Stress and Variation

Genetic Transilience

Cook: Aspects of Kinetic Evolution. Wash. Acad. of Sci. 8: 236-237. (1907)
Differences under New Conditions (Neotopism).—Variations induced by the transfer of organisms to new and unwonted conditions. Three stages of new place effects may be distinguished, (1) those in which there is merely a stimulation of growth, (2) those in which there is also a definite mutative change of the hereditary characteristics of the variety, (3) those in which the new conditions call forth a promiscuous mutative diversity.

Klebs: The Influence of Environment on the Forms of Plants (1909)
It is, however, a fact that if a plant is removed from natural conditions into cultivation, a well-marked variation occurs. The well-known plant-breeder L. de Vilmorin (L. de Vilmorin, "Notices sur l'amelioration des plantes", Paris, 1886, page 36.), speaking from his own experience, states that a plant is induced to "affoler," that is to exhibit all possible variations from which the breeder may make a further selection only after cultivation for several generations. The effect of cultivation was particularly striking in Veronica chamaedrys (Klebs, "Künstliche Metamorphosen", Stuttgart, 1906, page 152.) which, in spite of its wide distribution in nature, varies very little. After a few years of cultivation this "good" and constant species becomes highly variable. The specimens on which the experiments were made were three modified inflorescence cuttings, the parent-plants of which certainly exhibited no striking abnormalities. In a short time many hitherto latent potentialities became apparent, so that characters, never previously observed, or at least very rarely, were exhibited, such as scattered leaf-arrangement, torsion, terminal or branched inflorescences, the conversion of the inflorescence into foliage-shoots, every conceivable alteration in the colour of flowers, the assumption of a green colour by parts of the flowers, the proliferation of flowers.

Garner & Allard: Journal of Agricultural Research 18(11): 553-606 (1920)
Exposed to light from 9 a. m. to 4 p. m. Planted May 15, up May 19, and placed in dark house on day of planting. The test plants grew more slowly than the controls for a time and then appeared to grow no further. All but two of the test plants, of which there were a large number, became diseased and finally died without forming seed stalks. The two survivors developed a crown of large leaves, and the roots also reached much larger proportions than those of the controls. Apparently enlargement of the roots had not ceased as late as October 15, when one of them measured nearly 4 inches in diameter while its rosette of leaves measured 30 inches from tip to tip. Flower stems did not develop. The controls grew more rapidly from the outset, and all except three or four to be considered later formed flower stems in June, the first blossom appearing June 21. See Plate 75, B. [CybeRose note: This looks like an opportunity for the experimenter to select a short-day radish from an existing strain.]

Babcock: Remarkable Variations in Tarweeds. Journal of Heredity. 15 (3): 132-144. (1924)
DURING the course of a genetical investigation of two of the Hayfield Tarweeds* a number of interesting variations have appeared among populations only one, two, three, or four years removed from the wild state. Some of the variations which were found among about 400 plants during the first season of garden culture are the following: Size of plant, habit (form) of plant, color of flowers (corolla and stamen tube), structure of flowers (petallody), size of leaves (width and length), pubescence on leaves (long, appressed or short, spreading), and other less striking differences. From the first year garden cultures numerous pairs of plants were chosen and crossed because the individual tarweed, like the individual sunflower, is self-sterile. From their progeny similar pairs were again chosen and crossed, and this was repeated, so that inbreeding by brother and sister mating was practiced in some strains for two or three years, when these strains became so weakened that very few progeny were obtained. It was in these inbred strains that most of the other variations herein described were found.

Kettlewell: Temperature effect on pigment; Heliothis peltigera and Panaxia dominula (1944)

Clausen: Phenotypic expression of genotypes in contrasting environments. Report of the Scottish Plant Breeding Station 1958 pp. 41-51 pp.
By reference to examples from the literature, it is shown that many plants possess a considerable amount of usually unexpressed variability. This variability may become evident if the plant is moved to a markedly different environment or if ecotypes from different environments are crossed; under these conditions, latent genes may be activated.

