USDA Yearbook of Agriculture (1937)
Improvement of Flowers by Breeding
S. L. Emsweller, Principal Horticulturist,
Philip Brierly, Associate Pathologist,
D. V. Lumsden, Associate Physiologist,
F. L. Mulford, Associate Horticulturist,
Division of Fruit and Vegetable Crops and Disseases, Bureau of Plant Industry
One of the most popular flowers for both breeding and cytological research is the stock, Matthiola incana (L.) R. Br. This plant is unique in that it has two distinctive types, the single- and double-flowered forms, the latter containing no anthers or pistils and making no seed whatever. Reproduction, then, is from seed produced by the single flowered plants. The growers of stocks, both florists and amateurs, are interested almost exclusively in the double forms. For some time plant geneticists have known that there are three types of single plants. One when self-pollinated produces no doubles, the second produces about 25 percent of doubles, and the third from 54 to 56 percent. Obviously the third type is the one to use for seed. It is called eversporting because of the high percentage of doubles it produces.
The problem of explaining the peculiar behavior of the ever-sporting stock attracted many workers. One quite logical explanation assumes some condition that kills half the pollen grains and approximately 6 to 8 percent of the egg cells. This might be caused by what is called a lethal gene located in the same chromosome as the gene for singleness. The lethal gene would have to be recessive since if it was dominant all plants with this gene would die. Being recessive a plant can carry the gene and its normal allelomorph and be able to live. Let us use the symbol a for the lethal gene and the symbol A to represent the normal allelomorphic gene that is dominant to a; then all plants of the aa type would die, but Aa plants would live.
Now singleness in these stocks is dominant to doubleness. Single-flowered plants may be pure (homozygous) for singleness, or they may be hybrid (heterozygous) for this character. A pure (homozygous) single gives only singles when it is self-pollinated. A heterozygous plant gives 3 singles to 1 double. According to the above theory, the third type, the ever-sporting race, should be hybrid (heterozygous) for singleness and for the linked lethal, which would result in 54 to 56 percent of doubles instead of only 25 percent.
|6 This work has recently been questioned by Vestergaard and may need further investigation.
7 This actually happens occasionally, as discussed earlier, when a diploid sex cell is formed.
In 1931 Philp and Huskins (408) published a cytological study on ever-sporting stocks that apparently explained the situation.6 In order to understand their work it is necessary to recall that only one of each pair of chromosomes occurs in a sex cell. The pair of chromosomes of which one member carries the gene for doubleness and the other member the gene for singleness, therefore, never get into the same sex cell.7 Philp and Huskins are able to show that ever-sporting single plants had one pair of chromosomes that differed from one another in appearance, one lacking a small knob on its end. This is very significant when it is remembered that the two chromosomes of a pair normally have an identical shape. From their work, they stated that the pollen grains getting the chromosome lacking the knob did not function. In other words, the absence of the small knob acted as a lethal to the pollen grains in which it occurred.
If only the "double" pollen grains can function, while both "double" and "single" eggs are good, it is easy to see how such plants produce about half doubles and half singles. The fact that the ratio is not exactly 50-50 is explained by assuming that a few egg cells getting the chromosome lacking the knob do not function. This explanation was verified to some extent by a study of pollen germinations from pure and ever-sporting singles, which showed that germination of eversporting pollen was only about half that of pollen from pure singles. Evidently about half the pollen grains of an ever-sporting single were being killed in some manner.
The entire situation is much clearer if figure 15 is studied. At the top are shown the seven pairs of chromosomes of a single-flowered ever-sporting stock plant. The members of each chromosome pair look exactly alike with the exception of the first. One lacks the little knob on one end, which cytologists call a satellite. A large S is printed before the one that lacks the satellite. This is to represent the gene for singleness, which it carries. Its mate (homologue) has a small s to represent the gene for doubleness, which is recessive. The chromosomes of the eggs and pollen grains produced by this single-flowered plant are also shown. They are of two types, I and II, and half the eggs are of one and half of the other type. This is also true of the pollen grains. According to Philp and Huskins, however, only those pollen grains with the satellited chromosome function, and all the eggs live except about 6 to 8 percent of those lacking a satellite. Since only one type of pollen grain lives, there will result just two types of plants. They are shown at the lower part of the diagram. One will be single, since it receives the knobless chromosome with the single gene, and the other will be double, receiving two double-flowering (s) genes and two satellited chromosomes. Since there are not quite so many eggs carrying the large S for single flowers, the result is about 54 to 56 percent of doubles and 44 to 46 percent of singles. Phulp and Huskins report that all doubles examined by them had two satellites, while all singles had but one. This refers, of course, only to the ever-sporting races.
