International Congress of Genetics Aug 20-27, 1958
Basic Research in Plant and Animal Improvement
Donald F. Jones1
|1The Connecticut Agricultural Experiment Station. New Haven, Conn.. U.S.A.|
HETEROSIS, the stimulating effect of crossing related organisms was known in ancient times and was described in scientific journals in the eighteenth century. But it was not until Darwin's time that it was recognized as having biological importance. After writing the Origin of Species. Darwin turned to domesticated animals for evidence of change by selection. In this study he learned the importance of inbreeding in fixing the desirable characters in breeds of livestock. These named breeds had originated in England and on the continent of Europe in the century before his time. Animal-breeders, recognizing the value of close mating, also warned against the dangers of inbreeding. Their conclusions were somewhat contradictory and confusing.
In his characteristic, thorough manner Darwin set out to explore the effects of cross- and self-fertilization, choosing plants rather than animals because fertilization was easily controlled, large numbers could be grown, and successive generations followed annually in most plants. These investigations covered a period of ten years and were reported in a book published in 1876.
Nearly all species of plants worked with were benefited in some degree by cross-fertilization, and were reduced by self-fertilization. Darwin thought that continued inbreeding eventually ended in extinction, as indeed it often does.
One of the plants used by Darwin in his experiments was Indian corn, maize from the New World. These experiments were known to Professor Asa Gray, at Harvard, and through him to one of his students, James Beal, who was soon to experiment and to teach at the first state agricultural college in the United States at Michigan.
Since corn is pollinated by the wind and the flowers are separated in different inflorescences on the same plant, Beal conceived the idea that it could be easily cross-fertilized on a large scale. On the Michigan State College farm the first crossed corn was grown by pulling out the tassels, the male inflorescences, before pollen was shed from all plants used as the seed parents. Pollen was furnished by a different variety used as the pollen parent. For his crosses Beal followed Darwin's advice and selected varieties that had been grown for a period of years under widely different environmental conditions. These varieties also differed in genotype.
From these crossed plants Beal obtained significant increases in yield. The largest increases came from crosses of flint and dent varieties, one grown in the northern, the other in the southern part of the state. The crossed plants, in maturity, suited very well the central part of the state where the trials were made. Beal was so impressed by the sturdy growth and good grain-yields of his first generation crossed plants that he advocated this method of producing seed at farmers' meetings and in the agricultural press.
For various reasons this method was never used commercially. Farmers, seed-producers, and experimental agencies continued to employ the conventional methods of selection long used by animal-breeders but made little progress with corn, largely because they had no control over the inheritance from the male parent.
Beal was also interested in following up Darwin's experiments on the effects of inbreeding. He devised a method of covering the flowers of the corn plant with bags and applied the pollen to the plant's own pistillate lowers at the right time to obtain completely self-fertilized seed. In turn, Beal's student, Perry Holden, continued these studies on the effects of inbreeding at the Illinois Experiment Station up to about 1900. These experiments were reported by Shamel in the Yearbook of the U.S. Department of Agriculture for 1905. Continued inbreeding was very disastrous to the size and seed-production of the corn plants Holden worked with. Most of the inbred plants were unable to survive more than three generations. As a result of these experiments the Illinois station published bulletins on methods to avoid the undesirable results of inbreeding.