Clausen: Gene Systems Regulating Ecological Races. X International Congress of Genetics (1959)
J. P. Cooper (1954) found that when Wimmera ryegrass is sown outdoors in spring it will initiate flowering at the seventh to eighth node (mean 7.2). When it is sown in a heated greenhouse and given continuous light, considerable previously hidden variability is disclosed, and the seedlings initiate flowering at various nodes, ranging from the fourth to the twenty-first, as shown in Table V (mean node, 12.5). The variability will again be hidden if, before the continuous light treatment, the seeds are vernalized for six weeks at 3°C. When the cold treatment is followed by continuous light, the plants will initiate flowering at the fourth to the sixth node (mean 5.1).

Clausen: The balance between coherence and variation in evolution. Proc. Natl. Acad. Sci. USA. 46(4): 494-506. 1960.
As a result of their dynamic coherence-variation balances, natural entities remain unchanged so long as the environments remain essentially the same. Major changes in the environments cause migrations of races, interracial crossings, release of variability, and changed selective pressures. These events can lead to establishment of new races adjusted to new environments. Certain species have nevertheless been able to retain their basic coherence mechanism relatively unchanged throughout geologic periods, as has been demonstrated, for example, in the chromosomal homology of hybrids between the Old World and New World sycamores of the genus Platanus.

McClintock: The Significance of Responses of the Genome to Challenge. Science. 226: 792-801. (1983)
There are "shocks" that a genome must face repeatedly, and for which it is prepared to respond in a programmed manner. Examples are the "heat shock" responses in eukaryotic organisms and the "SOS" responses in bacteria. Each of these initiates a highly programmed sequence of events within the cell that serves to cushion the effects of the shock. Some sensing mechanism must be present in these instances to alert the cell to imminent danger, and to set in motion the orderly sequence of events that will mitigate danger. But there are also responses of genomes to unanticipated challenges that are not so precisely programmed. The genome is unprepared for these shocks. Nevertheless, they are sensed, and the genome responds in a discernible but initially unforseen manner.

Rutherford & Lindquist: Hsp90 as a capacitor for morphological evolution. Nature. 396(6709): 336-42. (1998 Nov 26)
The heat-shock protein Hsp90 supports diverse but specific signal transducers and lies at the interface of several developmental pathways. We report here that when Drosophila Hsp90 is mutant or pharmacologically impaired, phenotypic variation affecting nearly any adult structure is produced, with specific variants depending on the genetic background and occurring both in laboratory strains and in wild populations. Multiple, previously silent, genetic determinants produced these variants and, when enriched by selection, they rapidly became independent of the Hsp90 mutation. Therefore, widespread variation affecting morphogenic pathways exists in nature, but is usually silent; Hsp90 buffers this variation, allowing it to accumulate under neutral conditions. When Hsp90 buffering is compromised, for example by temperature, cryptic variants are expressed and selection can lead to the continued expression of these traits, even when Hsp90 function is restored. This provides a plausible mechanism for promoting evolutionary change in otherwise entrenched developmental processes.

Lerner: Response of plants to environmental stress (1999)

Molinier et al. Transgeneration memory of stress in plants.Nature. 442(7106):1046-9 (2006)
Owing to their sessile nature, plants are constantly exposed to a multitude of environmental stresses to which they react with a battery of responses. The result is plant tolerance to conditions such as excessive or inadequate light, water, salt and temperature, and resistance to pathogens. Not only is plant physiology known to change under abiotic or biotic stress, but changes in the genome have also been identified. However, it was not determined whether plants from successive generations of the original, stressed plants inherited the capacity for genomic change. Here we show that in Arabidopsis thaliana plants treated with short-wavelength radiation (ultraviolet-C) or flagellin (an elicitor of plant defences), somatic homologous recombination of a transgenic reporter is increased in the treated population and these increased levels of homologous recombination persist in the subsequent, untreated generations. The epigenetic trait of enhanced homologous recombination could be transmitted through both the maternal and the paternal crossing partner, and proved to be dominant. The increase of the hyper-recombination state in generations subsequent to the treated generation was independent of the presence of the transgenic allele (the recombination substrate under consideration) in the treated plant. We conclude that environmental factors lead to increased genomic flexibility even in successive, untreated generations, and may increase the potential for adaptation.

See also
Inheritance of Acquired Characters
Epigenetics, Gene Silencing, RNAi
Dominance Modification
Delayed Fertility (each attempt at flowering provokes an internal stress)
Domesticating Wild Plants