Selective Pollination
Donald Forsha Jones (1928)

Chapter 1

INTRODUCTION

The exercising of choice in mate selection is exhibited throughout the plant and animal kingdoms as well as in human affairs. Sexual reproduction promotes the union of different individuals, and the outcome depends upon the hereditary potentialities that have been brought together. The opportunity for discrimination exists both in the individuals which are brought together and in the gametes which take part in fertilization.

The compatibility of different organisms depends in a general way on their similarity. The taxonomic groups in both animals and plants define, broadly, the limits beyond which sexual fertilization is not possible. Selective fertilization, preventing the union of individuals of too great dissimilarity, is obvious in nature. In many cases reproduction in widely different matings is possible but the offspring are sterile. Even where fertilization is followed by continued propagation, there is a tendency for such assortative mating to take place that individuals which are alike are more likely to reproduce, other things being equal, than individuals which are more diverse.

A selective action may also occur among different gametes produced by the same individual. In the early development and verification of the Mendelian principles of heredity, the tendency was to overlook or ignore cases of this kind. Chance recombination of the germ cell which take part in sexual reproduction is one of the main tenets of Mendelism.

Ample evidence shows this to be true generally for matings of organisms which are closely alike in germinal constitution; but as the parental forms become more diverse, certain gametic combinations in the hybrid oilspring are formed less readily than other combinations, and many of these associations of gcrmplasm which are brought together are not viable.

1In 1901, W. J. Spillman published a report on Quantitative Studies on the Transmission of Parental Characters to Hybrid Offspring. His results, obtained before Mendel's experiments became generally known, demonstrated the recombination of inherited characters in definite numerical proportions.

Although some of the early hybridizers, particularly Gartner, Knight, Goss, and Seton, noted the segregation of characters in the offspring of hybrid plants, no one had thought it worth while to record the numbers in which the individuals of different type appeared until Mendel proposed a simple theory to account for the appearance of different individuals in the progeny of hybrids on the basis of chance recombination of units of inheritance which pass from generation to generation relatively unchanged.1

In the investigations of heredity undertaken at the time of the appreciation of Mendel's theory, first attention was given to establishing the fact of recombination. Later much interest has been given to the association of factors in transmission and the interpretation of this behavior on the basis of the chromosome mechanism.

Selective fertilization has been employed to account for exceptions to the usual Mendelian mode of inheritance that needed no such interpretation. Yellow mice, giving a ratio of two yellow to one non-yellow, were first supposed by Cuénot (1908) to be due to inequality in fertilization of the gametes carrying different factors. Later it was found by Castle and Little (1910) and others that the homozygous combination of the factor for yellow was lethal in the embryonic state. Other cases of unusual Mendelian ratios, at first thought to be due to unequal fertilizing ability of different gametes, have been accounted for in other ways so successfully that little attention has been given to unequal fertilizing ability of different gametes from the same individuals and from different individuals.

If we limit the definition of fertilization to the fusion of two cells, the gametes, to form the zygote, there is no clear case known of a selective action within the species where the individuals are all compatible with each other. But in the evolutionary process, species are differentiated to a point where the gametes of one are no longer able to fuse with the gametes of another, and it is difficult to imagine that this incompatibility was acquired completely in one generation, even in those organisms in which the fusion of the sexual gametes takes place outside of the body. According to the evidence from the higher plants, it seems more probable that there is a transitional stage in which the fusion of gametes from increasingly divergent forms takes place less readily than in the case of individuals of the same type.

The accomplishment of fertilization requires the bringing of the gametes together, and in those animals and plants where fertilization takes place within the body of the organism, much more is involved than the penetration of the egg membrane by the sperm and the fusion of the male and female nuclei. In the spermatophytes an elaborate mechanism has been developed whereby the male gametophyte grows as a parasitic organism in the sporophytic tissue of the seed parent. This is the pollen tube which brings the generative nucleus in contact with the egg. In these plants fertilization is dependent upon the ability of the pollen tube to grow, and discrimination between the gametes from different individuals and between different gametes from the same individuals is made before the germ cells come in contact.

There is some objection to the use of the term "selective fertilization" to denote such a discrimination made before fertilization takes place. "Gametophytic selection," "differential pollen-tube growth," and selective pollen-tube stimulation" have been used to designate such departures from independent assortment. De Vries has used the term "gamolysis" to denote a preferential fertilization of one type of gamete with another. For differences in pollen-tube growth Heribert-Nilsson (1920) has proposed the term "certation." In our language this word by itself has no meaning, and as a specific term it is open to the objection that its definition may have to be changed when the phenomenon it is supposed to designate is more fully known. Many special words have been coined in this way to designate facts which have been interpreted differently, as, for example, "xenia" and "heterosis." Terms of this kind are very apt to become confusing. For that reason the writer prefers to use the general term "selective fertilization" to cover broadly all forms of discrimination in reproduction.

In its effect it makes little difference where the selective action takes place. Any discrimination, however brought about, has an important bearing on evolution. Bringing the pollen to the stigmatic surfaces, germination of this pollen and growth of the pollen tubes in the sporophytic tissue of the seed-bearing plant, the liberation of the generative nucleus, and the penetration of the embryo sac are all necessary steps in the accomplishment of fertilization and sexual reproduction. In animals the choice of mates, mating instincts, and structure of the sexual organs determine which types shall reproduce as much as the compatibility of the germ cells.

In the lower plants and animals, where fertilization takes place by the fusion of gametes liberated from the bodies of their parents, very little is known about differences in fertilizing ability of unlike gametes. A marked selective action occurs, since this is one means by which species are kept separate from each other. Along with selective fertilization will be discussed related subjects having similar importance. 'These will include incompatibility of gametes, differential pollen-tube growth, and assortative mating.