In common genetic practice we cannot know of the existence of any particular gene unless we have two recognizably divergent forms (alleles). Much of the logic used in past genetic terminology was derived before and during the period when radiation was thought to be a major cause of genetic changes. Many mutant changes were dramatic clear-cut loss of function and often clusters of both related and unrelated functions and behaved as recessive alleles which were mostly dead ends. This was true because, especially ionizing radiation was an essentially destructive process that caused breakage and rearrangement or deletion of chromosome pieces carrying the functional genes. The discovery of transposable elements that transformed the function of components of DNA in a non-destructive way and the finding of chemical reagents which induced discrete changes in the DNA molecule at high frequencies made it possible to obtain large numbers of heritable changes. Because of the nature of the DNA molecule these changes are limitless in variety and provide the evolutionary basis for biology.

It has been the privilege and good fortune of the senior author to have begun his career preparation during the pinnacle of discovery of radiation biology with its master, L J Stadler; to have followed in his footsteps and logic and eventually inherit his outstanding research project with its world class resources, traditions and goals; to have continued his own research career into the mutagenic behavior of transposable elements under the inspiration of Barbara McClintock; to have perfected a technique for chemical mutagenesis in maize which has produced the largest collection of maize mutants in existence; and to have spent the remainder of his career recording, evaluating and making good use of this marvelous resource. This and many other exciting tangential projects became available as a result of these opportunities. It was also a special advantage to have been involved in the collection and preparation of a photographic record of the phenotypes of all the early collection of naturally occurring and induced mutations in Maize with Loring Jones and Marcus Zuber and later with Ed Coe and Sue Wessler and presenting them in two editions of “The Mutants of Maize”. The opportunity to be intimately involved with cutting-edge research projects and with highly skilled colleagues working in cytogenetic, Ionizing radiation, transposon behavior and chemical manipulation of DNA all during the “Golden Era” of genetic discovery was especially rewarding. Finally, having made a good photo record of most of the mutant phenotypes over many years has provided the basis for this presentation. It all began with a Doctoral Thesis about the analysis of the progeny of a single unique colorless kernel with many purple dots on its surface which proved to be transposon activity at the A1 locus produced by the activator Dt1. This collection consists of the best images of a photographic record of all the mutant phenotypes that it was possible to capture in the many experiments from then until now.

Earlier studies of gene mutation based on rare wide ranging single naturally occurring events and more frequent mostly destructive radiation induced cytological events produced new phenotypes which only partially reflected the broad range, intricacy and potential value built into the DNA molecule. Having large and repetitive numbers of discrete genetic change afforded by the discovery of the mutagenic activity of ionizing radiation (Stadler) (E G Anderson) then UV light (Stadler and Uber) then Transposable elements in the genetics system (McClintock) and finally the use of chemical reagents to directly change DNA configuration discretely in the germ line of the maize plant (Neuffer) has made it possible to have a wide array of mutant phenotypes that can be directly tied to mutational events of specific genes. It helped a great deal to have had the foresight and resources to make a good photographic record of a majority of those events.

The treatment experiments were initially designed to test the effect on targeted well known genes that could be followed through succeeding generations so that the changed phenotypes could be objectively observed and recorded. Genes affecting anthocyanin pigments such as A1 and kernel structure such as Sh2 were selected for ease of handling. Targeting of genes in transposon studies was achieved by starting at known sites and moving to other selected loci. The frequency of events which appeared to be quite high was confused by the fact that they occurred at many stages of development of the germ lineages often having mutant sectors which produced multiple copies of a single event that could not be distinguished from single events in the test M2 progenies. The frequency as a result of transposon activity appeared to be very high when measured in the M2 but was not accurate because events happening during somatic cell divisions were duplicated many times before they were recorded. The rate per cell generation is quite low when multiplied by the number cell generations in the reproductive life of the plant. Also transposons rarely ever produce alleles that were dominant to wild type in the M1.

