Elective Expression and Variegation

Guo, et al. (2006)
Heterosis, or hybrid vigor, has been widely exploited in plant breeding for many decades, but the molecular mechanisms underlying the phenomenon remain unknown. In this study, we applied genome-wide transcript profiling to gain a global picture of the ways in which a large proportion of genes are expressed in the immature ear tissues of a series of 16 maize hybrids that vary in their degree of heterosis. Key observations include: (1) the proportion of allelic additively expressed genes is positively associated with hybrid yield and heterosis; (2) the proportion of genes that exhibit a bias towards the expression level of the paternal parent is negatively correlated with hybrid yield and heterosis; and (3) there is no correlation between the over- or under-expression of specific genes in maize hybrids with either yield or heterosis. The relationship of the expression patterns with hybrid performance is substantiated by analysis of a genetically improved modern hybrid (Pioneer hybrid 3394) versus a less improved older hybrid (Pioneer hybrid 3306) grown at different levels of plant density stress. The proportion of allelic additively expressed genes is positively associated with the modern high yielding hybrid, heterosis and high yielding environments, whereas the converse is true for the paternally biased gene expression. The dynamic changes of gene expression in hybrids responding to genotype and environment may result from differential regulation of the two parental alleles. Our findings suggest that differential allele regulation may play an important role in hybrid yield or heterosis, and provide a new insight to the molecular understanding of the underlying mechanisms of heterosis.

Tower: Environmental influence on dominance (1910)
With this cross of an L. signaticollis ♀ X ♂ L. diversa, the determination of dominance and the ensuing type of behavior is clearly a function of the conditions incident upon the combining germ plasms. In the repetition of Exp. No. H 409/411, the experiment was varied so that in some cases it was the first laid eggs that gave the behavior of Exp. No. H 409, and in others, it was the last laid eggs, or those of the middle of the reproductive period—showing that the results are not "age results," nor due to segregations, nor orthogenesis giving one kind of germ at the start, another at the middle, and others at the close of the reproductive period.

E. B. Ford: Genetic Polymorphism (1965)
Kettlewell (1944) bred Panaxia dominula (Arctiidae) from typical parents found at Deal, Kent. He divided the brood into two batches, one was raised in normal conditions with the usual varying temperatures. It produced 20 imagines, all ordinary dominula like the parents. The second batch was reared at a constant temperature of 70 +/- 5 deg. F. It gave rise to 23 adults, comprising 6 normal dominula, 5 homozygotes [bimacula] representing an extreme variety in which nearly all the white spotting on the fore-wings was obliterated while there was an excess of black markings on the hind pair, and 12 intermediate heterozygotes [medionigra]. The latter were the most variable group and were nearer the mutant than the normal homozygotes in appearance. That is to say, the effect of a gene to which the species seems buffered in natural conditions had been uncovered when the larvae were reared at a constant temperature; an environment which, of course, the moth never experiences.

Davis: Malnutrition as a cause of irregularities in segregation (1921)
The experimental cultures (from seed developed under condition of malnutrition) showed uniformly a smaller percentage of brevistylis segregates, and there were such extreme ratios as 1:17, 1:10.5, 1:7.4, 1:5.4, etc., when the ratios in the normal cultures were close to 1:3. These lower ratios of brevistylis in the experimental cultures following malnutrition of seed were consistently correlated with lower percentages of germination (one exception), and with the presence in the experimental cultures of large numbers of abortive seedlings, called "stumpy" because a root failed to develop from the tip of the hypocotyl. This is the evidence for the writer's view that malnutrition of developing seeds in the F1 generation of this material lowers the ratio of brevistylis segregates in the F2 by increasing the mortality of brevistylis zygotes or embryos.

Wright: Dwarf Prairie Rose (1937)
The foliage of my thirty-nine hybrids [Hansa x Rosa suffulta] formed a most interesting study. Variation was all the way from Suffulta to Rugosa, and with some the size of the Rugosa leaf was combined with the type of Suffulta.

Budd & Hansen: Rugosa hybrids (1893)
R. Arkansana X General Jacqueminot.— Of thirty-seven flowers crossed thirteen plants were saved. All show much mildew except four. The four plants with perfect foliage follow the General Jacqueminot closely. All the plants with the mother type of leaf show the mildew. The largest plant of each type is eighteen inches in height.

Amateis (1961)
So far I have found that primary crosses in rhododendrons produce quite variable seedlings, breaking up in what would appear to be second generation segregation. Unlike the well-behaved pea and snapdragon of Mendel, they refuse to lend themselves to neat little mathematical ratios. On only one occasion have I seen consistent uniformity in a primary cross. This was a Leach cross, catawbiense album Glass x yakushimanum. It was a joy to behold. All so uniform in height that they looked sheared and each smothered with its merry pink buds and white blossoms.