Shortly after the rediscovery of Mendelism, George Shull at the Carnegie Institution in Cold Spring Harbor on Long Island, New York, began a study of the inheritance of a quantitatively variable character. He selected the number of kernel rows on the ear of maize for this study. Maize has from eight to thirty or more rows of grain on the ear. Different varieties fluctuate around a mean number of rows that may differ widely. Shull selected ears of different row number from a field of white corn of one local variety. In order to reduce the variability of his parental stocks each ear-row line wits self-fertilized. As expected, the first generation self-fertilized plants were somewhat reduced in size and vigor and each progeny varied around its own mean row number. There were large differences between different progenies all from the same variety. Several of these inbred lines that differed in row number were crossed after one selfing. Shull was surprised at the large increase in vigor and in productiveness of these first-generation crosses of somewhat inbred plants. He started the experiment with no thought of improving the yield of corn, but he saw in his experimental plantings a new method of controlling heredity to obtain better corn plants with enhanced yielding ability. Shull was getting the same results with a cross-fertilized plant that Johannsen had obtained shortly before in the self-fertilized bean plant. He was simply sorting out different genetic lines as Johannsen had done in beans, and Hansen before him in yeast, but in addition Shull's plants were highly heterozygous before inbreeding. It was shown later that these self-fertilized lines could be reduced to uniformity and constancy, and separated into different genotypes by progeny-testing. Since his corn plants were weak and low in productivity in the inbred condition, Shull proposed that they be crossed each generation to obtain maximum growth and yield. Crossing was easily done with maize by pulling out tassels before pollen was shed from plants grown in a seed-producing field, as Beal had done thirty years previously.
This novel idea was developed further by Shull in several articles in the Proceedings of the American Breeders Association and received much attention from agronomists and plant-breeders in all parts of the world. Shull did much more than Beal or Holden had done. He showed why it was important to inbreed first, to control and fix superior germplasm, before crossing, to obtain maximum vigor.
|2Holden in a letter stated that East was one of his students. I never heard East mention Holden as one of his teachers at the University of Illinois, although he did mention Hopkins and Burrill.|
In the meantime, a student of Holden at the Illinois College of Agriculture, Edward East,2 also began an experiment on the effects of inbreeding. He self-fertilized corn plants by Beal's method of using paper bags in the same year, 1905, that Shull self-fertilized his plants. East, who had graduated as a chemist at the University of Illinois, was employed at the Illinois Experiment Station to analyze ears of corn for protein and oil in an experiment to improve the commercial and feeding value of corn. In this selection experiment, designed and carried out under the direction of Cyril G. Hopkins, head of the Department of Soils and Agronomy, naturally pollinated ears of Burr White corn were analyzed and selected for high and low protein, and high and low oil in the kernels. After this experiment had been underway for several generations, it was noted that the yields, compared to the original unselected variety, were decreasing.
East studied the pedigrees of the selected lines and noted that all of the high-protein selections traced back to one plant at the start. He recognized that they were getting an indirect inbreeding effect by their close selection, he then proposed to Hopkins that they should make a further, more direct study of inbreeding. Hopkins was not interested in this, as Beal, Holden, and Sharnel had already shown that inbreeding was detrimental to corn. Fortunately, on his own initiative and without the approval of his chief, East started an inbreeding experiment of his own.
For his material East selected a different variety, a high-yielding strain of yellow dent corn known as Chester's Leaming. Yellow corn had been recognized as having higher feeding value than white corn and mid-western farmers were rapidly changing from the white to the yellow grain in spite of the fact that the experiment station chemists could find no differences at that time. All of the Illinois inbreeding and selection experiments, up to that time, had been made with white corn, using a variety known as Burr White.
When federal funds for fundamental research became available to the state experimental stations, East left Illinois to work at the Connecticut Agricultural Experiment Station at New Haven (September 1905). He left before his hand-pollinated, self-fertilized ears of Leaming were mature. He asked his colleague, H. H. Love, to harvest these for him and send them to New Haven when they were properly matured. These self-fertilized lines have been grown and self-fertilized continuously in Connecticut to the present time. Three of them, out of sixteen or more original lines, have survived and have now been self-fertilized fifty generations.
When Shull presented his classical report to the American Breeders Association meeting in Washington, D.C., in January 1908, under the title, "The Composition of a Field of Maize," East was in the audience and recognized immediately the importance of Shull's findings. By this time East's inbred lines had been selfed twice, but no intercrosses had been grown. However, a few crosses of these inbred lines had been made in 1907. These were grown in 1908 and in the following years. Crosses were also made between various inbred lines out of white and yellow dent, flint, pop, sweet, and floury maize. Some of these crosses were made between inbreds out of the same varieties. East confirmed Shull's findings and showed that very high yields could be obtained from crosses of certain inbreds from different varieties of corn. All of Shull's inbreds were out of one variety.
