Corn: Both a Crop and Genetic Model Species
Geneticists such as Dr. David Holding, in the Agronomy and Horticulture at the University of Nebraska, use corn in basic genetic studies that help lay the foundation for additional crop species (Fig. 1). Let’s examine one experiment.
Corn (Zea mays, maize) is not only an important crop; it has also been a valuable organism used in basic genetic studies to understand genetic principles. David Holding is a maize geneticist at the University of Nebraska-Lincoln who uses insights gained from this species to plan and carry out practical genetic improvements for popcorn and sweet corn. David Holding also has the mission to conduct experiments that generate new insights and he shares these discoveries through peer reviewed publications.
Figure 1. Dr. David Holding making controlled corn crosses with tassel bags. (copyright by University of Nebraska, 2019).
Maize geneticists like David have worked with this collaborative approach since the 1920’s when Mendel’s genetic principles were rediscovered by biologists and applied to other species of sexually reproducing living things. Back in the 1920’s George Beadle worked in a collaborative approach. George Beadle was a recent UNL graduate at the time and a young graduate student at Cornell University when he joined one of the pioneering maize genetics teams. Two future Nobel Prize winners (Beadle and McClintock) are pictured in their early career amongst their research team that was headed out to make crosses in their maize genetics nursery in Figure 2.
These maize genetics teams were confirming from their crossing experiments that genes were controlling seed and plant traits and were inherited predictably using the principles of segregation and independent assortment. These teams were also discovering a new idea – that genes were a part of chromosomes and therefore groups of genes were linked together as they traveled on the chromosomes during mitosis and meiosis. The community of maize geneticists were outstanding at collaborating and continually summarizing their collective discovery progress.
Figure 2. Cornell, 1929. Researchers dressed for work in a corn nursery and equipped with tassel bags. Left to right, standing -- Charles Burnham, Marcus Rhoades, Rollins Emerson, and Barbara McClintock. George Beadle is kneeling by the dog. (From Cold Spring Harbor Laboratory Archives, photo ID 16670).
The Summary of Linkage Studies in Maize bulletin published by Cornell University in 1935 provided this maize genetics team a platform for sharing maize genetics data involving genes that were linked together on a chromosome.
Newsletters and bulletins along with peer reviewed research articles were all ways to support each other’s progress in maize gene discovery.
This team combined their gene inheritance data in maize with a new idea called gene mapping proposed by drosophila (fruit fly) geneticists and assembled the first gene maps (e.g. Fig. 4) that showed the order of and the relative distances between genes that were linked together on the same chromosome in corn.
Figure 3. The cover of a bulletin called “A Summary of Linkage Studies in Maize” from June 1935 by Emerson, Beadle, and Fraser. Published by Cornell University Agricultural Experiment Station.
Figure 4. A depiction of a genetic linkage map of chromosome 3 in maize that could be generated by compiling the experimental data from many different maize genetic studies. Map units are listed at the top and the corresponding traits listed at the dash mark for each map unit. Recreation of image by James, 2010 on James and the Giant Corn.
While these first gene linkage maps were published in the 1930s, they are still useful in guiding decisions for someone like David Holding who might be planning crosses between parents to create genotypes that have combinations of these genes linked to chromosome #3. David Holding might plan and conduct experiments to create types of corn with new combinations of seed trait genes. Knowing that genes are linked can guide predictions that lead to a successful result.