|Figure 15.—The chromosome situation in ever-sporting, double-flowering stocks.|
Another cytologist, Frost (135), working with stocks, has also demonstrated the value of cytology to flower breeding. He has shown that the proportion of double-flowered plants may be increased very materially. Among the various types of stocks he had in his experimental plot was an ever-sporting form known as Snowflake. In this variety he found a peculiar single-flowered plant that had very narrow slender leaves. When this plant was self-pollinated it gave about 47 percent of singles and 53 percent of doubles. About 37 in every 100 plants had the narrow slender leaves of the parent. Also, these plants were very weak in growth and had no decorative value. The remaining normal plants descended from this narrow-leaved parent were found to be about 90 percent of doubles. Since the slender leaves are conspicuous even in the seedling stage, the plants having them may be discarded at that stage, and among the remaining plants there will be only about 10 percent of singles. In order to continue this situation, however, it is necessary that seed be saved only from the slender-leaved types.
The explanation for this interesting condition was worked out by making a cytological study of the chromosomes of the slender plants. In addition to the 14 chromosomes expected, these plants showed a small chromosome piece, which cytologists commonly call a fragment. These plants, then, had 14 + 1 chromosomes. Evidently this fragment was the part that carried the gene for singleness or doubleness. Since only slender-type plants had this fragment, it was concluded that the slender leaves were also caused by its presence. Since a gene for singleness was also in the fragment, these two characters, singleness and slender leaf, behaved as linked genes do; that is, they tended to remain together from generation to generation. Thus if the slender plants are discarded in the seedling stage, it means the removal of most of the singles. This very unique scheme is not in practice as yet, probably because it is not understood by the average flower breeder.
STOCK, DOUBLE-FLOWERED (p 969-973)
|John Parkinson, 1629: This kinde of Stocke gilloflower differeth neyther in forme of leaves, stalkes, nor flowers from the former, but that it oftentimes groweth much larger and taller; so that onely it beareth flowers, eyther white, red or purple, wholly or entire, that is, of one colour, without mixture of other colour in them (for so much as ever I have observed, or could understand by others) which are eyther single, like unto the former, or very thicke and double, like unto the next that followeth; but larger, and growing with more store of flowers on the long stalke. But this you must understand withal, that those plants that beare double flowers, doe beare no seede at all, and is very seldome encreased by slipping or cutting, as the next kinde of double is: but the onely way to have double flowers any yeare, (for this kinde dyeth every winter, for the most part, after it hath borne flowers, ans seldome is preserved) is to save the seeds of those lants of this kinde that beare single flowers, for from that seede will rise, some that beare single, and some double flowers, which cannot bee distinguished one from another, I meane which will be single and which double, untill you see them in flower, or budde at the least. And this is the only way to preserve this kinde: but of the seed of the former kinde was never known any double flowers to arise, and therefore you must be carefull to marke this kinde from the former.|
The first mention of a double form was in 1568, when a Belgian botanist, Dodoens, described it in a paper dealing with sweet-smelling flowers suitable for chaplets or garlands; and in 1581 an actual illustration appeared. At this time the flower was described as being so double that it was completely sterile. It is not known just when the double form appeared, but it was probably a mutation from the single. From the scant records of the period it seems that the only method of reproducing it was by cuttings. It was not until 1629 that any statement appeared indicating that double-flowered plants could be obtained from seed of singles.
Unfortunately, one of the earliest descriptions of double-flowered stock stated that the doubling was the result of special treatment and frequent transplantation. This belief was held for a long time and many special practices based on superstition developed in the culture of stocks. One of the most interesting descriptions of such practices appeared in a book on gardening in 1675:
Single Flowers Doubled
Remove a plant of stock when it is a little woodded and not too greene, and water it presently; doe this three days after the full, and remove it twice more before the change. Doe this in barren ground, and likewise three days after the new full Moon remove againe, and then remove once more before the change. Then at the third full Moon, viz., eight days after, remove againe, and set it in very rich ground, and this will make it bring forth a double flower; but if your stocks once spindle, then you may not remove them. Also, you must shade your plant with boughs for three or foure dayes after the first removing; and so of Pinks, Roses, Daysies, Featherview, etc., that grow single with long standing. Make Tulipees double in this manner. Some think by cutting them at every full Moone before they beare to make them at length to beare double.
