University of Florida, Brookhaven National Laboratory and Rutgers University
Noted maize geneticist Oliver Nelson of the University of Wisconsin died in a Madison hospital November 6, 2001 after a long battle with both osteoporosis and Parkinsonís disease. Having become acutely ill that afternoon, Oliver refused transport to the hospital in order to finish the communication of a paper concerning enhanced starch synthesis to the Proceedings of the National Academy of Sciences. He died soon after reaching the hospital.
Born in Seattle, Washington, Oliver was raised in the vicinity of New Haven, Connecticut and first experienced maize genetics as a summer high school assistant in the Department of Genetics, Connecticut Agricultural Experiment Station. Oliver graduated from Colgate magna cum laude in Botany at the age of 21. He subsequently received his M.S. and Ph.D. degrees from Yale under the direction of D. F. Jones.
Oliver joined the faculty at Purdue University as an Assistant Professor in 1947 and was promoted to Professor in 1954. To the great delight of R. A. Brink, Oliver left Purdue in 1969 for the University of Wisconsin. He spent the rest of his career there, having "retired" to emeritus status in 1991.
Throughout his long career, Oliver pursued many aspects of maize genetics and biochemistry. Initially hired at Purdue as a popcorn breeder (at which he was quite successful -- some of his better lines were used in the popcorn industry long after he ended his breeding efforts), he found that most popcorn lines carried the Ga1-s allele and as such were cross-incompatible with dents, carrying the ga1 allele, when used as females. This discovery led to a systematic study of the ga1 locus. With L.R. House he showed that non-reciprocal cross-sterility was the result of poor ga1 pollen tube growth on Ga1-s silks.
Oliver soon focused on the waxy1 locus to address questions of fundamental importance to biologists of the time, as well as to test predictions about transposable elements, then in vogue. Exploiting the finding of R.A. Brink and M. Demerec that the wx1 gene functions in the gametophytic genotype, Oliver realized that pollen populations of sufficient size to detect rare recombination within the gene could be quickly examined. If independent mutations of the waxy gene occurred at different sites, he could place these relative to each other by constructing a fine structure recombination map through the occurrence of rare wild-type recombinant grains. Looking at 50,000 pollen grains at a time he first showed that no wild-type pollen grains could be observed in plants that had two copies of the same waxy allele, but that rare, blue-black staining grains could be observed at frequencies between 0 to 80 per 100,000 in plants heterozygous for two independent waxy alleles. Characteristically, he also showed for one of these combinations that he could observe the same frequency of recombination in the seed. As he used stocks with flanking markers in this important control, this also allowed him to orient the mutations he was mapping relative to the other genes on the chromosome. Even though the frequency of recombination was not additive across the locus, he was able to construct his map using overlapping deletions. All told, the sites of 31 waxy mutants were mapped in this way. Among them were a number of stable transposable element-induced alleles. Oliver showed that these were distributed throughout the locus rather than being concentrated in a control region at one end of the gene. The construction of the first fine structure map of a plant gene, plus that done with the rosy locus of Drosophila, provided us with the most detailed glimpse of the structure of a gene before DNA sequencing. When the wx1 gene was cloned, Wessler and Varagona remapped 13 of the mutants at the DNA level. An excellent correlation between the genetic and physical maps was observed.
Oliver considered himself a biochemical geneticist. He began his career at a time when there was great excitement to learn how genes worked and what they could tell us about biochemical pathways. Oliver saw no reason that such studies could not also be pursued in higher plants. Since starch was the major component of corn seed, it stood to reason that mutations of this process would have a visible mutant phenotype. He and his colleagues conducted a systematic investigation of the deficiencies of a number of kernel mutations and over the course of three decades identified the biochemical defects associated with eight starch mutants including those responsible for wx1, sh1, sh2, sh4, bt1, bt2, du1 and su1. Placing these mutants in the biochemical pathways allowed us to have a better understanding of how this process operates in the corn kernel. This accomplishment is made even more impressive when one realizes that virtually all of this was done before recombinant DNA methodology was commonplace.
The starch work produced a number of "firsts". Identification of the lesion with wx1, a starch-bound ADP-glucose glucosyl transferase, to our knowledge, represented the first case in a higher plant in which the biochemical lesion of a gene with a visible mutant phenotype was elucidated. At the pathway level, the work with wx1 provided the first and unexpected demonstration that amylose, the straight-chained glucose polymer is not a precursor for the branched polymer of glucose, amylopectin, a fact still not presented correctly in many plant physiology textbooks.
