An unsuccessful attempt to hybridize Guatemalan Tripsacum and corn.
Having been successful in obtaining hybrids between diploid and tetraploid forms of corn and Tripsacum dactyloides native in the United States, the possibility of obtaining similar hybrids involving corn and Tripsacum species which are native in Central America was investigated. Tripsacum dactyloides is not known to occur in Latin America. Of the various species which do occur there, all that have been studied have proved to be tetraploids with approximately 72 chromosomes.
Since very special conditions are required to obtain hybrids of diploid Tripsacum dactyloides and diploid corn, the possibility seemed very remote that the tetraploid Tripsacum of Central America would hybridize with the diploid corns of that region. However, in developing an hypothesis of the origin of modern varieties of cultivated corn based on the assumption that teosinte resulted from the hybridization of Tripsacum and corn and that the chromosome knobs and various other important characters of corn came from Tripsacum by way of teosinte, Mangelsdorf and Reeves assumed that natural hybridization of Tripsacum and corn did occur in Central America. Hypotheses are of little value unless they can be tested. Fortunately, a direct test of this hypothesis, formulated nearly 10 years ago, involved no special difficulties. Tripsacum and corn were found to be in flower at the same time in readily accessible areas in the neighborhood of Guatemala City and Antigua at altitudes of approximately 5,OOO feet. More than 200 ear shoots of native corn plants from three different fields were carefully pollinated with Tripsacum pollen from plants collected in their natural habitat in the same region. In making pollinations by applying a mixture of Tripsacum and corn pollen directly to the bases of the corn silks and in culturing the embryos of resulting aborted seeds, the same technique was used that previously had been successful at Ithaca in obtaining a considerable number of Tripsacum‑corn hybrids. From three to four weeks after pollination each ear was carefully scrutinized for possible hybrid seed, the embryos of seeds suspected of being hybrid were cultured in a sterile nutrient agar and flown directly to Ithaca where their chromosome number was determined from root‑tip counts. There were no hybrid seedlings. All had 20 chromosomes.
This test failed to confirm the assumption of Mangelsdorf and Reeves that in the recent past Tripsacum and corn hybrids occurred in western Guatemala, subsequently designated by Mangelsdorf and Cameron as the secondary center of origin of cultivated maize. However, it would be desirable to make additional tests employing other species of Tripsacum which are found elsewhere in Central America. Also, a careful search should be made for diploid Tripsacums throughout Central America.
L. F. Randolph