From Out of Old Fields Comes All This New Corn: An Historical Perspective on Heterosis in Plant Improvement
Farming has always depended upon human-induced modifications of the natural world and heterosis, meaning to alter in Greek, is a prime example of technology in the service of agriculture. Unlike many key biotechnologies, however, its biological underpinnings remain poorly understood. Heterosis, a manifestation of the superiority of F1 performance relative to parental performance, is fundamentally concerned with inbreeding and outbreeding. The significance of outbreeding in the development of superior animals such as the mule has been noted since Mosaic times. Religious and moral teachings regarding human inbreeding likely preceded observations drawn from the experience of animal domestication by pastoral nations. Although relatively close inbreeding took place in early Egyptian and Greek cultures and among ruling classes in European societies, recognition of an incest taboo was commonplace in many of the world's cultures. By the 18th century, hybridizers were careful to note the paradox between the dangers of close inbreeding and wide outbreeding. Koelreuter was the first hybridizer to comment upon heterosis and the first to suggest it resulted from outbreeding. Knight, Mendel, and Darwin made significant contributions to the understanding of these phenomena during the 19th century, however they did not recognize the beneficial effects of outbreeding as opposite to the detrimental effects of inbreeding. Beal was influenced by Darwin and among the first to report increased yields with hybridization. Beal also trained Davenport and Holden who, along with East at the University of Illinois in 1900 pioneered inbreeding theory. Prior to this period, the primary aim of breeding programs was avoidance of inbreeding; thus hybrid vigor was said to work via the prevention of self-pollination. Breeders in this period focused on mass selection as inspired by Darwin's selection principle. Inspiration for the study of inbreeding may have come indirectly from an association between East and the botanist Hottes, recently back from study in Europe with Correns. The rediscovery of Mendel's laws by Correns and others may have influenced East to think of line purification from inbreeding. It was not until a landmark article by Shull in 1908 that the fundamental principle of inbreeding and its role in heterosis was clarified. In this work, Shull argued persuasively that selfing isolated homozygous lines and that these lines could be crossed to capitalize on heterosis; a fact that influenced East and helped to overturn the negative image of inbreeding to a positive benefit obtained by hybridizing inbreds. Shull coined the word heterosis in 1914. Commercially feasible F1 corn hybrids were developed following Jones' (East's student) 1918 proposition of the double cross. Since the 1930s, exploitation of heterosis has spread to include many other crop plants. For many crops, the full benefit of heterosis has yet to be realized because inbreeding efforts are still in their infancy from a genetic point of view. Although some have criticized the move toward F1 hybrids as driven more by profit rather than science, this argument does not address other potential benefits of Fl hybrids such as uniformity. Use of heterosis has fostered the development of a worldwide seed industry and dramatically altered the landscape of professional plant breeding, undoubtedly making a substantial contribution to research and development efforts in this discipline. The success of F1 hybrid technology has affected increases in food production in many regions of the world.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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