Epigenetic Program

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Karl K
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Epigenetic Program

Post: # 69912Post Karl K
Tue May 21, 2019 12:06 pm

It has become increasingly apparent in recent decades that there is much to heredity that doesn't fit into the simplistic neo-mendelist model. Dominance, for example, is not a simple matter of presence vs. absence. Often, there is an element of chance involved in deciding which of two alternative characters will dominate in a particular individual, while in siblings the relationship will be reversed.

In addition, there is an epigenetic program (for lack of a better term) that shapes the expressions of genes across the genome. The program is strengthened by repetition. That is, a specimen or population exposed to the same environmental conditions year after year will "set in its way", so to speak. The pattern of gene regulation becomes more stable and more difficult to disrupt.

However, some extreme conditions - internal as well as external - can provoke a "genomic shock", making it easier to push to the species or strain into new expressions.

There's a lot to this subject, so I will start with en example from humans.

"For decades, scientists thought that Down’s syndrome resulted from genes carried on the extra copy of chromosome 21 that causes the disorder. More recently, researchers have recognized that gene expression becomes altered throughout the entire genome. Now, researchers led by Stylianos Antonarakis at University of Geneva Medical School, Switzerland, examine these genetic changes in minute detail. By examining a set of monozygotic twins, one of whom had Down’s syndrome, they were able to see that cells with an added chromosome 21 exhibit a distinct pattern of gene dysregulation across the genome. Chromosomal areas that are usually highly expressed quiet down, while those that are normally silent perk up. As the researchers report in the April 17 Nature, the findings suggest that epigenetic changes could be at play. "
Image
https://www.alzforum.org/news/research- ... ire-genome

This is just a starting point, though, because breeding experiments with humans are not encouraged among decent people. Fortunately, there are abundant records among plant breeders. Van Mons (1835) explained that pears vary more and can be pushed further by selection when they are brought into cultivation. In just five generations he had pear trees blooming and fruiting in just three or four years from seed. Left in the forests, the offspring would have grown up like their parents, bearing in 10 to 12 years.
http://bulbnrose.x10.mx/KKing/VanMons.html

Breese (1965) found that perennial ryegrass (Lolium perenne) that had been vegetatively propagated for many years did not respond to vegetative selection. Seedlings from the same strain did respond favorably, while second generation seedlings were even more responsive.
http://bulbnrose.x10.mx/Heredity/Breese ... n1965.html

Michurin, at various times, discussed a method he called "vegetative approximation". When two species failed to cross in the usual way, he budded or grafted seedlings of one onto the other. Sometimes the cross could be made immediately, though in others the scion-species had to be grown from seeds of grafted shoots for a few generations before the cross would take.

This is not mysticism, and there is no need to suppose that genes migrated from stock to scion. It is simply a matter of one organism (scion) being forced to adjust its own physiology to that of another (stock). The adjustment involves epigenetic changes, which are at least partially hereditary, and become more so with each generation.

Alternatively, when two species differ in their habitat-adaptations, it can be useful to force one to become better adapted to the habitat of the other. A species that is commonly found in wetlands may not enjoy life on dry land, but a generation of two or three should make it more comfortable in the new environment. And such a former-wetland plant may be more willing to mate with another dry-land species.

Karl K
Posts: 1249
Joined: Sat Jun 02, 2012 4:49 pm
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Re: Epigenetic Program

Post: # 69922Post Karl K
Fri May 24, 2019 1:26 pm

Re: vegetative selection. As I discussed in my last note, the ability to respond to vegetative selection (for good or ill) seems to decline with age. In other cases, some strains and specimens appear to be less responsive than others.

