Gigantism, Polyploidy and Hybrid Chinas
Karl King
Hybrids produced from the French rose impregnated with the China rose, are not of such robust and vigorous habits as when the China rose is the female parent.
Thomas Rivers, 1846

It is now a well-known fact that doubling of the chromosome number brings about rather great physiological changes. There are considerable changes in the relative amounts of the substances involved. Amongst other things, there is a relative increase in the amount of water, with consequent changes in suction power and osmotic properties.

Folke Fagerlind, 1958

These two statements, separated by 112 years, refer to different aspects of the same fact: that there must be interactions between the chromosomes and the cytoplasm which affect the size, vigor and appearance of the larger organism.

When a diploid species is "colchicinized" to give a tetraploid, there is no change in the types of genes present in the nucleus, nor in the proportions of genes relative to each other. The increase in nuclear material alone disrupts the balance between nucleus and cytoplasm.

In hybrids, there is also a difference in the interactions between nuclear genes and cytoplasm. In such cases reciprocal hybrids may be distinctly different even though the chromosome sets are identical. Foreign chromosomes have an effect similar to extra chromosomes in changing the osmotic value of the cell. And where the parents also differ in chromosome number the differences in reciprocal hybrids may be due as much to the relative change in chromosome number as to nuclear gene-cytoplasm interaction.

Teas, Chinas and Noisettes are mostly diploids, while Gallicas, Centifolias and Damasks are commonly tetraploids. Crosses between the two groups usually give triploids which should be genetically equivalent. Yet as Rivers observed, where the diploid types are the seed parents, the offspring tend to be larger and more vigorous.

A triploid derived from a diploid seed parent represents a 50% increase in chromosome number, with 67% foreign chromosomes. In the reciprocal cross, the chromosome count is decreased by 25%, and only 33% of these are foreign to the cytoplasm. A triploid hybrid from a diploid seed parent is more likely to exhibit gigantism than an autotriploid, such as the giant triploid form of R. blanda. A triploid derived from a tetraploid seed parent may also show some physiological disturbance, but gigantism (or "hybrid vigor") would be less pronounced.

Gigantism and reduced fertility are common results of autopolyploidy, and both qualities can be reduced. Fagerlind wrote, "Most descriptions of experimentally produced doublings give no information concerning impairment of fertility as a direct consequence of the general physiological state of the new type." He continues, "... the low fertility often found in the newly formed polyploids is not solely, and perhaps not even chiefly, due to the disturbances of the reduction division, but to the fact that the sporophyte possesses a general physiology such that it exercises an injurious effect upon the young spore cells, or upon their mother cells." This may also contribute to the reduced fertility of triploids.

Fagerlind found that his autotetraploid Rugosas improved in fertility as they got older. He also cited Wettstein's work with Bryum caespiticium, in which chromosome doubling led to gigantism and sterility. Over the course of 11 years, "The restoration of normal fertility ran parallel with the successive elimination of the gigas properties." The new form was named B. Corrensii, and was later identified in the wild.

Fagerlind also cited Schlösser's work in stabilizing a 52-chromosome tomato. Diploid tomatoes have 12 pairs of chromosomes, so this unusual clone was doubled and had 4 extra chromosomes. The original plant shed all its flower-buds, and was propagated by top cuttings. "With each generation the bud-shedding tendency was reduced." By the 5th vegetative generation there was 80% "lethal" pollen, which dropped to 35% in the 6th generation.

In the 9th generation fruit formed, and the seedlings proved to be normal tetraploids. Yet the chromosome count of the clonal plants remained constant with 52 chromosomes. Despite having no opportunity for sexual segregation of nuclear genes, the vegetative tissue had "learned" to ignore the extra chromosomes, and to exclude them during meiosis.

To Fagerlind "This arouses the suspicion that every new system arising from an older one is characterized by a greater or lesser disturbance of the equilibrium, which may eventually be restored through auto-adjustment."

Gloire de Dijon is effectively an autotetraploid. Judging from old descriptions, this variety has suffered a general decline in vigor that cannot easily be attributed to viral infection. Perhaps this is another example of auto-adjustment reducing the imbalance that originally conferred gigantism. The current form is not degenerate, in this case, but has a normalized balance between nucleus and cytoplasm. Some other cases of "replicative decline" may also be due to this process of normalization.

It is the prevailing paradigm of genetics that all such changes must be "random", at least in regards to the organism's needs. Changes occur because they can, and are then either favored or eliminated by selection.

