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Breeding Strategies |
Although there is no magic formula for successful hybridizing, there
are some time-tested strategies that have been widely used, not just
by iris breeders but by plant and animal breeders of all sorts. This
article is a short review of some of these.
Breeding Strategies
Line breeding refers to making crosses between closely related plants (such as siblings) for a number of generations. Its purpose is to combine the best qualities of the parental stock, bringing out the full potential of the gene pool used. Some beginning hybridizers are disappointed when seedlings from a cross do not combine all the desirable characteristics of the parents in a single perfect plant. In fact, it's not unusual for the first generation seedlings to be inferior to the parents, apparently lacking their most impressive qualities. This is where line breeding is useful. The genes for the desirable characteristics are still there, but may not express themselves fully when combined with genes from the other parent. The answer is to line breed the seedlings. In time, the gene combinations that gave each parent its special qualities will resurface. This is most obviously true for recessive alleles, such as the tangerine pink factor and the plicata pattern. Crossing a pink iris with an iris with no pink ancestry will produce no pinks in the first generation, but the trait will reappear in successive generations if enough seedlings are raised.
For this reason, it is important not to give up on a cross too soon. If you had a good rationale for making the cross in the first place, it should probably be carried forward (by sib crossing) to at least the next (F2) generation, and possibly well beyond. The aphylla lines of Ben Hager and Jim and Vicki Craig were carried on through many generations of crossing seedlings that had much common ancestry. This was necessary because the project was to combine aphylla branching, bud count, and small size with the varied color patterns, modern form, and good substance of the tetraploid TBs and BBs. The genes for all of these things were present in the parental material, but bringing them all together in the right combination in a single plant is a multi-generation goal.
The development of the tall bearded pinks is another example of line breeding. Because the pink factor is recessive, simply crossing a pink with another iris with desirable branching, form, or vigor will not produce improved pinks in the first generation. Hall and Fay bred their pinks for many generations to win such improvements.
In iris breedling, "line breeding" is a rather lose term. Rarely do breeders carry on for three or more generations interbreeding the progeny of a single initial cross. Rather, other more or less distantly related irises are added to the mix periodically. This is done to make use of improved breeding material as it becomes available, or to remedy shortcomings that appear in the line as breeding progresses. There is an unfortunate tendency to use the term in an even looser sense, where all of a particular hybridizer's work is referred to as a "line", whether or not the seedlings are closely related.
To follow a program of line breeding, one obviously needs fertile seedlings. This is perhaps the most important rationale for working within the fertile families. If one crosses an arilbred with an SDB (the classic cross for arilbred medians), the seedlings will be sterile or nearly sterile, which closes off the potential for bringing out the best qualities of the two parents by line breeding. One must simply hope to be lucky with the initial cross.
Line breeding in the amphidiploid families has some special limitations that breeders should be aware of. Because amphidiploids always retain two chromosome sets from each of their two parental types, it is not possible to combine the parental genes in arbitrary combinations. For example, many generations of line breeding amphidiploid arilbreds have yet to produce an arilbred iris with three or four branches and a dozen buds, although such are certainly found among the TBs. Why? Because two of the genes governing branching and bud count will always be aril genes, inherited from the single-budded oncocyclus or the two-budded regelias. Certainly, the TB genes for improved branching and bud count can be brought to bear and arilbred branching improved by deligent selection, but the presence of the two aril genes will also bound what is possible in this area.
If line breeding is so effective at bringing out the full potential of the genes of the parental stock, why do anything else? There are two reasons, one positive and one negative. The positive reason is that no two irises (or small group of irises) capture all the interesting possibilities that breeding in the genus has to offer. However close to perfection one gets with line breeding, it will be perfection as defined and limited by the potential of the original parental material. Eventually, most of us will want to throw something new into the mix.
The negative reason is that line breeding tends to bring out recessive genes, and this is often detrimental. While we may delight in the expression of some recessives, such as the pink and plicata factors, most recessive alleles are "damaged" versions of the dominant forms, unable to perform their function in the biochemistry of the cell. As long as some of the dominant forms are present, the cells can continue to function more-or-less normally. But when too many of these nonfunctioning recessives are present, the health of the plant can be compromised. Line breeding thus tends to degrade the health of the plants (or animals) involved. Breeders must be on the lookout for this, and ruthlessly "cull" seedlings that show degradation, regardless of any appealing qualities they might have.
