Gametic Selection by Glyphosate in Soybean Plants Hemizygous for the CP4 EPSPS Transgene

Abstract

Glyphosate application to glyphosate‐tolerant soybean [Glycine max (L.) Merr.] cultivars expressing a CP4 EPSPS transgene has become the most common method of soybean weed control in the USA. The objectives of this research were (i) to determine whether atypical segregation ratios for glyphosate tolerance and glyphosate sensitivity in the progeny of hemizygous plants resulted from gametic selection caused by glyphosate applications, (ii) to investigate the effects of different glyphosate application rates and timing of application on segregation ratios, and (iii) to determine whether male gametes, female gametes, or both were sensitive to the levels of glyphosate used. Segregation for glyphosate tolerance in a no‐glyphosate control treatment fit the expected 3:1 segregation ratio, indicating segregation of a single transgene. Glyphosate applications at three stages of plant development (V3, V3 + 2 to 3 wk, V3 + 3 to 4 wk) eliminated the glyphosate‐sensitive phenotypic class irrespective of plant maturation stage in a field experiment. Variations observed in the relative proportions of lines with either all‐tolerant or segregating phenotypic classes depended on glyphosate dosage and stage of plant development at the time of application. Overall, the data suggest that glyphosate application to hemizygous plants shortly before the onset of flowering was lethal to male gametes that do not carry the glyphosate tolerance transgene.