Imprinting, Disruptive Selection, Antithetical Dominance

Heslop-Harrison: Gene expression and parental dominance in hybrid plants (1990)
Genomic imprinting, where the genes from one parent have different expression properties to those of the other parent, occurs in plants. It has potentially significant consequences because of the importance of hybrids in plant evolution and plant breeding, and provides a mechanism that can hide genetic variation for many generations. The study of nuclear organization shows that chromosome and genome position relates to imprinting in F1 hybrids, with peripheral genomes tending to be expressed preferentially. In some inbred, polyploid hybrids, such as Triticale (a wheat x rye hybrid), treatment with the demethylation agent azacytidine releases hidden variation, which was perhaps lost because of imprinting phenomena.

Gustafsson (1942, 1944)
Rosa canina
x R. rugosa. No apparent influence of pollen parent, but not entirely maternal. Missing one chromosome.

Nicolas (1937)
Sport of ‘Margaret McGredy’. "The foliage strongly resembled Rugosa but the plant characteristics also leaned toward R. cinnamomea." Both species were in the ancestry of ‘Margaret McGredy’. This sport then sported again to a HT with more intensely colored flowers.

Hurst (1925)
In the most complex case studied, in the octoploid species BBCCDDEE (R. acicularis Lindl.), the four double septets seem to work more or less in relays in different parts of the plant at different times and seasons, resulting in a periodic predominance of one septet over another in certain parts of the plant, the general result being more or less a mosaic of the four septets of characters arranged end to end or side by side.
    Naturally with four double septets working equally and independently in an octoploid species, only about one-fourth of the characters of each septet can be represented at one time. An analysis shows that in a plant of R. acicularis Lindl. carrying four years' growth of surculi, stems, branches and branchlets, about one-half of the characters of each of the four septets B, C, D and E were represented (fig. 174 e and f).
    An interesting case was observed the author's experiments in which several plants of the tetraploid species AACC (sub-sp. R. centifolia L.) grown in a greenhouse for a genetical experiment, developed temporarily in the second season the climbing character of the sub-tropical A septet, a feature usually absent when grown under natural conditions.

Clarke & Sheppard: Evolution of dominance, disruptive selection (1960)
IN a paper on the effects of disruptive selection, Mather (1955) pointed out that if there are two optimum values for a character and all others are less advantageous or disadvantageous there will be disruptive selection which can lead to the evolution of a polymorphism. Sheppard (1958) argued that where such selection is effective and the change from one optimum value to the other is switched by a single pair of allelomorphs there will be three genotypes but only two advantageous phenotypes. Consequently if dominance were absent initially it would be evolved as a result of the disruptive selection, the heterozygote and one of the homozygotes both coming to resemble one of the two optimum phenotypes (see Ford, 1955, on Triphaena comes). Thoday (1959) has shown by means of an artificial selection experiment that, even when a character is, at the beginning, controlled polygenically (sternopleural chaeta-number in Drosophila) and there is 50 per cent. gene exchange between the "high" and "low" selected sub-populations, a polymorphism can evolve.

[CybeRose note: Evolution of dioecy appears to be like disruptive selection. Antithetical Dominance is also similar, though operating on recurring species hybrids.]

Anderson: Antithetical Dominance (1941)
We have named this the hypothesis of antithetical dominance which can be stated in general terms as follows: In hybrids between extremely diverse parents, natural selection will tend to encourage those modifiers favoring one parental extreme or the other and suppressing intermediates.

Mangelsdorf: Hybridization in the Evolution of Maize (1952)
The plants in this field therefore furnished unmistakable evidence of hybridization, both present and during the recent past, between maize and teosinte. One plant out of every 167 plants in the field was a vigorous F1 hybrid shedding abundant pollen which became part of the general pollen mixture in the field. The F1 hybrids themselves, in spite of their vigor, have a low survival value. The Mexican farmer makes no distinction between teosinte and the F1 hybrids. Both are left standing in the field when the corn is harvested. The pure teosinte disperses its seeds which are enclosed in hard bony shells, and a new crop of teosinte plants appears the following spring. But the F1 hybrids have no effective means of seed dispersal, and their seeds, only partially covered, are quite vulnerable to the ravages of insects and rodents.

Yunis, Yasmineh: Heterochromatin, Satellite DNA, and Cell Function (1971)

Vig: Sequence of centromere separation (1983)

Allshire: RNAi, Heterochromatin (2002)


Disrupting Imprinting

Apparently prolonged inbreeding also may disrupt imprinting.

Proceedings of the National Academy of Sciences, 90: 287-291 (1993)
DNA Methylation, Vernalization, and the Initiation of Flowering
JE Burn, DJ Bagnall, JD Metzger, ES Dennis and WJ Peacock
Late-flowering ecotypes and mutants of Arabidopsis thaliana and the related crucifer Thlaspi arvense flower early after cold treatment (vernalization). Treatment with the DNA demethylating agent 5-azacytidine induced nonvernalized plants to flower significantly earlier than untreated controls. Cytidine at similar concentrations had no effect on time to flower. In contrast, late-flowering mutants that are insensitive to vernalization did not respond to 5-azacytidine treatment. Normal flowering time was reset in the progeny of plants induced to flower early with 5-azacytidine, paralleling the lack of inheritance of the vernalized condition. Arabidopsis plants, either cold-treated or 5-azacytidine-treated, had reduced levels of 5-methylcytosine in their DNA compared to nonvernalized plants. A Nicotiana plumbaginifolia cell line also showed a marked decrease in the level of 5-methylcytosine after treatment with either 5-azacytidine or low temperature.* We suggest that DNA methylation provides a developmental control preventing early flowering in Arabidopsis and Thlaspi ecotypes. Vernalization, through its general demethylating effect, releases the block to flowering initiation. We propose that demethylation of a gene critical for flowering permits its transcription. We further suggest, on the basis of Thlaspi data, that the control affects transcription of kaurenoic acid hydroxylase, a key enzyme in the gibberellic acid biosynthetic pathway.

