Morphological differentiation in plant species is correlated in general with reproductive isolation and genetic differences. Very different-looking species are usually also reproductively incompatible (hybrids cannot be made, or if obtained, are sterile). Nevertheless, some plant species which are closely related in the genetic sense that they are completely interfertile may look very different morphologically. This situation has compelled plant evolutionists to hypothesize that these exceptional plant species must have undergone rapid morphological evolution. They have changed greatly in appearance in a period of time that was too short to allow them to evolve mechanisms of reproductive isolation. Cultivated maize is one such exceptional species, compared to its wild progenitors, the teosinte corns of Mexico. The genetic basis of such rapid morphological change may depend upon two contrasting modes: changes at a very small number (one to four) of genes with major effects, or numerous changes of many genes, each with small effects. Dr. John Doebley is beginning to test these two hypotheses in his study of genetic effects on morphological changes in the ear of corn, Zea mays subspecies mays. The ear of corn is remarkably different from that of its ancestor, Mexican teosinte, despite the fact that corn and teosinte are completely interfertile and are thought to have diverged only 10,000 years ago under Indian domestication. Tests will be conducted on progeny between corn and teosinte races, analyzing associations between molecular markers on the chromosomes and genes controlling morphological features in corn and teosinte ears. By these means it should be possible to determine the minimum number of genes controlling key morphological differences between corn and teosinte, the chromosomal location of these genes, the mode of action (dominant or recessive), and the percentage of phenotypic variation for which each gene accounts. The data will allow discrimination between two competing theories on the morphological steps involved in the recent origin of the corn ear.