As mentioned earlier, the double-flowered stock plants were propagated by cuttings, but how they came from seed of the singles remained a mystery until it was cleared up by the genetic and cytological research of Saunders (463), Frost (135), and Philp and Huskins (408). For a long time it was generally believed that the double-flowers produced some pollen, which fertilized the singles and formed seed that produced doubles. Directions are still occasionally given for selection of seed from single-flowered plants surrounded by doubled-flowered. An examination of the double flowers, however, discloses no pollen whatever, and it seems certain that if any is ever produced, it is only on exceedingly rare plants.
The differences between a single and a double flower are very striking. The single has four petals, four stamens, and a pistil. When fertilized it produces a long, narrow, flattened fruit containing from 30 to 60 seeds. The double flower is composed entirely of petals, which vary from 40 to 70 per flower. There is no trace of stamens or pistil, and, of course, no seed is formed.
The double-flowered plants are desired by both florists and gardeners, and because of this there is active competition among growers of stock seed to produce high double-throwing strains. Accurate counts made by seedsmen have revealed many strains with 80-percent and a few with as high as 90-percent doubles. Usually, however, the proportion secured by florists and home growers is far less. The seedsmen themselves encounter sharp fluctuations; a strain producing as high as 80-percent doubles one year may drop to 50-percent or less the next. As a result of this apparent instability, seedsmen, florists, and gardeners have entertained a belief that doubleness must be controlled by some external environmental factor or factors.
|8 It has been shown that occasional pure single plants appear even in type 3. They, of course, bring down the percentage of doubles expected from this type and are an additional source of confusion.|
Modern genetic research has now found the fairly simple explanation of this situation. It also points the way to production of reasonably nonfluctuating, double-throwing strains that produce the maximum percentage of doubles. When a large number of single plants are self-pollinated and all seedlings of each one saved, it has been discovered that the singles are of three sorts. Type 1 produces only single-flowered plants, type 2 produces 3 single-flowered plants to 1 double-flowered, and type 3 produces about 54 percent of double-flowered plants and 46 percent of single-flowered. The single-flowered progeny of type-1 singles never produce any doubles in their selfed progenies; they are pure for singleness. The single-flowered progeny of type-2 singles are of two kinds, one-third being pure for singleness and two-thirds like type 2, that is, producing progenies with 3 singles to 1 double. Most of the single-flowered progeny of type-3 singles repeat the performance of their parents, each again producing about 54-percent doubles.8
It is now easy to understand how fluctuations in percentage of doubles may occur from generation to generation. Even though a seedsman practices careful plant selection and saves seed only from the high-double strains, he cannot predict with accuracy the ratio of doubles to singles from year to year. It seems highly probable that nearly all stock seed is a mixture of all three types of singles. The percentage of doubles that will develop in any strain, then, is influenced by the number of pure and heterozygous singles that were in the seed field. Since at present there is no certain method of distinguishing the three types of singles except by a progeny test it seems that with ordinary methods of seed production the number of doubles will continue to fluctuate from generation to generation.
The preceding explanation accounts for yearly changes in the proportion of doubles, but it does not explain the occurrence of strains with more than 54 to 57 percent. In fact, it sets such an amount as the maximum proportion that can be secured. How can the strains with over 80 percent of doubles be explained? Miss Saunders has given the explanation. About 20 years ago she noticed for several years the high percentage of doubles developing in a bed of stocks. It occurred to her that some sort of artificial selection could account for it. A few years later she planted 8 to 10 seeds in each of a large number of pots. When the seedlings were well established, those in each pot were numbered according to their size. When the plants finally bloomed, it was found that most of the large ones were doubles and the small ones singles. In 1923 White (546) at the Maryland Agricultural Experiment Station conducted a very similar experiment. He grew a large number of seedlings and then graded them into groups on the basis of size. When the plants bloomed, he too found that most of the large ones were doubles.