Burbank: Hybrid of Tomato and Currant Tomato (1914)
The fertilization was effected without difficulty, and an abundant supply of seed was produced. The hybrids that grew in the next generation were many of them pretty clearly intermediate in form and appearance between the parents. But some of them were almost ludicrous in appearance. They took on twisted and contorted forms, and in particular their leaves were curled and twisted into fantastic shapes.
    As to fruit, some of the plants produced long clusters with tomatoes much larger than cherries; others furnished small fruit like that of one of the parents. And in some cases a plant that had retained the short stocky tree form of the common tomato bore clusters of small tomatoes in bunches similar to those of the other parent.
    The foliage varied astonishingly between the two types. In some there was an exact compromise that was very curious. The dark, blistered leaves of the ordinary tomato, combined with the long, slender leaves of the currant tomato, produced a most interesting effect. Other specimens showed every possible gradation between the parent forms.
    Here, then, was a case in which there was no conspicuous dominance of one parent or the other as regards any individual character that could be segregated and classified.
    Neither as to size and form of plant-stalk, nor as to leaf, nor as to the fruit itself, was there clear prepotency or dominance of one parent over the other.

Edwards's Botanical Register, 8 (1846)
When the bright yellow flower of the white turnip is crossed with the dull golden of the Swede, an intermediate colour is not obtained, but some of the mules (as to the colour of the flower) follow one parent, and some the others.

Noble: Clematis jackmanii alba (1888)
The flowers produced from the old wood during the months of May, June and July, are double or semi-double, solitary, and of a bluish French gray; while those produced from the young shoots, in August and September, are single and white, in pairs on a long raceme, showing as many as ten pairs and a terminal on a string.

Brown: Form and Structure of Certain Plant Hybrids (1913)
In the great majority of cases the cells of the hybrids studied were intermediate between the corresponding cells of the parents. This indicates that there must have been a complete blending of parental elements. But exceptional cases showed that a blending had not taken place, or if it had, it must have been followed by a segregation of characters. For instance, certain epidermal cells of the calyx of the Calceolaria hybrid [Calceolaria hybrida x C. integrifolia] formed glandular hairs, and other epidermal cells simple hairs, one parent having glandular hairs on its calyx and the other simple only.
     A hybrid between Wild Goose Plum and Troth's Early Peach showed that segregation had advanced a step farther; whole patches of epidermal cells of the twigs were exactly like the epidermal cells of either parent. Hildebrand (1889) reports an interesting case of segregation taking place within a single cell. In a hybrid between Oxalis latifolia and O. tetraphylla he found the characteristic hairs of the two parent species might both arise from a single epidermal cell of the hybrid.

Lysenko: "Maternal" Hybrids in Wheat (1954)
In 1939, the offspring of these first-generation plants, the "legitimate hybrids" (i.e., the awnless), as well as the offspring of four awned plants, were planted separately at Gorki Leninskiye, the Experiment Base of the Lenin Academy of Agricultural Sciences of the U.S.S.R. I will not deal here with the question of how the progeny of the different first-generation plants varied, because the analysis is not yet complete. Suffice it to say that in the second generation, the progeny of one of the four awned plants (i.e., those that had seemed to be of the purely maternal type) produced several plants with characters which in many ways resembled those of the paternal form Lutescens 062. Of the 180 plants in the progeny of this plant, 172 were found to be of winter habit resembling the maternal form, and 8 of the plants eared; 5 of these were awnless and 3 were awned.

Meehan (1900)
Fuchsia fulgens had been introduced a year or so before, and someone sent my father a few flowers. I was the "garden boy" under my father. Fond of garden experiments, I had decided to repeat Knight's work in hybridising garden Peas, when the large amount of pollen on the Fuchsia flowers interested me. It was applied to the stigma of Fuchsia longiflora. From the one berry that resulted several dozen plants were raised. The largest and best of these subsequently appeared in the trade through the agency of Youell & Co., of Yarmouth, as 'St. Clare.' An interesting lesson followed. From this one berry no two of the many seedlings were alike. Some nearly approached the female, some the male; none could fairly be said to be intermediate. It was evident that the action of the pollen had not alone to do with the variation. Some physiological force, to this day not understood by me, must have been co-ordinate in the production of these results.