*CybeRose note: Prokofyeva-Belovskaya (1947) reported, "The development of young sc8 larvae at 14 and 30° C. converts the heterochromatic state of section 1AB1-20ABC into the euchromatic one (see § B). The thymonucleic acid content of the chromonemata decreases, while their conjugational properties increase. These observations agree completely with Plough's (1917, 1921) and Mather's (1939) data on the effect of temperature on crossing-over. A decrease of temperature from 25 to 13°C. and an increase to 30° C. increases the percentage of crossing-over, which is maximal at 13 and 30° C."

FEBS Letters 353: 309-311 (1994)
Sequence-specific hypomethylation of the tobacco genome induced with dihydroxypropyladenine, ethionine and 5-azacytidine
A. Kovav̌ík, B. Koukalova, A. Holb, M. Bezdĕk
Higher plant DNA is methylated at CG and CNG targets. In this study we have investigated the tobacco methylation system in tissue culture using the methylation inhibitors 5-azacytidine (5-azaC), dihydroxypropyladenine (DHPA) and ethionine (Ethi), and methylation-sensitive restriction endonucleases HpaII, MspI, HhaI, EcoRII, ScrFI, and Fnu4HI. Surprisingly, CAG/CTG sequences, contrary to CG doublets and CCG/CGG triplets, appeared to be refractory to the inhibitory effect of 5-azaC. Thus 5-azaC cannot be considered a general inhibitor of DNA methylation in tobacco cells. On the other hand, DHPA, the inhibitor of S-adenosylhomocysteine (SAH) hydrolase, and Ethi caused hypomethylation of both CAG/CTG and CCG/CGG triplets but not of the CG doublets. The sensitivity of triplet-specific methylation to the inhibition of SAH hydrolase suggests the possibility that plant-specific DNA methylation at CNG targets might be modulated by alterations of the SAH/S-Adenosyhnethionine ratio in plant cells.

Capoa et al: 5-azaC demetalation of human heterochromatin (1996)

Can. J. Bot./Rev. Can. Bot. 77(11): 1617-1622 (1999)
Evaluating the potential of using 5-azacytidine as an epimutagen
M.A. Fieldes and L.M. Amyot
Abstract: A number of early flowering lines were induced when 5-azacytidine was applied to germinating flax (Linum usitatissimum L.) seed. The genetics of these lines indicate that the induced changes are epigenetic and probably result from demethylation of the genomic DNA at loci that affect flowering age. Although the growth and development of three stable early flowering lines are altered and the percentage of filled seed was reduced in all three lines compared with controls, measures of seed productivity demonstrated that harvest index was unaffected in two of the lines. In the third, harvest index was lower than normal and both seed set per capsule and seed mass per 100 seed were reduced. Furthermore, six generations after induction this line began to display relatively high levels of polyembryony. The late appearance of this twinning and other aspects related to working with lines induced by 5-azacytidine and using 5-azacytidine as an epimutagen are discussed.

Plant Physiology 127: 1418-1424 (2001)
Dioecious Plants. A Key to the Early Events of Sex Chromosome Evolution
Ioan Negrutiu, Boris Vyskot, Nicolae Barbacar, Sevdalin Georgiev, and Francoise Moneger
The situation in white campion and other dioecious species with established sex chromosome systems indicates that more than one locus is involved in sex determination, as shown by both crosses between dioecious plants and related monoecious or hermaphrodite species (reviewed by Westergaard, 1958), or by mutagenesis (Lardon et al., 1999 a, 1999b). In white campion, sex determination is controlled by at least three loci (Lardon et al., 1999b). The Y chromosome contains two of these loci: a female suppression function, negatively controlling cell proliferation during carpel initiation, and a male promoting function controlling the specification of male gametophytic cell fate. That these are independent pathways for male and female developmental arrest is reflected by the fact that changes in sex expression generate either hermaphrodite or asexual (neuter, both male and female sterile) mutants. The genetic analysis of gamma ray-induced mutants has enabled us to distinguish two loci with female suppression properties: a Y-linked locus (called GSF-Y) and an autosomal locus (called GSFA). In this context, GSF-A appeared as a potential enhancer of the GSF-Y locus. Phenocopies of such mutations were induced chemically when genetically male plants were treated with 5-azacytidine, a DNA demethylation agent (Fig. 3c), or trichostatin A, a potent inhibitor of histone deacetylases (Janousek et al., 1996; J. Hodurkova and B. Vyskot, unpublished data). We conclude that sex expression control in white campion can be added to the list of flower developmental processes that are regulated epigenetically (Finnegan et al., 2000; Jacobsen et al., 2000; Fig. 3).

Hetero-Fertilization