Hybrids and Hybrid Vigor

Hybrid vigor is sometimes astonishing in its expression. Gravatt's Radish-Cabbage hybrid is far beyond what one might imagine. It is certainly not intermediate in its growth, but how do we account for the extraordinary vigor? Sadly, maize breeders have cheapened the term, and sometimes we read that any expression of a trait that is greater than the mean of the parents should be called "hybrid vigor".

It is interesting to note that some valuable clues have been largely overlooked; perhaps they involve the difficult task of actually looking at the prospective parents. For example, Keeble and Pellew (1910) observed that half-dwarf peas were of two very distinct growth habits. One type resemble Tall peas in all ways, but had shorter internodes. The others have the narrow leaves, thin stems and reduced branching of the dwarf peas, but with longer internodes. When the two types are crossed, Short Internode is recessive to Long, and Thin Stem is recessive to Thick. The offspring look like normal Tall varieties, though the F2 generation breaks up into Tall, the two types of Half Dwarf, as well as Dwarf individuals.

To be clear, what Keeble and Pellew observed was not hybrid vigor. It is what Hugo de Vvries (1902) called "Artificial Atavism". Even so, it does provide one model for how hybrid vigor (in crosses between species) can occur. For example:

Tree Physiology, 24:217–224 (2004)
Phytohormones and shoot growth in a three-generation hybrid poplar family
David W. Pearce, Stewart B. Rood and Rongling Wu

The F1 hybrids displayed heterosis (hybrid vigor) for primary growth as a result of the complementation of dominance for increased internode length from the P. trichocarpa parent and dominance for increased diameter from the P. deltoides parent.

Webber (1912) and Norton (1915) discuss more complicated examples involving three recessive mutations that distinguish garden peppers from their probable parent. Plants homozygous for any two of these recess mutations are semi-dwarf. The full dwarf type is homozygous for all three. Analogous cases may occur in three-way hybridsk, where (A x B) x C) exceed the three parents as well as the F1 hybrids.

The neo-Mendelist dogma argues that evolution proceeds as random gene mutations pass through the filter of natural selection. This implies that species that are most closely related are also most similar in their habits and adaptations. This turns out not to be true.

Lechowicz and Wang (1998) have conducted one of the few studies evaluating phenotypic plasticity in a phylogenetic context (see also Pigliucci et al. 1999). In a study of 16 species of North American spruce growing in ambient and elevated CO2 and low and high water availability, they found that interspecific variation in many morphological and ecophysiological traits was not associated with the species’ phylogenetic relationships. However, relative growth rate, which is the outcome of interactions among many ecophysiological traits, showed consistent evolutionary trends across species. Perhaps most interesting, relative growth rate was also less plastic across environments than the many ecophysiological traits underlying variation in growth, but the levels of plasticity in growth rate did not themselves show any pattern of phylogenetic constraint. Evolution of function in extant spruces has apparently involved different patterns of diversification in the mean value of traits affecting growth and in the plastic expression of these traits in differing environmental regimes.

Thus, closely allied species may achieve the same ends by different means. Hybrids, partaking of two regulatory adaptations, may be doubly-bless and exceed the parents in fitness and/or vigor.

Another model for some instances of hybrid vigor was describe by Edgar Anderson (1944) in Tradescantia. One species has few leaves and long internodes. The other has many more leaves and short internodes. The hybrids grow about twice as tall as the parents. The same pattern is found in maize crosses involving Northern Flints (long internodes) and Southern Dents (many more leaves).

Naudin: Maternal Daturas (1863)
These plants were exactly like the mother plant, but were nevertheless abnormal, because of their unusual height, being nearly twice that of the mother species, and also in dropping all the flowers produced in the lower forks. Such increased vigor and partial sterility were observed by Naudin in all intermediate hybrids of Datura, but in this case the plant resulting from the cross showed no trace of the characters of the father species, and its seeds, when planted the following year, yielded the ordinary form of the mother plant.

Chappellier: Breeding Crocus sativus, Dioscorea, Mirabilis (1900)
Mirabilis longiflora fertilised by the pollen of M. jalapa has given me an interesting hybrid.
   This hybrid is, like many others, notably more vigorous than either of its parents — in fact, it has tubers of good size, about three years old, furnished with supports and forming bush-like plants, 2 metres in height, and if the branches, unsupported, are allowed to take their natural direction, a single plant can form a clump nearly 4 metres across.
   On a strong plant I have counted nearly 400 flowers, exhaling a sweet odour, and which, be it understood, are renewed every day.

Cook: Intensification/Suppression in Cotton Hybrids (1909}
In addition to the superiority of their lint, the vegetative vigor of the hybrids [with Kekchi and Upland] gives them a great advantage in the field. They are much more fertile than the parent types, often producing several times as many bolls as adjacent Egyptian plants. A single hybrid plant grown at Yuma in 1907 is reported by Mr. W. A. Peterson to have borne over 1,100 bolls. Plants with 200 to 300 bolls are frequent. The hybrids also appear to be much less susceptible to injury from adverse conditions, either in the yield or in the quality of the lint.

Burbank: Walnuts (1914)
At sixteen years of age, Mr. Burbank's new Paradox walnut trees were sixty feet in height and as much in breadth of branches — the trunk being two feet in diameter at about four feet from the ground. Meantime English walnuts on the opposite side of the street averaged only eight or nine inches in diameter at thirty-two years of age, and had a spread of branches only about one quarter that of the youthful Paradox.

Gravatt: Radish-Cabbage Hybrid (1914)
The hybrid produced a great abundance of large leaves. In size and shape they are nearer to the cabbage, but are much larger than those of either parent. The largest leaf of the cross was five feet nine inches from tip to point of attachment, and one foot seven inches broad. The leaf was measured at the time the plant started blooming.
   The stem of this hybrid is several times larger than the combined stems of the two parents. When the hybrid reached the size shown in figure 15, the circumference of the stem was 13 1/2 inches, measured at a distance of one foot from the ground. The stem holds its size for some distance from the root. The lower part or about the first two feet of the stem is very thickly beset with leaves, thus resembling the cabbage in habit.

Swingle: Citrus Hybrids (1927)
Almost all of the citranges grow faster than the combined growth of both parent species.