THE EFFECT OF INBREEDING UPON CORN. All of the main types of corn such as dent, flint, sweet, pop and flour corn have been inbred by self-fertilization for several successive generations. The results have been the same in general for all types. Particular attention has been given to several strains resulting from a variety of Leaming grown originally in central Illinois. Inbreeding was started by Dr. E. M. East in 1905. Four lines descending from three individual plants at the start have been continued to the present time under the direction of Dr. H. K. Hayes and later by the writers, and in 1923 they had been inbred by seventeen successive self-fertilizations. The results obtained have been reported from time to time. Particular reference is made to "Inbreeding in Corn" and the "Distinction between Development and Heredity in Inbreeding" by East, published in the report of the Connecticut Agricultural Station and in the American Naturalist, and "Heterozygosis in Evolution and in Plant Breeding" by East and Hayes in a Bureau of Plant Industry Bulletin. Later results are given in a bulletin of the Connecticut Agricultural Station under the title of "The Effects of Inbreeding and Crossbreeding on Development" and the "Attainment of Homozygosity in Inbred Strains of Maize" in Genetics by the senior writer. As the method of selection in self-fertilized lines TABLE I. Yield and Height of Four Inbred Leaming Strains of Corn Self-Fertilized Seventeen Generations. has been the direct outgrowth of these investigations as to the effects of inbreeding, a brief resumé of the results obtained to date will be given here. The method of inbreeding followed in the earlier experiments was to self-pollinate a number of plants at random and use one of these as the progenitor for the following generation. Such a family descending from a single self-fertilized plant in each generation is called a line or strain. The yield of grain and height of plant of four lines from Leaming during seventeen successive selffertilized generations compared to the non-inbred variety are given in Table I. The four lines A, B, C, D, were derived at the start from three different plants. One of these was separated in the third generation into two lines, B and C. These have been continued separately since. Other lines were started from the same variety but have since been lost on account of failure to secure self-pollinated seed. In some cases this loss has been accidental, but for the most part these strains were maintained previous to their extinction with great difficulty and showed a much greater reduction in growth and vigor than the other strains which survived. Although there is wide variation in yield of grain and height of plant from year to year the general direction is downward. After the ninth generation size and productiveness have remained on about the same level. The original variety yielded at the rate of eighty-eight bushels per acre the year it was first self-fertilized. In 1916 seed of the same variety was obtained from the original source and grown in comparison with these strains, then in the ninth or tenth generation. On account of its change to a new location under conditions to which it was not as well adapted as the inbred strains, which had been grown there for many years, no strict comparison can be made. In spite of their possible advantage the inbred strains were only from one-half to one-third as productive and were also noticeably reduced in height. This decrease in yield which results from a reduction in size of all parts of the plant and a lessening of the growth rate has so far been the universal result of inbreeding corn as far as known to the writers. Several hundred self-fertilized strains have been grown long enough to bring this out clearly. Accompanying the lessening of productiveness and growth vigor there has been a reduction in variability. From a variety that showed the usual variation in height, color of silks, glumes and leaf sheaths, number of ears, position of the ear and other details in all parts of the plants there resulted in the four self-fertilized lines a marked uniformity among all of the plants within each line. This similarity in type became noticeable in the earlier generations of inbreeding, and after seven or eight successive self-fertilizations every plant in any one line was as much like every other plant in that line as any two plants in a naturally self-fertilized species, such as wheat or tobacco, from seed from the same individual. In other words, the vari ability that resulted from the recombination of hereditary factors was in time eliminated. Figure 18. Two inbred strains from the same variety of flint corn, one with many tillers and the other without any. In Where the original variety had some plants with colored silks and others with uncolored, some of the lines now have all their plants with red silks while in others all the silks are green. some lines the foliage on all the plants is a bright glossy green, in others a dull bluish green. All the plants of one of the lines remain green and stand firmly erect throughout the season while in other lines the foliage turns yellow towards the end of the growing season and in still another the plants frequently go down on account of a weak root system. Differences in susceptibility to smut are shown in these four strains as brought out in table II. In every detail of structure of the plant, including tassel and ears, all the individuals of one line are remarkably alike and noticeably different from the other lines. Some of these differences are shown in the accompanying illustrations, figures 17, 18 and 19. The uniformity within the line and the differences between the several lines are brought out statistically in tables III to VI, which show the height of plant, length of ear, number of nodes and rows of grain on the ear for the original variety and the four strains derived from this variety. TABLE II. Per cent. of Plants Showing Smut Infestation in Four Inbred Leaming Strains. During the early generations of self-fertilization various forms of abnormalities appeared. The most frequent of these are seedlings wholly or partially lacking in chlorophyll, various types of striped plants, golden plants, dwarfs, plants with ears showing many poorly developed and aborted seeds, and others with sterile tassels and ears. These are a few of the more strikingly aberrant types. Some of these are able to produce seed and when self-fertilized come true to their abnormal condition. Others are wholly incapable of reproduction and are eliminated, but the inbred strains in which they appear may continue to produce them regularly as part of their offspring in the following generations. After several generations these abnormalities are usually no longer produced and the remaining plants are all normal in type although reduced in size and in rapidity of growth. Many of the abnormal forms which appear in large numbers in the inbred families are occasionally seen in fields of corn which have never been artificially selffertilized. Obviously, inbreeding is not responsible for their creation. They are recessive in mode of inheritance; that is, when crossed with other plants the following generation is all normal but the abnormality reappears in the subsequent generations. |