The elucidation of the enzyme associated with su1, in association with subsequent cloning and characterization of the gene in the Myers-James lab at Iowa State University, showed that debranching of alpha 1,6 bonds in starch is essential for the synthesis of wild-type levels of starch. This, too, was unexpected based on the then current view of starch synthesis. The cloning and identification of the protein associated with bt1, coupled with subsequent physiological/biochemical studies at Penn State University, provided the first definitive evidence that ADP-glucose, the precursor for starch, is synthesized in the cytosol (and not in the plastid) in the cereal endosperm.
As mentioned, Oliver's pursuit of wx1 was aimed initially at understanding the nature of the relationship between the gene and its associated transposable elements. This interest persisted throughout his career at Wisconsin. This line of investigation, to some extent with the sh2 locus, but in much more detail with the bz1 gene, also led to a number of "first" observations. These included the first reports that inserted transposable elements led to the production of a structurally altered protein. This was as expected if the elements were inserted throughout the gene as shown in his wx1 recombination investigations ó but was contrary to the idea that transposable elements act as normal regulatory elements of gene expression. Another first was the report that excision events led to a heterogeneous group of revertant alleles that displayed different protein properties, thus anticipating the alterations in coding regions created by transposon excision. These insights came before the application of recombinant DNA methodology to the study of transposable elements.
In the 80s, when the tools of molecular biology made it possible to address questions at the DNA level, Oliver's interest shifted to the transposable elements themselves. Another "first", in what immediately became classical work, his lab --in collaboration with Nina Fedoroff's-- isolated for the first time a plant gene by a then novel procedure of transposon tagging. The maize elements have been subsequently exported to other species and have revolutionized gene isolation procedures not only in maize but also in other plants. The second main contribution from Oliver's lab during his last decade of activity, and one that involved perhaps the largest number of students and postdocs that he had in his career at any one time, was the elucidation of many of the changes that these highly unstable elements are capable of undergoing.
Perhaps the greatest acclaim given to Oliver came for the work done during the period 1962 to 1969. These studies, done primarily in collaboration with Ed Mertz of Purdue University, showed that levels of the essential amino acids, lysine and tryptophan, could be enhanced by mutation. The discovery that certain amino acids were enhanced in opaque2 and floury2, was of tremendous importance to maize breeding programs; however, problems of kernel softness and yield drag precluded early use in the corn industry. Only now, through development of "modified opaque" maize, is the o2 mutation being incorporated into corn lines of commerce.
A little known fact about the early opaque2 and floury2 work exemplifies Oliverís uncanny ability at understanding biology through mutant analysis. Before the understanding of o2 and fl2, biochemists at Purdue were massively screening, with little success, maize lines for amino acid content. Oliver knew that lines selected at the University of Illinois for enhanced protein content exhibited an enhanced level of zeins. Because zeins contain little to no lysine, these lines were of little value for feeding monogastric animals. These lines also exhibited a translucent phenotype. Oliver reasoned that mutants with the opposite phenotype (opaque) might have reduced zein content. If non-zein proteins increased, lysine content would increase. Accordingly, four opaque mutants were analyzed: o1, o2, fl1 and fl2. Lysine was doubled in two of the four mutants.
In addition to his many research accomplishments, Oliver was a tireless worker for the maize genetics community. Oliver played an instrumental role in hosting the maize meetings following its move from Allerton House in central Illinois to the various sites in Wisconsin and the Chicago area. He chaired and wrote the substantially unchanged "A Standard For Maize Genetics Nomenclature". Oliver also served as Chairman of the Genetics Department at Wisconsin, an Associate Editor for Plant Physiology and devoted more than his fair share of time to grant panels in Washington.
Oliver won many prestigious awards during his career. These included an honorary Doctorate of Agriculture from Purdue University in 1973, The Thomas Hunt Morgan Medal from the Genetics Society of America in 1997, the Stephen Hales Prize from the Ameican Society of Plant Physiologists (1998), the Herbert Newby McCoy Award (1967), the John Scott Medal (1967), the Holblitzele National Award in Agricultural Sciences (1968) and the Commemorative Medal of the Federal Land Bank System (1968). He was elected to the National Academy of Sciences in 1972.
Oliver was reserved, unfailingly courteous, rigorously honest, and forthright.
He was an insightful and observant scientist. His strength was the use
of genetics - mutant phenotypes and segregational analysis - to answer
biologically and agronomically important questions. This careful and perceptive
analytical bent of his was the hallmark of his career. Oliver played an
instrumental role in maize genetics, plant biology, and the maize genetics
community. He taught by example. He provided our introduction to a life-long
career with the corn plant, and he gave us an everlasting appreciation
of the values of Mendelism and appropriate controls. Not only was he a
mentor to the three of us in the early stages of our careers, he continued
to provide guidance, advice, wisdom and an unconditional friendship to
us throughout the rest of his life. He will be missed.
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