The frequency from chemical mutagenesis using the parafin oil pollen method was also very high and could be accurately measured in both the M1 and M2 because the pollen grain at the moment of treatment contains only single germ cells each of which may become an individual M1 plant and an unequivocal progeny to be counted. In addition the chemical reagents did produce substantial numbers of mutants that were dominant to wild type and therefore easily seen in the M1 as whole plant cases and conveniently as half plant chimeras which permitted observation and preservation of lethal dominant mutants. The actual frequency of mutation from treatment of maize pollen with EMS (ethyl methane sulfonate) ranged widely depending treatment stocks, conditions, procedures, loci, etc. Our “rule of thumb” for a good average treatment was: 1 recessive mutant, per gene locus, per 1000 treated pollen grains that achieved fertilization. For dominant mutants it is, 1 per 200,000 in the M1 which is the average including a few like Oy1 (which have a uniquely high “recessive” rate of 1/1000). The fact that all the genetic components in the rest of the genome were randomly subjected to the same powerful force for change had serious consequences. Literally hundreds of new visible mutants (and unknown numbers of invisible cases) were produced in M1 treatment populations of as small as 3000. This lead us to change our approach from the study of selected individuals to mutation in general.

The Author has had the pleasure of working with all the above forms of manipulation of the Maize genome and has collected mutant data and images from all but the “MU Transposon System”, which was treated with great respect, and deliberately kept out of our fields as though it had something that could be catching. All of this information has been brought together over the years in our own local Mutant Database (current stored as a Microsoft Access database) which consists of a case number, symbol, name, phenotype, linkage, treatment, origin, description, seed source, photo images and other pertinent data. We periodically share our latest copies of this database with colleagues at MaizeGDB. This wiki links to data at MaizeGDB, including much of the data in Mutants of Maize. MaizeGDB, in turn, links to the Coop stock center, making it easy for interested parties to get access to seeds of the described mutants.

Sources; no implication of frequency just opportunity occurrences

              Spontaneous and unknown-------------------------------
              Radiation; Xrays, Nuclear Ionizing, Ultraviolet-----------
              Transposon;  Dt, Ac, Spm, Mr, Rmb, Rst, Mu---------
              Chemical; EMS, NG------------------------------------------
              Other (ploidy, cytogenetic consequences, etc)------

A collection of images of about 800 of the best mutants is presented at this web site as a guide to current maize mutant phenotypes. It is prepared with simple definitions and captions that will be clear and useful to the beginning student of Maize, to the worker in the corn field, to the graduate student, post doc, biologist and the seasoned maize geneticist; to give each a clearer understanding of this especially useful plant and how it provides special insight into all biology. The photos which were taken by an amateur photographer with an ordinary camera and processed by modern electronic means, are high resolution and can be viewed in the classroom, laboratory or field using computer, tablet or even smart phone. The images can be used to get a general view or enlarged to be seen at the cellular level and can be simply printed out for paper viewing or transmitted electronically as needed.

Having access to such a large collection of mutants with multiple copies of many genes and their associated phenotypes gives the observer a different perspective than that derived by observing rare single unique events. Having a single mutant event expressed as a definitive phenotype is like viewing a single frame of a motion picture film to determine the characteristics of the object framed. Each additional copy gives valuable data which leads to the complete picture. Each clue is very valuable but useful only to the extent of other knowledge available to the viewer. The purpose of this Guide is to provide as many visual clues as possible to the interested worker who may be looking at corn for whatever purpose. It also provides keyed access to the much larger collection of similar examples found with much more comprehensive data at MaizeGDB.

When viewing new mutant phenotypes we have found that there are many aspects to consider. It is true that while things that pertain to the welfare or survival of an organism may or may not have an impact on phenotypic expression there are many things that do in peculiar ways. Reporting some of these may be helpful to the user of this Guide. We add them here as text topics separately from the definitions and captions accompanying the images.

Visual expressions which appear to be phenotypes but are not of genetic origin: Abrupt changes in the appearance of single plants or members of a population that are unique often occur but on testing turn out to be the result of non genetic factors such as yellow leaf stripe (iron or nitrogen soil deficiency). Viviparous; premature kernel germination (fungal infection of the kernel). Morphological changes such as twinning of growth at the nodes and abnormal structures (smut infection). Mutations causing leaf lesions, chlorophyll streaks, or strong anthocyanin pigmentation each have a counterpart phenotype produced as symptoms of a disease caused by a specific pathogenic organism, insect pest or unusual environmental event. The truth of the matter in each case requires a pedigree analysis for proof and the results often provide interesting and valuable insight into the detailed workings of Biology. Note, I do have good photo image examples of each of these if we want to use them!

