Journal of Heredity 7: 3-8 (Jan 1916)
SOMATIC SEGREGATION
Variations in Plants May Be Divided in Two Classes, One of Which Breeds True While the Other Does Not-Modern Work Shows
Importance of Former Class in Practical Breeding-Further Study May Aid in Understanding Causes of Variation in General.1
E. J. KRAUS
State Agricultural Experiment Station, Corvallis, Oregon

1Read before the twelfth annual meeting of the American
Genetic Association, at Berkeley, Cal., August 5, 1915.

HORTICULTURAL literature generally, and particularly that which deals with ornamentals, abounds in references to bud variations, bud sports or node sports according to the notion of any particular writer. Anyone dealing with plants in quantity is impressed by the differences exhibited by individuals, differences which often it would be desirable to perpetuate, if possible. It has been argued that, if vegetative propagation means simply the dividing up of an individual into many independent parts, all of which are still the same individual, then the extent to which a plant which possessed particular merit could be multiplied and disseminated, would be limited only by the relative ease of propagation. As a result, considerable work has been attempted along the lines of selection of better strains of many kinds and varieties of plants. Practically the whole idea of pedigreed nursery stock, the selection of runners in strawberries, and the reworking to another strain of unprofitable trees or orchards for greater yields or better fruit, is based on the question of bud variation and bud selection. While the evidence from most of the experiments so far conducted along these latter lines is negative, it is by no means safe to conclude that there is nothing in the idea of improvement by such methods. The greatest difficulty which has arisen and led to confusion has been the failure to recognize the difference between those unstable variations due to purely local conditions and those which, while they may be due to local conditions to some extent at least, are a real part of the plant organization and persist even under widely changed environment.

Bud or somatic variations are subject to a broad classification, much as are seminal or seed variations. Three classes are recognizable: modifications (fluctuations), segregations, and mutations. The first class is observed by far the most commonly, and has furnished the material for the larger part of the experiments which have been conducted with an attempt to determine whether superior strains could be built up and maintained entirely by the selection of vegetative parts. The second class occurs in individuals of hybrid, or supposed hybrid origin (the term being used in its broad sense), or in those individuals in which some apparently new character has arisen, but remains associated with the original form and at times exhibits itself wholly independently to a greater or less extent. To illustrate: an example of this latter sort is the manifestation of pure white in the green and white variegation of many plants, or the production of self-colored flowers by certain individuals which normally hear striped flowers. To the third class belong sudden wide changes or departures from the normal character of a plant, such as double flowers, purple foliage, certain instances of the white areas in foliage, fastigiate forms and the like, which apparently are new to that individual. It is difficult, clearly to differentiate the second and third classes, and if one believes that mutations are merely a result of hybrid ancestry, class three should not be recognized. At times, even with breeding records, it would be difficult to determine whether a variation should be classed as a segregation or a true mutation; whether, in other words, the new or apparently new character became expressed through the removal of influences which suppressed or masked it, or actually arose anew.

INFLUENCE OF LOCAL CONDITIONS

As stated previously, fluctuating variation is by far the most common of all forms of variability. Such variation is greatly influenced by, and largely may be due to, environment in its broader sense. The production of inferior or superior fruit, whether it be size, color, or even quantity, may be the result of purely local conditions. A few instances of this type of variation and experiences with it are of interest.

Hedrick, in his discussion of pedigreed nursery stock, has pointed out the fact that fluctuations are not stable and, when environmental conditions are changed, characteristics change with them; and, in citing an example, states that "sixty Rome trees all propagated from buds from one branch show quite as much variation as could be found in an orchard of Romes propagated indiscriminately and growing under similar conditions" With nothing more than change of the name of the plant concerned, his statement fully summarizes a number of similar experiments.

An interesting example of range of fluctuation in size of fruit is furnished by a well-regulated Spitzenberg orchard which came under my observation. Generally the trees throughout the orchard produced average-sized fruits. Certain trees, however, consistently produced small, ill-shaped, inferior apples. Various treatments were recommended and tried but to no avail. Finally it was concluded that the trees were of an inferior strain and regrafting to a superior sort was recommended as the only remedy. Accordingly certain of the small fruited trees were severely cut back in the spring and top-worked, slightly less than one-half the number of branches being left so as not to destroy completely the balance of the tree. In the fall these remaining branches matured the finest fruits in the entire orchard; the companion trees which had not been so severely handled produced small fruit as usual. Apparently there was nothing in the idea of an inferior strain so far as these trees were concerned.

Selection for improvement in apples has been made on the basis of the color of the fruits—one of the most elusive of characters. In this regard, the following statement, which I made some time ago, since has been abundantly confirmed:

The red in Shiawassee, Mcintosh and Jonathan is composed of at least two reds, the one light and carrying with it the factor for striping, the other dark and associated with the factor for solid color. Now if these apples are grown under poor light conditions, they are almost without exception light red and striped, while in full sunlight the deep red factor further manifests itself and the fruit becomes self-colored dark red, though on close inspection the stripes are evident beneath the solid color. This is not a case of segregation; both characters are present, the one being simply overlaid by the other.