S. L. Emsweller has also investigated the problem in genetic studies with stocks. The seedlings were not graded by size, but as soon as the first true leaves were developed, about 150 plants from each of several varieties were transplanted into small pots. All seedlings of a progeny were saved. When the small plants were established, the height, spread, and stem diameter of each were measured each week until the plants began to bloom. They were then classified as doubles and singles, and the mean height, spread, and stem diameter for each group were computed for the weekly intervals. In all cases it was very clear that the double plants were more vigorous than the singles, even in the seedling stage. This does not mean that the smallest double plant was larger than the largest single; there were always a few plants of each type that overlapped. It was possible, however, by selecting only the very largest seedlings, to secure 85 to 90 percent of doubles (fig. 23).
Thus the occurrence of unusually high double strains is readily explained. If anyone, florist or gardener, has more seedlings of stocks than are needed, he will invariably discard the weak, small ones and save the largest. On the California flower-seed ranches, stock seed is sown in rows in the field. When the seedlings have become well established they are thinned by hand, and naturally the stronger plants are left. This readily explains the frequent occurrence of rows with 80 to 85 percent of doubles. Such rows, of course, came from parent plants that gave the highest possible percentage of doubles.
In the light of these facts, certain recommendations for growing seed of stocks can be made. In the absence of definite information on natural crossing in stocks, it is advisable to self-pollinate all plants selected. A small sample of seed from each selfed plant should be sown in a separate row. Random samples of these seedlings, the first 50 in each row, should be transplanted, the lots again being kept separate. These trial plantings will indicate the genetic type of the parent of each row. Seed from pure singles will give only single-flowered plants; that from simple hybrids, about 3 singles to 1 double; and that from the so-called ever-sporting type, slightly more than 50 percent of doubles. The seed of all pure singles and simple hybrids can then be discarded and a seed crop grown from plants that produced the maximum number of doubles. Such a procedure would require 2 to 3 years but would certainly give high-quality seed. In many sections of California it is possible to maintain such a planting for several years. Emsweller has done so and secured a greatly increased seed yield the second year. It is recognized, of course, that this method would involve extra expense, but it has been profitable with delphinium, hollyhock, and columbine, and it should be with stocks also. If seed of this type were generally available, and florists and gardeners rigidly discarded all weak seedlings, they should have little trouble in securing stocks running close to 90 percent double. This means that over twice the ordinary amount of seed should be planted, since slightly more than half the seedlings would be discarded in thinning out on the basis of size.
|Figure 23.—Stock seedlings selected for size after transplanting: A, Group of the extremely large plants, 90 percent of which were double-flowered; B, smallest plants, 18 percent of which were double-flowered.|
There are several types of stock plants (fig. 24) varying in habit of growth, earliness, and flower color. Unfortunately the importance of the problem of double flowers has retarded work on inheritance of these characters. Some data are available, however, on inheritance of tall versus dwarf plants and branching versus nonbranching. Tall is dominant to dwarf, and in the second hybrid generation there will be found three tall plants to one dwarf. The situation is not so clearcut for branching crossed with nonbranching. The first-generation hybrid is branched, and in the second generation there is a close approach to a ratio of 3 branching to 1 nonbranching. These nonbranching plants, however, have some tendency toward branching, which the original nonbranching parent plant did not have.
The future breeding work with stocks will probably be concerned with the inheritance of other important characters. There is also need for the discovery of some simple seedling characteristic to enable florists and gardeners to select with certainty double-flowered plants in the seedling stage.
|Figure 24.—Four types of double-flowering stock showing variation in growth habit. There are many varieties of each kind including a wide range of colors. (A) The tall ten weeks and (B) the dwarf ten weeks types are used mostly for bedding, are very early bloomers, and are easy to grow. (C) The imperial (branching) and (D) the column (nonbranching) types are grown mostly under glass by florists. The column sorts (D) are valued for their tall single spike and for adaptability for close planting, thus yielding more salable flower spikes per square foot of greenhouse bench.|
CybeRose note (5/13/05): This discussion appears to account for the facts mentioned, but neglects other facts discussed by De Vries (1904). He reported that experiments had shown that seeds in the upper 1/3rd of the capsule produced only 20-30% doubles, while the lower 2/3rds gave 67-70%. If the double-flowered strains were the same in their inheritance, segregation of knobbed and unknobbed chromosomes in the ova would not easily account for the preferential occurence of knobbed chromosomes in the lower 2/3rds of the capsule.
