Rapid amplification and fixation of new restriction sites in the ribosomal dna repeats in the derivatives of a cross between maize and Tripsacum dactyloides

Some of the derivatives of a cross of maize (Zea mays L.) × Tripsacum dactyloides (L) L (2n = 72) have abnormal development leading to strange and striking morphologies. The Tripsacum chromosomes in these “tripsacoid” maize plants (with Tripsacum-like characteristics) were eliminated and the maize chromosomes were recovered through repeated backcrossing to maize. As an initial attempt to analyze the DNA alterations in tripsacoid maize, we have detected a few restriction site changes in the ribosomal DNA repeat of these plants (Hpa II, Bal I, Sst I, Mbo II, and Sph I) and a new Sph I site was mapped to the spacer region between the 26S and 17S genes. Several possible mechanisms for the generation of a new restriction site are discussed, and we propose that the transient presence of Tripsacum genome during the backcrossing in some way induced a rapid amplification and fixation of new restriction sites in a relatively short period of time.