Mexico, the birthplace of corn, is edging toward the use of genetically modified varieties to lower its dependence on imports, but strong opposition among some growers and environmentalists, who see altered corn as a threat to native strains, has kept the wheels turning slowly. Monsanto Co., DuPont Co.'s Pioneer Hi-Bred unit and Dow Chemical Co,'s Dow AgroSciences recently completed small, controlled experiments in northern Mexico with genetically modified corn, and are seeking government authorization to enter a "pre-commercial" phase, expanding the growing area to nearly 500 acres from 35 acres.