Fagerlind found that autotetraploids of Rugosa and Multiflora were more susceptible to drought and frost than their diploid parents. Both conditions could be influenced by the increased quantity of water in the cells, which is also responsible (at least in part) for the gigantism. Natural selection alone would favor any reduction in gigantism, and thus indirectly favor an increase in fertility in the original plants. The plants did not "decide" to increase their fertility by reducing the gigantism.

Auto-adjustment

How the adjustment occurs is a matter for further research. There is compelling evidence that in some cases specific cytoplasmic factors—plasmagenes—are involved (Darlington). The relative abundance of these plasmagenes, and their differential distribution in the various tissues, may be at work. For example, careful bud selection has produced thornless rose plants from normally thorny varieties, new colors of flowers, forms with more or fewer petals, and even reblooming clones from once-blooming or rarely recurrent varieties.

Other research indicates that some genes are silenced—their expression suppressed—by methylation. The pattern of methylation in cultured cells may change, allowing previously silenced genes to be expressed. De novo methylation may also silence previously expressed genes. "Whereas cultured diploid cells progressively lose methylation, permanent lines retain a constant level. In such lines it may well be that loss is balanced by de novo methylation." (Holliday)

This continued change in the pattern of methylation seems to agree in part with Fagerlind's requirement: "The auto-adjustment would then constitute a kind of modification, one in which — in a way — the genetic material itself becomes an object for modification." Changes could continue until an optimum for the current milieu is achieved, as in the case of the permanent lines of cell cultures.

When Moreau-Roberts raised Commandant Beaurepaire from seed sown in 1864, it was enormously vigorous. "The second year, its luxuriant growth made it look like a Banksiana." ... "At length, the growth having slowed down, my hope in 1872, was to coax this variety into reblooming." (Dickerson) But not until 1876 did 40 or 50 plants rebloom, allowing him to reclassify the variety as a Hybrid Perpetual, and to rename it Panachée d'Angers. That the rebloom occurred only after the growth had slowed suggests a connection, and that both changes may have been a result of auto-adjustment and selection.

Graham Thomas becomes extremely vigorous in warm climates, while its ability to rebloom suffers. The change in external environment has also changed the internal balance of nuclear genes and cytoplasm. In time, and with careful bud selection, this beautiful rose might become adjusted to warmth, and bloom as well as it does in cooler climates.

Fagerlind mentioned two intergeneric hybrids, Mahoberberis and Fatshedera "in which the total lack of flowers renders all sexual reproduction impossible." The Hybrid Chinas might be considered as lacking flowers after the Spring bloom period. Even some of the Hybrid, like Duchess of Sutherland, "will not give autumnal flowers constantly, but often make shoots without a terminal flower-bud." (Rivers) The same complaint has been made about some modern Hybrid Teas (e.g., Chrysler Imperial).

Similarly, according to Rivers, "Antinous is a new rose, evidently between the French Rose and Crimson Perpetual [Rose du Roi], equalling that fine rose in form and fragrance, and surpassing it in beauty of colouring. This when first introduced did not bloom constantly in the autumn: it now, however, puts forth its fine crimson-purple flowers in September." (Rivers)

Mendelists would have us regard the ability to rebloom as a simple recessive trait. However, the existence of various degrees of intermediacy, and the ability of vegetatively propagated varieties to change their bloom habit, suggests that auto-adjustment and epigenetic regulation are involved. Natural selection has favored "genetic systems" in which the ability to bloom is strongly impaired except at specific bloom times. Artificial selection of reblooming types involves upsetting a balanced system of genes and cytoplasm.

Conclusion

A new polyploid or a hybrid is a "new system arising from an older one" (or two), and must be "characterized by a greater or lesser disturbance of the equilibrium, which may eventually be restored through auto-adjustment." The adjustments are random in regards to the organism's needs — the beneficial or useful may be retained while the harmful are eliminated — at the cellular level.

In the process of auto-adjustment, other changes may come about which are direct or indirect consequences of the adjustments; and these may also be favored or rejected by natural or artificial selection.

Summary

  1. Gigantism is a common result of an increase in chromosome number.
  2. Triploids raised from diploid x tetraploid are more likely to exhibit gigantism than those from the reciprocal cross.
  3. Reduced fertility of new polyploids is partially due to the gigantism.
  4. Gigantism and sterility may become reduced by auto-adjustment if the vegetative clone is maintained long enough.
  5. Auto-adjustment may also be responsible for some instances of "replicative decline".
  6. Auto-adjustment may involve changes in the interactions between nuclear genes and the cytoplasm (which also mediates gene-gene interactions), as well as epigenetic alteration of gene expression, and may be responsible for certain so-called bud sports.

Bibliography