of the Kansas Academy of Science 71(3): 373-378 (Autumn, 1968)
What We do not Know About Zea mays*
Missouri Botanical Garden, St. Louis, Missouri
|*Presented at the 100th Annual Meeting of the Kansas Academy of Science. University of Kansas, Lawrence, Kansas, May 3, 1968.|
During the years when research on corn and its relatives was my main interest, I was often introduced to audiences as "An authority on corn," to which I usually responded that I was NOT an authority on corn, but an authority on what was not known about it.
I meant every word. There were real authorities on corn; men who had spent their lives studying it: H. K. Hayes, R. A. Emerson, L. J. Standler, Donald F. Jones, E. G. Anderson, Paul Mangelsdorf, Ralph Singleton. I knew these men well and respected them greatly. All but E. G. Anderson were students of E. M. East, and he was one also at second hand, as a student of Emerson.
E. M. East, one of the two men who discovered the inbred-hybrid breeding method and the one who continued analyzing heterosis in cornbreeding plots, was the scholar with whom I took my doctorate, but I worked with him on Nicotiana rather than on corn.
After graduate work at Harvard, I came to the Missouri Botanical Garden and Washington University as a geneticist. The main interests of my colleagues and students were in taxonomy, morphology and physiology. I was keenly interested in natural history and the then little known Ozarks were at our back door. I taught genetics, but I explored the Ozarks with my students. They learned about genetics from me, and they convinced me that I should take a serious interest in taxonomy. With them and with other colleagues and students I have studied it ever since. We really explored together much more than the Ozarks; we explored the wide field between genetics and taxonomy, then a terra incognita. I began to study the species problem. I began to think about the importance of back-crossing in wildflowers and ornamental shrubs. I took a keen interest in the effects of heterosis outside of corn-breeding plots. With my students, I helped originate what is now called "biosystematics."
All this time I kept up a lively, personal interest in corn breeding and corn genetics. It was natural, therefore, that when the Pioneer HiBred Corn Company gave me the opportunity to study corn in any way I pleased, that I turned to problems suggested by my associations with taxonomists, morphologists, and students of the Ozark flora. I became interested in such unexplored fields as:
Let us review that history briefly. It was published in full, Anderson and Brown (1952), but in an historical journal and has not yet found its way into many elementary text books.
Southern planters from Virginia to Louisiana discovered the productive white dent corns of Mexico, some of them with grains so flat and white that they looked like the seeds of a squash and hence were called "gourd seeds." We know now to our sorrow, that just crossing any two different kinds of corn does not produce the abundant vigor that makes hybrid corn a possibility. However, crosses between the golden yellow flints our pioneers got from the Indians and Gourd-seeds have great hybrid vigor. By 1850, the mixing was actively under way from Pennsylvania to Iowa and south to the Gulf States. Thirty years later, the golden dents of the U.S. Cornbelt were the World's most important source of human and animal food; their origin, the gift of scores of hard-pressed pioneers, had been forgotten. Dr. William L. Brown and I had to fit the pieces together before we finally got the complete story of this greatest reservoir of heterosis in Zea mays.
The authorities on maize were sometimes puzzled or amused by these and various other kinds of our research with maize, but to a man they helped the program. Above all, they kept the pages of Maize Genetics Cooperation open for short accounts of work under way by my students and me.
Looking back at these studies, the one central theme seems to have been heterosis, hybrid vigor. In maize, its distribution is far from random, therefore I still distrust corn-breeding theories based on randomness, such as those advanced by Dobzhansky and other Drosophila geneticists at the Ames Conference on Heterosis (1952).
I suggested (1944) on detailed, statistically analyzed, cytological evidence that at least one other genus beside Zea and Tripsacum was involved in the evolution of maize as we knew it. I developed an original, basically simple mathematical method for predicting the numbers and kinds of multivalent associations to be expected in such complex pollen mother cells that I believe could be adapted to other problems concerning complex polyploids. Recently Galinat, Chavanti, and Hager (1967) reported new evidence pointing to Manisuris as the third genus. Galinat has since then published further observations suggesting that the mixing of these diverse genera may in part have begun before domestication. Meanwhile, Wilkes has published abundant evidence that Teosinte has long been growing wild over considerable areas in western Mexico, and that the pattern of its interaction with maize is complex but not chaotic. All these advances in our understanding have continued to keep me interested in finding out more about heterosis in maize.
I have never published my most critical and objective evidence. It was the by-product of a series of species crosses in Aquilegia (the columbines of our flower gardens), though after describing that evidence, I shall have confirmatory facts from corn-breeding. The columbine data are unique in the light they throw on this important problem, but the basic facts are simple.
In 1949-1950, I held a fellowship in England under the general direction of J. B. S. Haldane. I worked 11 months at the John Innes Horticultural Institution, then at the height of its influence in cytogenetics. My work was chiefly on Aquilegia and Iris, which I had been studying in Missouri. Under the informal atmosphere of "the old John Innes" I also participated in various projects long under way there. At Haldane's suggestion, I spent February and the following September at Rothampstead with R. A. Fisher, analyzing my statistics on variation between and within Iris species. In June, mid-way between these two sessions, I went up to Rothampstead to consult Dr. Fisher, who was then much interested in theories about the evolution of dominance. The Aquilegias which Miss Schafer of the John Innes and I were studying together, were then in full flower, and it struck me that our columbines could provide material of unusual interest to Fisher. I chose the three most different species in our collection (or in the entire genus for that matter) and their three F1 hybrids. One was the plain, blue Aquilegia vulgaris of Europe; another was the long-spurred, yellow columbine A. longissima, the species which produced all our "long spur hybrid columbines" when an English woman crossed it with other species. The third was a strange little plant from the Himalayas, A. ecalcarata. In various ways it reminded one of a meadow-rue (Thalictrum). It had even been placed in another genus when first discovered, but since it hybridized readily with other Aquilegias, producing partially fertile hybrids, it is now generally referred to as an Aquilegia.
