Age and Condition of Parents in Breeding

I was pondering the the odd fact, reported by Sepanov (1960) that the use of incompatible co-pollen while self-pollinating maize plants resulted in inbred lines that, "acquired morphological stability more quickly than inbred lines from pollination with self pollen only."

How could this be? It soon came to me that the co-pollen "tricked" the seed parent into providing more nourishment than self-seeds would usually receive. It was the extra nourishment that gave added vigor to the pollen tubes. And this added vigor was imparted to the seedlings. Compare this with Beaton6, where the reduced nutrient supply in pollen grains resulted in dwarfed (stunted) geraniums.

Other reports came to mind that seemed to be related. And all appeared to agree with Michurin's assertion that there is more to heredity than mere unit characters. Specifically, it now appears to me that the character of hybrids might be influenced to varying degrees to the relative health and vigor of the parents. That is, a strong plant growing in its native habitat will likely imprint its own character on its seedlings more strongly than if the same plant were struggling to survive in less congenial surroundings.

Michurin: Age and condition of parents (1929)
The age and health of the parent plants chosen for the crossing are of very great significance in practice. Young hybrid plants in their first bearings, or older plants that have been bearing for many years but which were weakened by a dry or unusually cold spring during the given vegetation period, possess a weaker individual capacity for hereditarily transmitting their properties, and, conversely, plants belonging to pure species and, particularly, wild forms in their prime, possess the greatest capacity of handing down their properties to the hybrids. Thus, for example, from the cross of the Crimean Kandil Sinap with the Siberian crab apple [Malus baccata Borkh.] hybrids were obtained with fruits the size of our ordinary orchard Kitaika [Malus prunifolia Borkh.],whereas the cross between Kandil Sinap with the seedlings of our orchard Kitaika in its first blossoming produced fruits of an excellent taste. In this case the maternal parent was the young seedling of the Kitaika, not the pure type of course, but a hybrid; this became evident later from the larger size of its fruit as compared with the ordinary size of the Kitaika. That is why its resistance properties were not transmitted with due intensity, and as a result the shoot ends of the seedlings obtained from this cross suffered from the frost. To eliminate this shortcoming the hybrids had to be placed once again under the influence of their female parent—Malus prunifolia Borkh.—by grafting cuttings of the seedling into the crown of the maternal tree, which soon gave the required degree of resistance to the new variety. This circumstance should he taken into consideration when choosing the parent plants.
    It has been likewise remarked that flowers chosen for fertilization on the maternal plant, if placed nearer to the main vertical branches of the trunk, give better hybrids with larger-sized fruit but such that tend to deviate considerably in structure in the direction of the maternal plant, and, conversely, flowers on the horizontal branches, placed nearer to the periphery of the crown generally give hybrids with fruit of smaller size and such that deviate in the direction of the male parent. The shady side of the maternal plant yields hybrids of poorer quality as compared to the sunny side. This is particularly clearly expressed by the depth of the outer colouring of the fruit and by the amount of sugar in the pulp.

1) Stepanov, K. I. (1960) A study of self pollinated lines of maize obtained by supplementary pollination with foreign pollen. Trud. 1. nauc. Konf. molod. Ucen. Moldav. (Proc. 1st sci. Conf. jun. Sci. Moldav.) pp. 199-205
The depressive effects of inbreeding in two maize varieties were reduced at Kišinev by adding pollen of Atriplex, Hibiscus, pumpkin, sorghum, African millet, rye, wheat or sunflower to obtain the first-generation inbreds. The inbreds produced in this way acquired morphological stability more quickly than inbred lines from pollination with self pollen only.

2) Couvilion, Gary A. Cercis canadensis L. Seed size influences germination rate, seedling dry matter, and seedling area. HortSciewnce 37(1): 206-207 (2002)
These data show that Eastern redbud seedlings derived from large seed are of greater vigor, have a higher percent germination, and are more uniform than seedlings from smaller seed. This has been reported for several annual crop plants. Separating redbud seed into seed weight fractions of various sizes and planting each seed size separately would provide growers with more uniform seedlings of various vigor levels and germination rates. The larger seed would be the more desirable because they would produce larger plants with the greater vigor. Although seedlings from smaller seed would be smaller and less vigorous, when planted separately from larger seed they would produce more uniform seedlings, and advantage that would maximize seedling growth in the seed bed due to minimization of mutual shading.

3) Young and Stanton: Influence of environmental quality on pollen competitive ability in wild radish. Science 248: 1631-1633. (1990)
Pollen of Raphanus raphanistrum produced under low nutrient conditions sired fewer seeds than pollen produced under better conditions when the two types were applied on a stigma together. ... If paternity is more strongly influenced by less visible pollen traits (pollen nutrients), then measuring pollen number and size is not sufficient for estimating male fitness.

4) Cook, O. F., Local Adjustment of Cotton Varieties (1909)
The Triumph cotton is known to breeders as one of the most regularly uniform varieties. It was originated at Lockhart by Mr. Alexander Mebane, and has been carefully selected by him for a considerable series of years. It would be difficult to imagine better examples of uniformity in a variety of seed-propagated plants than are afforded by Mr. Mebane's fields of Triumph cotton. A careful inspection of about 50 acres of Mr. Mebane's cotton resulted in finding only three plants that appeared to be definitely different from their fellows.
    Immediately after this test of diversity had been applied at Lockhart, and with the uniformity of the Triumph cotton at that place as a basis of comparison, a similar study was made of a much smaller field of cotton raised at Kerrville, Tex., from seed grown by Mr. Mebane at Lockhart. The difference in diversity between the two places was very striking. Adjacent individual plants were often obviously unlike, and a considerable percentage of the plants could be reckoned as showing distinct departures from the Triumph type.

