A COMPARISON OF CHROMOSOME KNOB FREQUENCIES BETWEEN SYMPATRIC AND ALLOPATRIC POPULATIONS OF TEOSINTE AND MAIZE

Unrestricted gene flow would be expected to lead to similar chromosome knob frequencies in sympatric populations of maize and teosinte. Chi-square tests were therefore used to compare chromosome knob frequencies of 45 collections of Mexican teosinte, grouped according to six geographic regions, with sympatric and allopatric populations of maize. Comparisons of knob frequencies between sympatric and allopatric populations within each region produced an initial classification for each knob-forming position. Further comparisons for each knob position over all regions, led to an interpretation of results for sympatric populations of maize and teosinte in terms of gene flow, selection, differentiation, ancestral similarities, and ancestral differences. Within regions, a high number of knob positions suggesting gene flow was associated with a low number of knob positions suggesting recent differentiation, and vice versa. It is suggested that the teosinte populations most likely to represent unique sources of germplasm are those most susceptible to future genetic erosion by maize. These relationships may represent a general model applicable to a number of crop plants and associated weed races.