Improvement of transgenic alfalfa by backcrossing

Two regenerative alfalfa genotypes were transformed with Agrobacterium tumefaciens with binary vectors containing the coding sequences for β-glucuronidase (GUS) and npt II (kanamycin resistance). The regenerative genotypes and their transgenic populations were agronomically inferior, and one was a somaclonal variant for flower color. GUS was used as a dominant genetic marker in a model system for studying backcrossing to improve transgenic alfalfa. Agronomic yield deficiencies and somaclonal changes were corrected by one to three backcrosses to cultivar genotypes, depending on the vigor of the original transformant. Three backcrosses were considered optimal because progeny contain 94% cultivar germplasm and could be used as parents of a new cultivar. Use of different cultivar genotypes each generation of back-crossing minimized inbreeding and maximized the heterotic potential of backcross derivatives. The improvement of transgenic alfalfa by backcrossing using a dominant marker required only as much time as the original transformation experiment.