrids has produced a race of tetraploid arils with more oncocyclus ancestry. Regelias were used in their breeding too, for the sake of gardenability, and because the original colchicine-induced tetraploids were very difficult to intercross. (Fertility tends to be low in induced tetraploids for several generations.) 

I think the great need in this family is to incorporate even more oncocyclus species into the family. Repeating the colchicine treatment on other oncocyclus species and hybrids should be pursued, and so should crossing tetraploid arils with diploid oncos. This will be slow and difficult work, but it is important in the long run. 

The present family can be improved by line breeding, of course, but we should try not to depend exclusively on the currently available tetraploids, which are still not many. They do show a nice diversity in form, pattern, and plant qualities, but most of them show their regelia ancestry to a moderate or great extent, and lack the refinement of the diploid oncocyclus.
 
The 40-Chromosome Amphidiploids
This family includes the SDBs, originally derived from TBs and I. pumila. These have been line bred nearly to perfection, and are the most popular medians. There is another important segment of this family, however: MDBs from SDB breeding. Many of the popular miniature dwarfs in recent years are actually diminutive SDBs, in much the same way that BBs are diminutive TBs. This approach to MDB breeding has potential, but also has problems familiar to BB breeders: plants that grow out of class and the difficulty of "fixing" the small size into a reliable line. 

A full family of 48-chromosome dwarfs and medians crossed with I. pumila would likely produce very dainty 40-chromosome amphidiploids that would be a great asset to MDB breeding. There are already some 40-chromosome MDBs from such breeding, but the program is still in its infancy.
Such 40-chromosome miniatures would also be useful in SDB breeding, as a way to reduce coarseness and other problems of proportion that sometimes arise within this class.
 
The 44-Chromosome Amphidiploids
This fertile family includes the amphidiploid arilbreds. Most current cultivars are descendants of C. G. White's original groundbreaking hybrids. Also included are amphidiploid arilbreds from regelia breeding. Now that there are tetraploid aril hybrids from colchicine-treated oncocyclus and oncogelias, it is possible to expand this family with far less difficulty than White encountered in creating it. 

First and foremost, the new tetraploid arils should be crossed with modern TBs, to broaden both the aril and bearded components of the arilbred gene pool. This line of work is underway, although progress is slow. More breeders need to get involved. 

Secondly, more attention needs to be given to the regelias. The tetraploid regelias can be crossed with a variety of modern TBs, and the resulting seedlings line bred or worked into more traditional arilbred lines. This sort of breeding has been going on for decades, but at a very slow rate. The reason is that the "regelia look" is not appreciated as an interesting alternative to the "onco look". Until regeliabreds are appreciated for their own distinctive qualities, breeders will not be inclined to work with them. Regelias are also more easily grown than oncos; hence they have potential in producing more adaptable arilbreds. 

Finally, 48-chromosome dwarfs and medians can be used with the tetraploid arils to create arilbred medians that are fully fertile with traditional arilbreds. (A couple have already been produced.) This development would really liberate arilbred median breeding from its current shortcomings. Most current arilbred medians are sterile unbalanced tetraploids, produced by crossing arilbreds with SDBs. They are thus only 1/4 aril, with no possibility for improvement through line breeding. Arilbred medians from tetraploid median x tetraploid aril breeding would be 1/2 aril and fully fertile. Furthermore, since they belong to the same fertile family as the traditional C. G. White-type arilbreds, the potential for improvement and refinement is enormous. 

One particularly interesting combination might be to cross tetraploid MTBs with regelias, which already show some of the grace and delicacy expected in the MTB class.
 
The 36-Chromosome Amphidiploids
This last family comes from crossing tetraploid arils with I. pumila. There are very few irises of this type in commerce. They are arilbred dwarfs, half aril but typically less than 6 inches tall. So far, most of these have been produced to satisfy breeders' curiosity, rather than as part of a determined program to create a new commercially successful fertile family. 

Systematic production and improvement of these "arilpums" should now be taken up in earnest. The I. pumila genes should confer adaptability to colder, wetter climates, a quality which most current tall arilbreds lack. Furthermore, being fully 1/2 aril, the arilpums can be expected to show strong aril flower characteristics. Ultimately, they may be the best vehicle for bringing the beauty of the arils into the gardens of nonspecialists. 

There are many clones of I. pumila available; there are also a fair number of tetraploid arils. All that remains is to make a lot of crosses and grow a lot of seedlings.
 
Personal Priorities
My two highest breeding priorities are to create more small 48-chromosome medians and to establish a family of 36-chromosome arilpums. The 44-chromosome arilbreds are also a strong interest, particularly working with regelias and tetraploid medians as parents. I am fortunate to live in a climate that is cold enough to grow I. pumila and other dwarf bearded species, but also dry enough to grow pure arils. 

The small 48-chromosome medians, actually, are a cornerstone in expanding three of the six fertile families described in this article. They are valuable in their own right, by virtue of being compatible with the highly developed TBs; they can vitalize MDB breeding by spawning a 40-chromosome fertile family of truly small dwarfs; and they can bring about a completely new family of fertile arilbred medians with all the drama and variety of the tall arilbreds.
I do indeed have big plans for these little irises, and I hope others will too. These are broths that now suffer from too few cooks, rather than too many. Much-needed progress is always slowed, and sometimes even reversed, because there are not enough hybridizers to create, share, and protect the products of their work.
 
Acknowledgments
Nothing in this article is original. All the ideas have been around for a long time, and some dedicated breeders are already at work in each of the areas I mention. Here are some of the important people in this story so far. This list is not all-inclusive. If I leave some out, it is not meant to minimize their contributions. 

Ben Hager recognized the importance of I. aphylla in median breeding, and decades ago launched a long and ultimately successful program to breed tetraploid MTBs from I. aphylla. Hager also led the way in breeding 40-chromosome miniature dwarfs.
Jim and Vicki Craig are now the unquestioned specialists in the world of 48-chromosome tetraploid medians. Using I. aphylla, they have produced not only MTBs but IBs and BBs in a full range of colors. In just the last few years, they have also produced SDBs from this line. This is an exciting development indeed. 

Peter Werckmeister was one of the first to appreciate and publicize the "fertile family" concept as presented in this article, particularly as regards the arilbreds. He produced a number of 36-chromosome arilpums, now apparently lost. 

Sam Norris, along with John Holden, developed a line of tetraploid arils based on colchicine-treated oncocycli and oncogelias. Without this revolutionary development, we would be limited to only one or two tetraploid arils with any oncocyclus ancestry, and half of the plans in this article would not be possible. 

Harald Mathes has used the tetraploid arils to produce a few fertile arilbred dwarfs and medians, in both the 44-chromosome and 36-chromosome families. Francesca Thoolen has taken a strong interest in the 36-chromosome arilpums. 

As I said at the beginning, these types of breeding projects are unlikely to bring commercial success or high awards to the hybridizer, at least not until many generations of seedlings have gone to the compost pile. Yet I'd consider my plans a success if someday I am deemed to have entered the company of the dedicated iris breeders listed here.

Related essay: Iris Chromosome Notation and Classification Thoughts
Return to Irises entry page
Return to Gardening entry page
Return to Telperion Productions entry page
Copyright © 1997 Tom Tadfor Little