Figure 1 displays this evidence rhombically as in the paper Miss Schafer and I published about these six plants and their overall resemblances and differences.
When I laid out the freshly gathered columbines on Fisher's desk, he studied them eagerly. Nothing delighted him more than new kinds of evidence on a problem he was working on. I suggested that he try to sort out the three parents and their three hybrids, and he worked at it for sometime before giving up the attempt. When I laid them out as in the diagram, he studied them until lunch, asking many intelligent questions.
The next day at tea time, back at the John Innes, I reported the whole matter. Several of the staff were amused, possibly because Fisher's new ways of studying dominance were outside their habits of thought.
When tea was over, Miss Dorothea DeWinton, one of the ablest members of the staff, who had worked closely with both William Bateson and J. B. S. Haldane, came up and asked if I would show her these six kinds of Aquilegias. They were growing in a corner of the experimental plots remote from her laboratory. She laid them out one at a time on her work bench in a good light and examined them carefully. She picked up one or two to compare with each other or with another of the six. Then within a few minutes she picked up the three hybrids and laid them out as spokes of a wheel (much like the diagram). With these in place, she took the three species one at a time and fitted each into its proper position. I asked her how she did it so quickly. She pointed to the three first-generation-hybrids and said, "Well obviously these have hybrid vigor. With them out of the way, the rest was easy."
|Figure 1. Three species of Aquilegia, E = A. ecalcarata, V = A. vulgaris, L = A. Iongissima. The same symbols are used (with an x to indicate hybridization) for the three F1 hybrids.|
From working with East, I was familiar with a good many examples of hybrid vigor but until that moment it had never occurred to me that there might be various subtle aspects of heterosis which, in combination with those sometimes listed, might eventually, to an able, experienced observer, be recognized (as a variable complex) almost instantly.
The speed with which Miss DeWinton could use her experienced eye with this complex material is a good hard fact, however we may interpret it. To me, it suggested strongly that there were things about heterosis that we did not yet understand theoretically. In teaching genetics I used this story as an example of what geneticists and agronomists do not know about heterosis.
While I am not a corn breeder and never taught plant breeding, several of my students are successful plant breeders, although only four of them work with maize or sorghum. In discussing with them the various "new' problems which have arisen when so many inbred lines were being "converted" rapidly, I began to return again and again to this story of Miss DeWinton. I finally wrote it up and mailed it off to several of my former students. It received serious attention from all of them, and Dr. Don Duvick had some interesting new evidence. He has had much practical experience converting established inbreds to various single-gene factors affecting animal nutrition or in adding sterility and fertility systems to eliminate de-tasseling by hand.
In intensifying and maintaining such "continuous back-crosses" there is the problem of "rogues." No matter how carefully pollinations are made, there is always the possibility that a stray pollen grain may float in, producing a hybrid seed with an unknown male parent.
If this program of back-crossing is being carried on in Iowa continuously from generation to generation, this rogue is usually a big husky plant, unlike its inbred siblings and easily eliminated before it is big enough to shed pollen. However, under pressure to produce the desired new inbred quickly, one generation is grown in Iowa, the next in Jamaica or Florida in the winter time, and so on, back and forth, until the new models have been produced and can be tried out. At such times, there may be light frosts in Iowa in late summer. They have little effect on fields of hybrid corn; weak little inbreds are another matter. Their seeds may well have been produced in Jamaica under conditions difficult for this inbred and may not have ripened off well enough to make sturdy plants in a cold Iowa spring. Hence, their improperly ripened seeds down in Jamaica the next winter may produce very weak plants. Even though they can be successfully selfed, even the rogues may not have enough heterosis so they can all be distinguished before they produce pollen. Then the precious pedigreed material has to be sent back to Iowa, where part of it is tested out and discarded if it is vigorous and hence thought to be a rogue. If it has not been contaminated, it is saved for planting in the next generation.
In carrying out this frustrating program, Dr. Duvick noticed that he apparently could recognize heterosis in these weak cultures. He accordingly entered his tentative judgments in the record book. When the voucher seedlings were grown and studied, his judgments were confirmed. He has since been busy with other matters, but he feels this problem is worth looking into.
For years, working with my students on hybrid populations of various kinds of plants, I maintained the attitude that any differences which could be seen, could be measured or scored objectively, but that learning how to do this was a research problem in itself, sometimes very discouraging in its earliest stages.
I believe heterosis deserves this kind of study. I suspect a deeper analysis of its nature has been retarded by those who felt that its makeup at the genetic level and its use in breeding programs should be learned by rote, like the multiplication table, and never speculated about. I believe Miss DeWinton's scoring of the Aquilegias and Dr. Duvick's experience with inbreds, should be carefully considered by students of heterosis.