On Imperfect Hybridity. Isaac Anderson-Henry, Esq. (1872)
Among the same batch of seedlings from which I obtained Veronica Andersonii,—V. salicifolia (syn. V. Lindleyana) + V. speciosa,—came one which, to all appearance, was a reproduction of the male parent pure and simple. And deeming it nothing else, I presented it to a friend, V. speciosa being then comparatively a new plant; and he, when he flowered it, came to tell me that it had come a very different thing in bloom to the true V. speciosa, having much longer flower-spikes and of a much lighter colour than those in that species, being of a light crimson instead of a dark purple, as in the V. speciosa.

Pearson & Lee: Intermittent Prepotency (1903)
While we suppose unit prepotency,—the tendency of one individual out of a pair to be prepotent,—to be chronic, there is another form of prepotency which we may describe as intermittent. One or other parent may at a particular mating, or may in certain individual offspring of one and the same mating, be prepotent. On another occasion, or in other offspring of one and the same mating, it may not be prepotent or even the other parent may be prepotent. Such prepotency might exhibit itself in "alternative" or "exclusive" inheritance, and is distinct from any unit prepotency or absolute or partial dominance. It does not depend on the possession by one mate of certain characters, but on the condition of the parents and other circumstances peculiar to a special mating.

Experiment Station Record, 30: 144 (1914)
On some hybrids of Vitis vinifera and V. berlandieri, GARD (IV. Conf. Internat. Génétique Paris, Compt. Rend, et Raps., 1911, pp. 395, 396).
In studying a number of hybrid forms of V. berlandieri X V. vinifera raised from seed of V. berlandieri it was observed with regard to the stem that the hairy character of the maternal parent and also the glabrous character of most varieties of V. vinifera occurred among the hybrids) together with a large number of intermediate forms. Transverse sections of the stem show that the structure is sometimes intermediate between the two parents and sometimes nearer that of V. vinifera. Most generally certain characters of the liber and of the secondary wood, and especially those of the primary wood, are nearer V. vinifera. In the roots, on the other hand, these characters are nearer the other parent and are in accordance with the power of resistance to phylloxera and the excellent qualities as stocks possessed by these hybrids.

Melons (Sageret, 1826)
     "The assumed product of the crosses made ought to have been intermediate: (1) Flesh very pale yellow, (2) seeds very pale yellow, (3) netting light, (4) sides slightly marked, (5) flavor at once sweet and sprightly, but the contrary was the case.
     As a matter of fact, in the two hybrid fruits reported upon, the characters were not blended or intermediate at all, but were clearly and distinctly those of one or the other parent."

Datura (Naudin, 1865)
     “He has confirmed that which Sageret already knew, that in a hybrid the characters of the two parents are often shown, not blended but approximated, in such fashion that the fruit of a Datura hybrid, born of two species, the one with a smooth, the other with a spiny capsule, presents smooth places in the midst of a surface generally spiny. This disjunction, as it is called, is explained according to him by the presence in the hybrid of two specific essences, which tend to be separated more or less rapidly the one from the other.”
     It is of further of great interest to note that the seeds gathered from the smooth side of the capsule reproduced only the smooth-capsule form, Datura laevis, while those taken from the spiny side gave rise only to the spiny form, Datura stramonium.

Dogs, pigs, stocks (Darwin, 1865)
     According to Rengger, the hairless condition of the Paraguay dog is either perfectly or not at all transmitted to its mongrel offspring; but I have seen one partial exception in a dog of this parentage which had part of its skin hairy, and part naked; the parts being distinctly separated as in a piebald animal. When Dorking fowls with five toes are crossed with other breeds, the chickens often have five toes on one foot and four on the other. Some crossed pigs raised by Sir R. Heron between the solid-hoofed and common pig had not all four feet in an intermediate condition, but two feet were furnished with properly divided, and two with united hoofs.
     Analogous facts have been observed with plants: Major Trevor Clarke crossed the little glabrous-leaved, annual stock (Matthiola), with pollen of a large, red-flowered, rough-leaved, biennial stock, called cocardeau by the French, and the result was that half the seedlings had glabrous and the other half rough leaves, but none had leaves in an intermediate state. That the glabrous seedlings were the product of the rough-leaved variety, and not accidentally of the mother-plant's own pollen, was shown by their tall and strong habit of growth. In the succeeding generations raised from the rough-leaved crossed seedlings, some glabrous plants appeared, showing that the glabrous character, though incapable of blending with and modifying the rough leaves, was all the time latent in this family of plants.