Naturally, there was much interest in this revolutionary new method of corn breeding that promised so much from a theoretical viewpoint: first, control over the heredity from both the male and female parents; and second, maximum hybrid vigor. The first crosses of inbreds which had become adapted to eastern conditions failed miserably in the mid-west and did not convince western corn-breeders that this method was practicable. Moreover, both East and his assistant and later successor at the Connecticut Station, F. K. Hayes, found that there were serious handicaps in the utilization of first-generation crosses of selfed lines, The inbred plants were so small and weak that seed yields were low and the kernels produced on inbred plants, although cross-fertilized, were so small and poorly shaped that they could not be handled in farmers' planting machines, and the young seedlings started with a handicap compared with large well-nourished seeds from a vigorous plant. After repeated trials both East and Hayes gave up trying to use crosses of inbreds for commercial production and reverted to Beal's method of making varietal crosses.
One further step was needed. By combining four inbreds in two successive crosses in what I have called a "Double Cross," all the handicaps were overcome, maximum hybrid vigor was not only maintained but actually increased because more diverse germplasms could be brought together. A further advantage was added in genetic homeostasis, a factor not fully recognized at the time but now more generally appreciated. The variability of the double cross as compared with the germinal uniformity of the single cross, at first considered to be a serious objection, is of great importance in giving wide adaptability to corn hybrids. The double-cross method is now being used with many cross-fertilized plants and with animals. It also has great promise for forest trees and in a modified form can be used to advantage with self-fertilized crops to give an initial heterozygosity and heterosis that is perpetuated by self-fertilization as long as possible.
While seedsmen and corn-growers were slow to recognize the values in hybrid corn, in time superior germplasm was found in a relatively few highly selected inbreds, as I predicted in a paper in 1920: "...the first-generation cross between inbred strains is handicapped at the outset of its growth. A means is at hand to overcome this. Even if many strains are found which give no appreciable increase in yield over the original variety, this does not vitiate the value of this method of selection, if sometime and somehow superb germplasm can be isolated which will give greatly increased yieldsÉ" After thorough testing in many different combinations, the most successful double crosses are now being grown over very wide areas in the United States and other countries. Some varieties are being grown from Maine to Virginia and west to the Missouri River; others produce well in the coastal plains of the Carolinas, the San Joaquin Valley of California, the irrigated valleys of Spain, Italy, Israel, and India.
Thus, basic knowledge of genetics contributed by Darwin, Johannsen, Shull, and East, plus a small invention of my own, the multiple cross, has made hybrid corn possible where the practical farmer, seedsmen, and many agricultural researchers did not succeed. These practical men did not succeed and possibly never would have succeeded without a knowledge of basic genetic principles. It is proof of the adage that a "practical man is often one who continues to practise the errors of his forefathers."
In the United States, where we formerly planted from 100 to 120 million acres of corn each year, we now plant less than 85 million acres and produce regularly more corn than ever. One of our serious problems now is over-production.
Not only has basic research revolutionized corn-production, but it has shown the seedsmen how to eliminate the cost of detasseling by utilizing a new principle of cytoplasmic pollen-abortion with restoring genes. This new method not only reduces costs but also eliminates the serious problem of finding adequate help to pull out tassels which has to be done in all kinds of weather and sometimes in areas where little labor of this type is available. By this new method of emasculation more seed is produced from the same genetic type of seed-parent plants and the farmer obtains still larger yields.
This method of using a sterile seed parent with or without restoring genes in the pollinator is being extended rapidly to many other crops in many different plant families. Other methods of utilizing biological information are available to be used in cross- and self-fertilized plants. I refer to the ring chromosomes as exemplified in Oenothera and apomixis as employed by the dandelion (Taraxacum) and many grasses. Nature uses these methods to obtain heterosis in self-fertilized plants. Why should we not use the same methods? The principles involved in cytoplasm and gene interaction, I believe, also help us to understand speciation. Thus basic research has made possible increased production and applied research has helped to clarify one of the most elusive problems in the whole realm of biology.