In other cases, such as the excess of females in Lychnis dioica (Melandrium), pollen tube growth factors are involved. That is, pollen tubes carrying the female factor grow faster and therefore fertilize more of the ova in the upper part of the ovary.
To the contrary, double vs. single flowers are presumably determined by factors segregating in the ovaries. The pollen grains are all of the same type, and should show no clear-cut difference in growth rate correlated with floral doubling.
An alternative interpretation is that "nutrition" is involved, as Devries suggested, and that a transposon is ultimately responsible both for the doubling of the flowers and for the abortion of pollen.
In the model discussed by Emsweller, et al., the ever-sporting Stock requires two "mutant genes". One mutant resides in the knobbed chromosome and produces double flowers when homozygous. A separate "mutant", carried by the unknobbed chromosome, is responsible for aborting all the pollen grains carrying it as well as some of the ova. How these two mutations came to exist within a single strain may be a matter of historical interest, but is not improbable in a plant cultivated for so long.
However, a transposon might be responsible for both phenomena. When active the transposon might over-stimulate an adjacent gene, producing one of the abnormalities. And when silenced, another gene (or multiple genes) might be silenced as well, thus accounting for the other anomaly. In fact, this very model has been identified in mice with yellow coat color. The active transposon over-stimulates the Agouti gene, giving a yellow coat color, diabetes, obesity and an increased tendency towards cancer. When the transposon is silenced, so is the Agouti gene.
It is not clear that ever-sporting Stocks carry such a transposon. It is just as possible that something else is involved, such as an in-line duplication that may or may not lead to heterochromatization of the region containing it. The change in heterochromatization might also account for the presence or absence of the chromosome constriction that produces the appearance of a knob or satellite.
Which state is correlated with the different expressions? The fact that a chromosome fragment in the Snowflake strain could be routinely associated with single flowers suggests that this is the more heterochromatic form of the chromosome. Fragments, which are typically heterochromatic, can increase the degree of heterochromatization in other chromosomes. In this case, apparently, the increased heterochromatization suppresses another gene (or genes) involved in the growth of leaves.
Single-flowered plants, in the model I am describing, would inherit a chromosome with a heterochromatic segment from the pollen parent, and a chromosome in which the segment is not heterochromatic from the seed parent. The state of the chromosome segments is, as in other examples that have been studied, established (more or less permanently) at the time of meiosis. However, in the next generation's meiosis the state of a chromosome may alter.
Example: Toss 100 coins onto a table, then separate all the Heads from the Tails (there should be roughly equal numbers of each). Put all the coins into a container, shake well, then pour them all onto the table. Again we see that there are roughly equal numbers of each type, but we know better than to suppose that segregation has occurred. The state (Heads or Tails) of each coin changes randomly, yet for the whole group we usually get out very nearly what we put in. Try it again with 50 dime Heads and 50 penny Tails. Not only do we still get very close to a 50/50 split, we also will observe "independent segregation" of Heads/Tails and Penny/Dime.
Something similar can happen with heterchromatic vs. non-heterochromatic segments. If only one of the chromosomes has the heterochromatic segment, about half of the gametes will carry a heterochromatic segment — though not necessarily the same one that was heterochromatic in the parent.
In a study of heterochomatization in fruit flies it was observed that the degree of heterochromatization was influenced by the state of the mother. If the mother was homozygous for the heterochromatic region, her heterozygous offspring showed increased size and number of white spots. Heterozygous offspring of heterozygous mothers showed fewer, smaller white spots.
This phenomenon cannot be tested in Stocks because neither type of homozygote is available for comparative testing. One might, however, attempt to culture pollen mother cells from the ever-sporting Stocks in hopes of securing a haploid carrying an unknobbed chromosome. Doubling the chromsomes should — in theory — give us a plant homozygous for the unknobbed chromosome. We may expect it to produce no viable pollen, but it could be pollinated by a another specimen. Its ova should be mostly fertile, except for the 4 to 8% that typically die. If a substantially larger proportion die we might interpret the results as an increase of heterochromatization — the parental effect.