Many roses have been hurt by too-rapid propagation without proper bud selection. In many cases, this damage can be repaired. In other cases on record, cultivars have been improved far beyond what they were when first introduced. 'General Jacqueminot', for example, originated as a once-bloomer in the garden, though it could be nearly everblooming in the winter when grown under glass. Other HPs and HTs have been improved as to vigor, size, health and color, as well as in freedom of bloom.
http://bulbnrose.x10.mx/Heredity/King/V ... ction.html

Some roses are too stubborn to be either improved or degraded by methods of propagation. 'Radiance', for example, stubbornly remains itself, and is also indifferent as to rootstock. And according to Nicolas (1937), ""Radiance is the most obstinate and selfish breeder. I have, perhaps, done more Radiance breeding than anyone else on earth, and got nowhere. It seems as if the color and plant characters were welded together, and, like Mary and her little lamb, one goes where the other does."

This does not mean 'Radiance' is useless as a breeder. One of its offspring, 'Ma Perkins', is a grandparent of Austin's 'Wife of Bath'. But no one has been able to breed a white or yellow rose that precisely duplicates all the virtues of 'Radiance'.

When breeders work with species, they are usually looking to extract a trait or two that might be useful in cultivated types. Efforts to "crack" a species are not always really successful.

Austin (1993) wrote, "The third line we pursued was by way of the Rugosa hybrid 'Conrad Ferdinand Meyer.' At first we harbored no great hopes of success, for we feared that the resulting seedlings from a cross with this excessively vigorous hybrid would be altogether too gross in character. 'Conrad Ferdinand Meyer' was itself a cross between the very popular and beautiful Climbing Noisette Rose 'Gloire de Dijon,' and an unknown Rugosa hybrid. It also had one of the most powerful and delicious fragrances. As before, we crossed with some of our better English Rose in particular 'Chaucer,' and had one of those pieces of luck that sometimes turn up in rose breeding. Some of the seedlings from this cross were of typical rugosa appearance, while others bore absolutely no resemblance to a Rugosa Rose. It seemed that some of our hybrids had taken the genes only from the 'Gloire de Dijon' half of 'Conrad Ferdinand Meyer,' while others had inherited those from the Rugosa side. What we had in many instances were in effect hybrids of 'Gloire de Dijon'."

Griffith Buck (1960) had similar results using Rosa laxa hybrids.
"Pollen from the 'Crimson Glory'-R. laxa seedling was effective in producing viable seed on a wide range of cultivars of the Hybrid Tea, Floribunda, and Grandiflora garden classes. The seedlings segregate into two sharply defined groups of approximately equal numbers. One group bears a pronounced resemblance to R. laxa in growth habit, foliage and prickle characteristics. The flowers, which are borne only in June, are single, two to three inches in diameter, and are in the lighter tones of pink, salmon, and yellow. All the plants in this group are as hardy as the species parent. The plants of the second group resemble the garden rose parent in floral and foliage characteristics. The plants are June-blooming; the everblooming habit of the Hybrid Tea and the remontance of R. laxa being absent. All plants of this group winter-killed during the winter of 1958-59, even though they had been given winter protection. All the seedlings retained the freedom from powdery mildew and the susceptibility to blackspot of the 'Crimson Glory'-R. laxa parent."

Michurin had similar problems when he first started. He raised hybrids between the best foreign plants he could obtain (American apples, French pears, English strawberries, Spanish filberts) with species native to central Russia. Following the advice of European experts, Michurin gave his seedlings the best possible care to "draw out the heredity" of the cultivated types. Sadly, this treatment drew out the weaknesses of the exotic plants. Most died during the dry heat of summer, and the rest were finished off by the brutal winter.

He decided to start over by raising his next batch of hybrid seedlings on poor dry soil. This time the plants survived, but were mostly not improvements over the locally adapted wild types.

Finally, he decided to make another attempt by deriving the "hardiness factors" from species that were as foreign as the cultivated types. Plants from North Korea, Manchuria and other frigid lands proved to be more useful for Michurin's breeding efforts than those form central Russia. The desirable traits were not so firmly "welded" to the undesirable traits when the two parent were equally "exotic".

The point of this last item is that the epigenetic program of two prospective parents can be disrupted more thoroughly when neither is closely adapted to local conditions.

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