5) Robertson, Reeve: Inbred variability, Drosophila (1952)
Size in Drosophila shows the same phenomena of decline under inbreeding and heterosis in crosses between inbred lines as other qualitative characters. This suggests that heterosis, or increased size, vigour, etc., and reduced susceptibility to environmental variations are both manifestations of the same phenomenon of heterozygosity.

The rest of these references are afterthoughts that are directly related to the above list. No. 6 and 6b connect with Michurin's second paragraph. Pollen from different anthers in the same flower can give different results, just as different flowers on the same plant may give different results. An important point in Beaton's discovery is that the dwarfness of the plants persisted despite large scale propagation, and was partially hereditary and cummulative.

6) Beaton: Influence of the pollen in the same flower (1861)
In the great bulk of the Scarlet or Horseshoe Geraniums there are but seven stamens, four long ones, one of medium length, but which is often wanting, and two almost sessile like the anthers of Wheat—that is, very short indeed, and opening at the bottom face to face. These two are they which reduce a whole family to beggary; first to dwarfs or Tom Thumbs, or better still, to minimums, or the smallest of that kind consistent with vigour sufficient to become a useful plant in cultivation, and, lastly, to the brink of ruin, and drive that race out of existence altogether, if there were not other means provided to arrest the decline, or keep it from manifesting itself at all in a state of Nature.

6b) Bradley: Hybridising at the Antipodes (1906)
Seeing that the top division of the [Hippeastrum] perianth is always the largest and best coloured, I generally use the anther, the filament of which is adnate to this division; whether this be the reason or not I do not know, but the progeny generally have more equal divisions to the perianth, and the bottom division is greatly improved.
    On the other hand, with a view to getting as white a bloom as possible, I use the bottom division (generally all white) from the white red-striped varieties; and in the seedlings the flowers have much less colour; but the shape of the bloom is spoilt, the divisions being narrow.

7) Morse: Pollen Selection (1902)
Pollen grains vary in size and vitality, though they may have been grown in the same stamen. In fact, I am selecting my pollen grains. Method used: A piece of unglazed paper is used, shaking the ripe pollen onto it and curving the paper, and at the same time elevating one end so that the pollen runs down onto a plate. On looking at the pollen with a lens we find that a certain amount of inferior grains are left on the paper, and by repeating the operation only the heaviest grains reach the plate. From experience I have found that these selected grains carry with them the general make-up of the plant bearing them, unless the energy of the stigma overpowers the pollen life. If such is the case the progeny is intermediate between the two in all points. But if the vigor of the pollen predominates it carries with it all the characteristic traits of its parent except color, which invariably leans to the seed-bearing parent.

8) Stephenson: Inbreeding depression, hybrid vigor and pollen performance (2001)
We found that sporophytic vigor (e.g., flower and fruit production) increased with the level of heterozygosity and that the level of heterozygosity of the sporophyte affects the in vitro and in vivo performance of the microgametophytes it produces. These findings are analogous to the "maternal environmental effects" frequently observed in seeds.

a) Jóhannsson: Pollen Vigor (1998)
b) Johnson & Mulcahy: Pollen vigor in inbred plants (1978)
c) Dajoz: Pollen quality, radish (1993)
d) Mulcahy: Gametophytic and Sporophytic Characteristics in Zea mays (1971)

9) Cook: Suppressed and Intensified Characters in Cotton Hybrids (1909)
The external conditions are able to affect the yield and the quality of the lint of the hybrids through the coherence of characters described in the previous chapter. The hybrids which resemble only the Egyptian parent in its unacclimatized form and fail to show any definite Kekchi or Upland characters or any improvement of lint have all been found under conditions favoring the development of the tall, erect form in Egyptian plants. In a planting at Coachella, Cal., in 1908, Mr. T. H. Kearney found that the hybrids resembled the pure Egyptian plants much more closely than at Yuma or at Sacaton, Ariz. Neither by the habits of growth nor by the more detailed differences, such as the form and color of the bolls, could the hybrids be recognized with the same readiness as in the Arizona plantings.

10) Morgan: Variability of Eyeless (1929)
It is not a little curious to find that the best fed and biggest flies are those less likely to develop eyes, while the late hatched small starved flies have large eyes.

11) Davis: Malnutrition and segregation in Oenothera (1921)

12) Seed Size
a) Arthur: Hot Water, Seed-size (1893)
b) Dimon: Cutting off parts of cotyledons of pea and nasturtium seeds (1901)
c) Zavitz: Influence of seed size on yield (1910)
d) Halsted: Planting depth for corn and beans, seed size (1917)

13) Nägeli: Achillea species and lime (1865)
I include this old note to suggest that pollen tubes, expressing segregated genes from a hybrid, might be strongly influenced by local conditions: temperature, salt, pH, etc.
Would a particularly well nourished, or perhaps a cross-bred specimen, be more able to resist competition from the other Achillea species?

14) de Vries: Nutrition and Selection (1909)

15) Old or Unripe Seeds biblio

16) Physiological Predetermination biblio