Variation in phenotypic expression: Ideally one might expect a fairly consistent relationship between phenotypic expression and each causal genetic event but reviewing large numbers of mutations (including both numbers of loci and repeats of single loci) has shown that such is not the case. Duplicate factor genes which generally appear alike usually produce mutants that are demonstratively different from each other. Likewise repeated mutants of the same locus appear in different allelic forms either because they are different or because they have different immediately adjacent neighboring genes .

Some loci have mutants that regularly express variations of only one phenotype (i.e. white seedling) but some mutant genes have multiple phenotypes. For example repeated mutants at the clf1 locus on the short arm of chromosome #1 have four distinct phenotypes: a colorless (no anthocyanin) kernel, a white floury endosperm, a small round defective embryo and an albino (can only be seen as chimera loss in DsClf/clf leaf tissue) plant.

The phenotype of induced dominant mutant alleles of most genes are highly responsive to many different genetic modifiers found in diverse breeding stocks such as Mo17, Mo20w, B73, and W23 where outcross will produce enhancement, suppression, or even elimination of the phenotype depending on the mutant gene and stock. For example Les1 produces very few lesions when crossed on Mo20w but an almost lethal profusion of lesions on W23. Almost the reverse is true with when Les10 is crossed on the same stocks. Similarly when certain recessive lethal defective kernel mutant heterozygote’s are selfed there are recessive segregants in some of the F2s that are viable and produce viable though modified mutant plants. This is probably true because, especially, the elite inbreds all carry an assortment of dominant alleles of genes that promote vigor, yield etc. or cover weak alleles when making hybrids.

We were able to produce good numbers of valid mutants when treating corn kernels with EMS but found that the difficulty of identifying and validating individual cases made it much less efficient so we abandoned that effort. It is apparent that seed and organ treatment can be successful in other creatures whose male gametes are not as convenient as those in corn.

We were surprised at the large number of lesion mimic mutants obtained ( ). Our rough estimate was that more than 200 such loci are available for mutation. Also there were a few loci in which the dominant rate (1/1000) was in the same range as the common recessive rate; Oy1 for example which also has only one site in the corn genome. Another surprise was the fact that there were 19 different hcf (Miles) loci with mutants affecting a single operation: the transport of a single electron across the chloroplast membrane. There were many other interesting things that have been or still are available for inquiry. Soon after we recognized the great value of our material and had accumulated sizable M1 and M2 populations we knew that we could not handle even a small part of the opportunities provided. We decided to share with others and invited them with the statement: “ If you have interest in the genetic control of any biological function and can imagine what its phenotype will be, then come and sample our M2 population of selfed ears because it will be there. Our calculations are that if you take a 30 seed sample from 3000 or more ears and test them you will have a 95% chance that it will be there.” Several colleagues did so and they were never disappointed.

Our considerable experience and collected data clearly show that each of the mutagenic agent used produced a different and unique spectrum of heritable and non-heritable phenotypic events:

• UV light; for which we have only a little information added to L J Stadler’s personal communications and publications produced mostly discrete point mutations and some cytological aberrations.
• X-rays; Our results substantiate that of Stadler and others who found that this type of radiation caused mostly destructive loss and re-arrangement of chromatin material carrying the genes in blocks and

pieces many of which were heritable as such and produced phenotypical variations often associated with cytogenetic changes transposon induction and Hodgkin’s Lymphoma in humans which caused Stadler’s untimely death.

• Atomic radiation; from mutant cases and data provided to the maize genetics community by E G Anderson and students from the Bikini Atom Bomb Test; through MaizeGDB. The mutants we used were the most likely point mutations from a broader spectrum than that produced by from X-rays because the treated material received a much wider range of ionizing wave lengths.

• Tritiated Thymidine; no data or images because treatment failed due to failure to reach the pollen nuclei.

• Transposon System; most of our research was conducted with the a1-m, Dt system; a variation of the a1 Dt system from M Rhoades and the Ms system in our stocks which apparently is actually Mc Clintock’s Ac-Ds system.