Darwin: The Variation of Animals and Plants Under Domestication
Hybrids were raised by Gärtner between Tropaeolum minus and majus116 which at first produced flowers intermediate in size, colour, and structure between their two parents; but later in the season some of these plants produced flowers in all respects like those of the mother-form, mingled with flowers still retaining the usual intermediate condition. A hybrid Cereus between C. speciosissimus and phyllanthus,117 plants which are widely different in appearance, produced for the first three years angular, five-sided stems, and then some flat stems like those of C. phyllanthus. Kolreuter also gives cases of hybrid Lobelias and Verbascums, which at first produced flowers of one colour, and later in the season, flowers of a different colour.118 Naudin119 raised forty hybrids from Datura laevis fertilised by D. stramonium; and three of these hybrids produced many capsules, of which a half, or quarter, or lesser segment was smooth and of small size, like the capsule of the pure D. laevis, the remaining part being spinose and of larger size, like the capsule of the pure D. stramonium: from one of these composite capsules, plants perfectly resembling both parent-forms were raised.

  1. 'Bastarderzeugung,' s. 549. It is, however, doubtful whether these plants should be ranked as species or varieties.
  2. Gärtner, ibid., s. 550.
  3. 'Journal de Physique,' tom. xxiii., 1873, p. 100. 'Act. Acad. St. Petersburgh,' 1781, part i. p. 249.
  4. 'Nouvelles Archives du Museum,' tom. i. p. 49.

Orange x Pomelo (Swingle & Weber, 1897)
FIG. 10—Leaves of orange and pomelo and of their hybrids of the first generation, showing close resemblance to one or the other parent: f, St. Michael Blood (Citrus aurantium), female parent m, Bowen pomelo (C. decumana), male parent; hf, hybrid, resembling female parent hm, hybrid, resembling male parent.

Citrus x Poncirus (Swingle, 1911)
The citremons, or crosses between the lemon and Citrus trifoliata, on the contrary, show nearly 20 per cent of seedlings with an exaggerated development of hypophylls, and in the majority of cases never produce any normal foliage leaves at all, dying from starvation shortly after the reserve food material of the seed is used up. One parent, Citrus trifoliata, has a few hypophylls along the base of the stem of the young seedling, while the lemon, like the orange and all other common citrous fruits, shows a pair of rather large, rounded, sessile, opposite leaves. Some show five leaflets (this occasionally occurs in citranges also), others show unifoliate leaves or leaves with very much reduced side leaflets (which is rarely or never seen in citranges). Most of the citremons have trifoliate leaves with large lateral leaflets, which is the type characteristic of citranges.

Paphiopedilum (Hurst, 1900)
When several hybrids from the same pair of species are compared together, this variation of the parts, of “Partial Prepotency,” as I propose to call it, becomes even more apparent and more diverse. For example, in three hybrids raised from the same parents, in the first, the pollen-parent may predominate in form in a certain part; in the second, the seed-parent may prevail in that part; while in the third, that part may be fairly intermediate between both parents; while in regard to colour, these conditions may be exactly reversed. But this only includes one part of the hybrid, and the same law applies equally to every one of the parts so that when the changes are rung on twenty or more different parts by the two parents in both form and colour, we can well understand the many possibilities of variation in hybrids of the same parentage; and I venture to suggest that this law of Partial Prepotency, founded on actual facts observed in hybrids of Paphiopedilum, may perhaps throw some light on the question of variation in offspring of the same parents. Yet, notwithstanding this variation in the parts, it is a remarkable fact that in primary hybrids the whole plant taken together is fairly intermediate between the two parents, the balance of power being well maintained in the whole.

Peas, Datura (Sutton, 1903)
If each cell contains maternal and paternal potentialities in regard to each character, and if dominance is not a common function of one of these, there is nothing to show why as a result of some disturbing factor one body of chromatin may not be called into activity in one group of cells and its homologue in another. This would produce just the sort of a mosaic which Bateson and Saunders found in Datura or as Tchermak's pied yellow and green peas obtained by crossing the Telephone pea with yellow varieties. Correns describes the condition as poecilodynamous and his conception of the causes of the phenomenon as I understand it is parallel with that which I have outlined above. The logical possibility suggested by Bateson that the recessive islands in such cases as the mosaic pea may be due to recessive allelomorphs in the paired state does not accord with the theory of a chromosomic basis for those allelomorphs, since the chromosome groups, both of cells showing the recessive character and of neighboring cells showing the dominant one, are derived, so far as we know, by longitudinal or equation division from the chromosomes of the same original cleavage nucleus and hence must be alike.

