Before considering specific problems, we must answer the questions, What are tetraploids, and why the interest in them? The term “ploidy” or “ploidy level” refers to the number of sets of chromosomes in every cell of an organism, be it plant or animal. Every plant cell has a specific number of sets, with two sets representing diploids, three sets triploid, four sets tetraploid, etc. In a specific plant each set contains exactly the same number and type of chromosomes; the number varies with different plants. The chromosomes contain the genetic material, or DNA, the chemical code which determines the identity of every living thing, plant or animal (including humans). The greater the number of chromosomes or sets of chromosomes, the greater the genetic content – and the greater the possible genetic expression upon reproduction.
We are definitely saying that four sets of chromosomes in a given plant, the tetraploid plant, is superior to two sets of chromosomes in the same plant. This was demonstrated conclusively by the transition of the bearded irises from diploid to tetraploid, approximately over the interval 1900-1940. All tall bearded iris introductions today are tetraploids. They show larger size, heavier flower and foliage substance, more intense colors, more variable color combinations and greatly enhanced breeding possibilities.
The same desirable features are quite evident in tetraploid Louisiana irises, although they have not been developed to nearly the same extent as the bearded irises; this remains for the future. But there are other undesirable features or problem areas which are also noted with the Louisiana tetraploids. These problems must be solved if the future potential is to be realized. There are four main problems: (1) Limited fertility, (2) limited genetic variability, (3) difficulties in opening, and (4) reduced cold-hardiness. These problems are considered a direct result of tetraploidy, since they are observed in diploids only infrequently.
Limited Fertility
Almost all tetraploid Louisianas are somewhat fertile but not nearly as fertile as the diploids. This is by far the most serious problem for hybridizers. Fertility problems have been in evidence since the very beginning of the work with colchicine. Tetraploids were not produced directly but by chimera X chimera hybridizing; chimeras represent partial conversion to the tetraploid state and are unstable (they tend to revert back to diploids). Almost all the early chimeras were sterile. Special hybridizing methods (pollen examination and segregation) and hundreds of crosses produced only four tetraploids over an eight-year period. Once stable tetraploids became available, it became easier to produce more by chimera X tetraploid hybridizing, but fertility remained the major problem.
Tetraploid X tetraploid hybridizing became a reality about 1975. This further enhanced fertility to a slight extent. Serious limitations remained in (2) setting seed and (2) far fewer seed per pod than diploids. During the 1975-1980 period a combination of these two items made it 12-15 times more difficult to produce tetraploid seed. Some improvement was realized with time and by 1990 the degree of difficulty had stabilized at 5-6 times that for diploids. These odds are simply not satisfactory for producing new tetraploids in quantities necessary for future exploitation.
Limited Genetic Variability
Four original tetraploids plus two or three additional chimeras produced the entire line of tetraploid Louisianas available today. This involved much in-breeding which limited genetic variability despite the fact that tetraploids are inherently more genetically variable than diploids. This resulted in certain undesirable features, the most obvious of which was color. Diploids cover the entire range of color from pure white to almost black. Most tetraploids are dark-colored, deep and medium purple, red-purple and red – beautiful colors, but mostly dark. The dark colors are genetically dominant because of the extensive inbreeding. We do have a few yellows, thanks mostly to the recessive yellow gene in WHEELHORSE which had a limited period of use as a chimera. There are no whites, pale pinks or other light colors. It is pretty obvious that the availability of new, light-colored tetraploids will greatly improve this. Otherwise it will be a long and arduous task to achieve the spectrum of colors from the current breeding stock.
Difficulties In Opening
Tetraploids generally have thicker and heavier floral segments than diploids. This translates into better overall substance in most cases. However, the thick, heavy segments often cause a problem in the opening of some flowers. Such flowers may open more slowly, and the unfurling of the segments may be less complete than with diploids. This has an adverse effect on flower form and renders the form less pleasing. With dark-colored flowers the problem may be aggravated, particularly in warm climates. This problem does not exist with all tetraploids and it is considered correctable by hybridizing and selection.
Cold Hardiness
There are indications that tetraploid Louisianas available today may be less hardy to severe cold than the diploids. There may be problems when temperatures fall below 10° F late in the growing season, particularly following a period of considerably warmer temperatures. This is based on very limited observations in Baton Rouge and on comments from growers in colder climates. Reasons fr this are not clear and it is not known how serious this problem may be. It is also unclear to what extent this may be correctable by hybridizing and selection.
The best approach, at least the first step in these matters, seems to be (1) to provide more varied breeding stock, particularly light-colored new tetraploids; and (2) developing interest among hybridizers to work with all available stock. The accent here is on new tetraploids, as unrelated as possible to the current line and resulting from conversion of diploids. White, pale yellow and pink new tetraploids probably offer the best chance in solving problems which are considered the most serious.
There are three basic routes to desired new tetraploids: (1) the conventional colchicine route; (2) the use of new, hopefully more effective, chemical agents for inducing polyploidy; and (3) interploidy hybridizing, or what is termed meiotic polyploidization.
Note from Samuel N. Norris: Joe Mertzweiller sent me a start of two interploidy plants, C-75-88E X PROFESSOR HEDLEY and I.brevicaulis (Rowan) X C-75-26A. The first one checked out as a diploid both from the stomata size and from the chromosome count, while the other checked out as a tetraploid. The converted plants are always suspect but there was no sign of diploid cells in the slides.
