90 years ago: the beginning of hybrid maize.

IN early 1908, George Harrison Shull, then at the treatment was thorough, careful, and thoughtful. His Cold Spring Harbor Laboratory, published a paper book, The Effects of Cross and Self Fertilization in the Vegetable with the unimposing title, “The composition of a field Kingdom, was published in 1876. Its 490 pages include of maize.” This marked the beginning of the exploitamyriad examples, with the overall conclusion that intion of heterosis in plant breeding, surely one of genetbreeding is generally deleterious and cross-fertilization ics’ greatest triumphs. It is appropriate, on this 90th generally beneficial (Darwin 1876). As pure descripanniversary, to look once again at Shull’s great contrition, his writings could hardly be improved upon, but an bution and its sequelae. interpretation had to await the rediscovery of Mendel’s The increased size and vigor of hybrids between plant laws in 1900. varieties and species had been known for centuries. A The development of hybrid maize: In his 1908 paper, well-known example of hybrid luxuriance was found Shull reported that inbred lines of maize showed genin crosses between two species of Jimson weed, Datura eral deterioration in yield and vigor, but that hybrids stramonium and D. tatula, in which the hybrids were twice between two inbreds immediately and completely recovas tall as either parent. The most spectacular example ered (Shull 1908); in many cases their yield exceeded that I know of is the radish-cabbage hybrid; a single that of the varieties from which the inbreds were deplant filled a greenhouse and grew out the roof (East rived. Furthermore, they had a highly desirable uniforand Jones 1919, p. 192). It was sterile, however, and mity. In a subsequent paper in 1909, he outlined the only in this century were fertile derivatives obtained by procedures that later became standard in corn-breeding polyploidization. A time-honored showpiece of hybrid programs (Shull 1909). vigor is the mule, also sterile, but known for 4000 years At the same time, E. M. East did similar experiments for its hardiness and longevity. Even in ancient times it at Connecticut State College. He also recognized the was widely known that inbreeding leads to weakness and deleterious effects of inbreeding, but didn’t realize the small size. The avoidance of incest was practiced by value of crossing inbred lines. Breeding weak parents many human societies, and there was speculation about held no appeal for him until he heard Shull’s report such behavior in animals. Zirkle (Gowen 1952, p. 5) in January 1908. In February, he wrote a letter to Shull, quotes a fanciful mixture of science and myth from saying: “Since studying your paper, I agree entirely with none other than Aristotle. your conclusion, and wonder why I have been so stupid as not to see the fact myself.” His report (East 1908) was A story goes that the king of Scythia had a highly-bred mare, and that all her foals were splendid; that wishing characteristically generous to Shull, and he added conto mate the best of the young males with the mother, he siderable corroborating evidence (Gowen 1952, p. 17). had him brought to the stall for the purpose; that the Nevertheless, East was not convinced of the useyoung horse declined; that, after the mother’s head had fulness of the idea, because the puny inbred lines probeen concealed in a wrapper he, in ignorance, had interduced such small quantities of seed. The great cost of course; and that, when immediately afterwards the wrapper was removed and the head of the mare was rendered seed, he thought, negated any increased yield of the visible, the young horse ran away and hurled himself hybrids. This led to a strong disagreement with Shull, down a precipice. but in 1910 they agreed not to let this become an open debate nor to let personalities intrude. They remained The literature of the nineteenth century is full of true to their word. examples, mostly from plants. As usual, Charles DarThe limitation of poor seed production from inbred win got in the act. In his words, “Nature thus tells us, lines was overcome by an idea from D. F. Jones (1918, in the most emphatic manner, that she abhors perpetual self-fertilization.” And, as we have come to expect, his 1922), who, while still a graduate student, advocated