If then cions are taken from branches bearing superiorly colored fruit, due to purely local conditions, such as light, air, or moisture, the trees resulting from them when subjected to another environment might or might not prove superior, depending upon whether the new conditions were favorable to the development of the one set of colors or another. As will be brought out subsequently, however, certain color changes, the result of segregation are transmissible.

An experiment by Howard and Whitten, to determine whether the tendency toward greater yield is transmissible, is reported as follows:

Three crops of apples have thus far been harvested from trees, part of which were grown from cions selected from high-producing parents and part from trees of low-producing parents. There was practically as much variation in yield, size, and color of fruit between trees from the same parent as there was between trees of different parentage. Bud selection from high-producing and from low-producing strawberries carried on through twelve previous years showed absolutely no gain m productiveness by selecting runners from high-producing parents.


A CAUSE OF FREQUENT MISTAKES
Yellow Newtown apples produced by cross-pollinating with Roxbury Russet. The upper fruit has two distinct russet bands, whose appearance is merely a coincidence, and due to the segregation of color originally possessed, not to any direct influence of the Roxbury Russet pollen. Russet is a rather uncommon segregation character in the Yellow Newtown, and it would be easy to suppose that this is a case of immediate influence of pollen were the true explanation not known. Many such erroneous conclusions have been drawn in similar cases. (Fig. 1.)

More evidence and experiments of the same general nature might be adduced. The one main conclusion to be drawn from them is, that when an attempt is made to propagate a modification it is not transmitted as such alone, but its offspring are capable of developing the entire group or range of variations of which it formed a part. Evidence and examples of segregations are abundant, and many are being recorded. From this class of variations real advance or regression may be expected. Bateson calls attention to this form of variation and offers, quite correctly, an explanation on the basis of a somatic segregation of parental characters. He cites, as one example, two Sweet-Pea vines, each of which, normally having borne heterozygous purple flowers, produced a lateral shoot which bore red flowers, showing that 'the factor B has been omitted in one of the cell divisions by which they were produced." An interesting similar instance of my own observation is worth recounting. A friend who is interested in Sweet-Pea culture crossed two varieties, a pink and a white. From the second generation he secured one form which he has termed an impure dominant. It is white with pink edging, and has given rise through seed to many forms, light pinks, dark pinks, whites, a glowing salmon and a soft salmon rose, this latter being the only form which has not proven variable on further testing. Three years ago this same impure dominant produced a node-sport, a beautiful glowing rose. Seeds from this sport produced the same rose-colored form, which has remained absolutely fixed. It is of interest to add that through seeds the impure dominant has produced similar pink forms which have not bred true.

Some three years ago Groth called attention to a seeming occurrence of 'xenia' in the peach tomato. This variety normally bears fruits which have much the appearance of very red peaches. The plants in question bore an occasional smooth fruit. Through correspondence, I later learned that experiments demonstrated certain fruits were smooth even when enclosed and protected absolutely from pollination by a smooth soft. Here again, evidently, was a case of somatic segregation, worthy of further testing. A few seeds, said to be from a fruit showing an approach to smoothness, were kindly sent me. Seven plants have resulted from the sowing of a part of the packet; six of them have a few smooth fruits, many intermediate and a few which are rough. One plant has so far produced fifty-four fruits, all very rough and pubescent. It will be of interest to determine the behavior of seedlings from the several types of fruits.

Tufts has pointed out a somatic segregation of characters in the Le Conte pear and Transcendent Crab apple, a segregation which closely approaches a simple Mendelian ratio. He ascribes this segregation to a hybrid origin of the two varieties.

VARIATIONS IN BARTLETT PEAR

A color variation of the Bartlett pear was found several years ago. It occurred as a branch on a normal tree, and differed from the normal in that the bark was a golden yellow, striped green and brown, and the fruit had prominent yellow stripes from calyx to stem. Buds from this branch have reproduced trees of two entirely distinct types. The dark form is scarcely to be told from the ordinary Bartlett; the light form is a much less vigorous grower, has golden-yellow bark tinged with pink and grows very shrubby, producing large numbers of short laterals. Normal Bartletts of the same age as the variegated trees bloomed to a limited extent last year, and profusely this year. The light form produced but two fruit clusters on a total of seven trees; the dark form had no fruit buds at all. The light form so far has not produced bloom from axillary buds on one-year wood, though normal trees have done so in abundance. This case is mentioned since it may possibly represent a case of perpetuation of a change in form, productiveness and color.

The color of fruits of apple furnishes an interesting example of segregation. By comparing with breeding records the various colors expressed in the socalled banded fruits, it was concluded that in a number of varieties there were present several factors for color; that it is possible for any color to appear pure to any extent from a small segment to the entire fruit; and that if such segregation extends to a part which may be removed and propagated vegetatively, the segregation may be perpetuated. Among the varieties of apples so propagated are Red Gravenstein, which has arisen at least twice, Red Rome, which arose three times in one orchard within the last four years, and a blood-red Spitzenberg. Many other striped or banded forms among a wide range of fruits might be listed. Attention again is called to the fact, however, that among the apples of which a study was made, no colors truly new to the variety have appeared in the somatic segregations.