Veronica (De Vries, 1904)
     In my garden, I cultivated, for many years, a Veronica longifolia which was a hybrid from the blue species and the white variety, and correspondingly had blue flowers. But from time to time splittings occurred; either one single spike bloomed white, or a few isolated white flowers appeared on an otherwise blue spike. During the entire life, up to the time of the formation of the reproductive cells this internal dualism manifests itself in this way. Sometimes proofs of it are even found in the anatomical structure of the tissues, and of the individual cells, where the parental characters are set free and a mosaic-like structure results. MacFarlane, who has made the most thorough study of the anatomical structure of hybrids, recognizes everywhere the principle of duality, and goes so far as to regard every individual vegetative cell of a hybrid as a hermaphrodite formation. And the renowned French investigator of hybrids, Naudin, also expressed himself about forty years ago in a similar manner. "L'hyhride est une mosaique vivante," said he; we do not recognize the individual parts as long as they remain intimately blended, but occasionally they separate and then we are able to distinguish them.
     We therefore regard it as established that, in the children, the inheritances from the fathers and mothers are indeed combined, but not fused into a new entity. Acting always conjointly under ordinary circumstances, they yet do not lose the power of separating occasionally.

Cistus (Gard, 1911)
     Cistus ladaniferus x C. monspeliensis.—15 hybrids were obtained, and one plant resembling the seed parent. The former consisted of two types, one of which nearly resembled C. ladaniferus and the other C. monspeliensis.
    C. albidus x C. polymorphus subsp. villosus.—170 hybrids were obtained. Some of these possessed the leaf characters of the seed parent, others those of the male parent, while a third category consisted of intermediate forms.

Sea Urchins, etc. (Loeb, 1912)
It is well known that Herbst and Tennent have made experiments in which the paternal influence in the hybrid embryo was diminished. Tennent states that in the cross between Hipponoe and Toxopneustes, Hipponoe characters become dominant in sea water of a high OH concentration and Toxopneustes characters in sea water of a low OH concentration. The amount of acid or alkali Tennent needed to accomplish his result was very small; namely about 2 cc. N/10 acetic or hydrochloric acid to 500 cc. of sea water.

Burbank: Variegated expression of purple and green in plum leaves (1914)
This is a crossbred seedling, descended from purple-leaved and green-leaved ancestors. The segregation of colors in the second generation has been referred to, and is illustrated here in a very interesting way. Most specimens have leaves that are all green or all purple; but here and there one mixes the colors, in a way that has peculiar interest for the student of heredity.

Radish x Cabbage (Karpechenko, 1924)
"The radish-cabbage hybrids towards autumn appeared to be strictly polymorphic in their habit of growth and, irrespective of difference in the variety of cabbages participating in the crossing, were generally of three types. The 1st type was a small plant with a very short stem about 30 cm. in length, densely covered, especially at the base, with small leaves; the stem was often branched and bore short flowering shoots 15-17 cm. in length with clustered flowers. The 2nd type of plant consisted of big leafy rosettes of 1 1/2 metres in diameter, or bushes with short thick branches and big leaves. They flowered sometimes, developing small clusters of flowers. The 3rd type was represented by plants of a vigorous growth with a strong stem with many branches of 2-2 1/2 metres in height. They flowered abundantly and their flowering shoots reached a length of 1 1/2 metres and more."

Drosophila (Serra, 1949)
     These constitute one of the most interesting types of position effects. In what concerns the phenotype they provoke are of the mimic class, but in many cases they show an additional characteristic, mottling or variegation. After the first case of variegation was explained as an effect of the "wild" allele being relocated in the chromocenter in the salivaries (Schultz 1936) many other cases of mottling have been demonstrated to have this causality. The variegation appears in heterozygotes, where the mutant loci corresponding to genes contained in the segments inserted near the heterochromatin may manifest in some patches of tissue, while adjacent ones remain of the wild type or of an intermediary phenotype between the + allele and the extreme recessive mutant. However, not always does the neighbourhood of heterochromatin near + alleles causes variegation: sometimes there is a uniform phenotype like the recessive.

Japanese-Siberian hybrid Iris (Hensler, 1992)
     The first crosses were made in 1992 using 2 JI pod parents (2n=24) and 2 Siberian pollen parents (2n=28). Seeds from 3 pods gave me 2 distinctly different types of fertile hybrids.
     Chromosomes were counted on 4 of the F1s (2 of each seedling type). Three of these showed 2n=26. One showed 2n=24.
     The F1 "A" types appeared to be pure species Ensata except for behavior. These plants prefer the growing conditions of Siberians and are at their best when given good soil and water with good drainage. Most of the F1 "B" types appeared to be pure Siberian. Two individuals however, showed non-typical ridges in their leaves once established and preferred the growing conditions of the JIs.

Also, Epigenetics, Gene Silencing, RNAi