Backcrossing (crossing a seedling with one of its parents) is really a special case of line breeding, but it has a particular function that is worth mentioning. When one seeks to introduce just a single quality from an iris into a line, backcrossing can be helpful. Suppose I have a line of seedlings I am quite happy with, but would like to put a blue beard on them. I have a plant with blue beards available, but its other qualities are inferior to those of my seedling line. Line breeding by crossing siblings might get me to the desired result, but I would be wading through many generations of those inferior qualities from the blue-bearded parent. Instead, I can make the cross and look for seedlings that show the blue beard (I might have to go to the F2 by sib crossing to find them). Now the blue-bearded seedlings can be backcrossed to my superior seedling, repeatedly if needed, each time making sure I use seedlings that show the blue beard. In this way, I can obtain a plant most of whose genes come from the superior seedling, but with the blue beard added.
In iris breeding, backcrossing is seldom used in its pure form as described above. More likely, the seedlings showing the blue beards would be crossed to a different variety or seedling from the line, rather than the precise one that was used initially as the parent. This is because later generations in the line are likely to have improved versions of the original parent. Still, the basic approach is the same--bringing one or two particular characters into a breeding line by injecting them once and then crossing repeatedly back to the main line.
Outcrossing is the practice of crossing seedlings from line breeding to unrelated (or distantly related) irises. This can be done to bring in particular desired traits (in which case it may be followed up with backcrossing), or to launch an entirely new line. Outcrossing can reinvigorate a line that has been weakened by too much line breeding. "Hybrid vigor" results when two dissimilar types are crossed, because they are unlikely to carry the same recessive genes, so the resulting seedling have functional dominant forms of all their genes.
Outcrossing is important to maintain the diversity of the gene pool. Outcrossing opens up new possibilities through the combinarion of different genes that may not be apparent from the appearance of the two parental types. It is outcrossing that lays the groundwork for line breeding, mixing the raw material whose potential will appear in advanced generations of breeding.
If the parents are very distantly related, from different species or groups of species, the cross is referred to as a wide cross. Wide crosses have great potential, because they combine genes that have not been combined before. They can also be quite difficult, because the more distant two parents are, the less likely they are to cross successfully and produce viable seedlings. This is where an understanding of the fertile familes is especially valuable. Some wide crosses have the potential to create or augment fertile families that can be line bred; many others do not. These latter type of wide crosses can be made for curiosity's sake, but will be dead ends as far as a breeding program is concerned. Some breeders, excited by a new species imported from some exotic locale, cross it indiscriminately with anything available in their garden. Although there is nothing wrong with such exploratory pollen daubing, it is also a good idea to have, in advance, an understanding of which crosses are likely to yield fertile offspring, so that these can be given priority and the number of false starts reduced.
In the larger scheme of things, this is why tetraploidy is so important for hybridizing; it is not that the plants are huskier or the flowers are larger (sometimes they are, but often they are not). It is only at the tetraploid level that truly wide crosses (arils and bearded, TB and I. pumila, etc.) can be expected to lead to fertile families that can be line bred. Wide crosses between diploids are mostly dead ends, or at least require many generations of frustrated attempts at breeding before a lucky breaktrhough is obtained.
Unless one uses species exclusively, the work of other hybridizers will come into the breeding program. There is nothing wrong with this, of course; that is how the established families have been built up, through many generations of both irises and hybridizers! I do, however, give some thought to whether I am building on other hybridizers' work, or simply duplicating it. It is tempting, when another hybridizer shares your breeding interests, to stock up on that breeder's irises and use them to start your own program. The problem with this is that they will already have made similar or identical crosses, and may be even two, three, or more generations further on in the work. Certainly, each seedling is different and you may grow one that offers something new. But for the most part, this seems to me to be duplication of effort.
There are some exceptions, of course. If a hybridizer is deceased or no longer breeding on a significant scale, it is a service to the iris world to carry on their work. And sometimes, duplication is unavoidable if the original hybridizer's irises are no longer in cultivation.
Duplication of work can sometimes be inadvertant. Perhaps you have two irises from different hybridizers that you choose to cross, unaware that they come from the same breeding lines a generation or two back. Again, this may be quite fine, you may get something new that neither of the two other hybridizers has produced. On balance, though, I prefer to combine very different lines and species when planning my own crosses. It seems this is where the greatest potential lies to make a contribution and pass on something different to the next generation of breeders.
November 2010
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Unless otherwise noted, all text and illustrations copyright Tom Waters and all photographs copyright Tom or Karen Waters. Please do not reproduce without permission.