As stated in the beginning, if mutations are regarded as the mere result of hybrid ancestry, then any variation which might be classed as such could be considered only as a segregation. Still there are instances of variations which do seem to be unexplainable on the basis of segregation and which certainly are not mere fluctuations. They are readily perpetuated by vegetative propagation, are decidedly distinct from the form from which they sprang, and seem to possess characters new to the individual.

The case of the green and white variegation of foliage deserves particular consideration. It is of extremely common occurrence and generally may be observed by anyone who will spend a short time in search for it. On the campus at Corvallis there appeared on one of the several hundred bushes of Portugal laurel, cultivated as an ornamental, a green and white variegation which now represents fully one-third of the entire shrub. From the green and white shoots several which are entirely white have sprung, but, as yet, none which is entirely green. The same variegation in connection with the normal plant from which it sprang, has been observed in more than a score of widely scattered species; among them red clover, maple, dandelion (Taraxacum), carnation, radish, bean, ox-eye daisy, ash, holly and Trillium. Such green and white variegations are readily propagated vegetatively. Many ornamentals also come to mind and furnish readily accessible material for study. On almost any individual it is possible to find shoots which are entirely white and frequently those which are entirely green. This behavior is clearly a case of segregation. The green shoots are readily propagated and, if left to remain on the plant from which they sprang, often outstrip the remainder of it in growth. The white form is generally vegetatively weak and usually is incapable of independent existence.

But to account for the original appearance of the "white" character. Fortunately we are in a position to know that the white exists as a recessive in some plants, as shown by Emerson in his work on maize. Chapin found, in the case of variegated Amaranthus, that seed from green branches produced green seedlings while self-fertilized seed from white branches produced white seedlings, which facts led to the conclusion that the green and white variegated plants are the result of the crossing of green and white gametes. If, then, green and white gametes exist in the case of those variegated plants (hybrids) resulting from the cross between them, the appearance of the green and the white shoots is a case of segregation resulting from hybrid ancestry. On the other hand, where the green and white forms spring directly from a form which always has been green, so far as recorded, the appearance of the white character probably would be considered as a mutation. The question arises: Is there a difference between the green and white combination which arises as a mutation and that produced by crossing green and white gametes? Whether the variegated form arises in one way or the other they are apparently indistinguishable. In either case entirely green and entirely white shoots are frequently produced in the same manner and may be similarly propagated. It is in instances of this kind that the line of demarcation between segregations and mutations is drawn only with greatest uncertainty. A further study of them may aid us also in arriving at a more definite knowledge of the nature of mutations themselves.

Double flowers from single flowers, laciniated foliage, weeping and fastigiate forms, purple foliage, and other less common variations are considered as mutations. It seems that, in general, many wide changes in the color of the flower should be classed as a segregation rather than a mutation, especially so if breeding records show the same colors among the progeny, though, even under these circumstances, how to distinguish the two absolutely is a problem, since it may be argued that the appearance of new colors among the seminal offspring may likewise be due to mutation.

Like true segregates, mutants may be perpetuated vegetatively, and scores of horticultural varieties exist. The exceptions to this rule are the pure white forms which soon perish when severed from the parent plant and may be budded or grafted into other stocks only with greatest difficulty.

SUMMARY

To sum up, then, vegetative variations are first of all of two distinct sorts—modifications or fluctuations, which do not remain true when propagated and subjected to varying conditions, and segregations or mutations which may be propagated and expected to remain reasonably constant under a wide range of conditions In the past experiments have largely dealt with modifications, and the conclusion reached that little or no advance could be expected from the selection of vegetative variations. There is certainly good evidence accumulating, as the work of Shamel shows, that among citrus fruits, at least, distinct advance is being made in plant improvement through bud selection alone. This is no doubt a case in which true segregates or mutants have been utilized and, if so, they can be expected to remain constant under a wide range of conditions. There is a broad field for work on the genetics of bud-variations, and when their nature is better understood and the lines of discrimination more dearly drawn between the several classes, more rapid advance in their utilization may be expected. It seems likely that many of the laws pertaining to the perpetuation of fluctuations among seminally propagated pure lines will apply with equal force to vegetative fluctuations in a clonal variety. If so, a study of the one may well be taken into consideration in connection with a study of the other in any attempt to arrive at the fundamentals underlying either. And, as suggested previously, it seems entirely possible that a closer study of the cases of vegetative segregations and mutations, as they come to hand, will aid materially in the advancement of knowledge concerning the nature of variation in general and the discovery of the principles underlying it.


Alaskan Berry Hybrids

At the Sitka Experiment Station in Alaska a strain of hardy strawberries is in the making, the result of crosses between the native of the Alaskan coast region and cultivated varieties. Several thousand seedlings have been grown, all very vigorous and most of them productive and of high quality. The native variety of the interior of Alaska is now to be used in similar crosses.

The Cuthbert raspberry has been crossed with its relatives the native Salmonberry (Rubus spectabilis Pursh.) and the Thimbleberry (R. parviflorus Nutt.). The only interesting fact so far developed is that the hybrids of the two species first named are almost entirely sterile.

Somatic Segregation Biblio