Exceptional Segregation of a Selectable Marker (KanR) in Arabidopsis Identifies Genes Important for Gametophytic Growth and Development

Genes transformed into plants are usually inherited in a regular Mendelian manner. There are, however, transformants in which the selectable trait fails to segregate as expected. Genetic analysis of the kanamycin-resistance (KanR) trait in >900 independent transformants of Arabidopsis revealed that 9% produced progeny families with an enormous deficiency of KanR individuals. Self-pollination of individual KanR plants from these families revealed lines that continued to segregate for a deficiency of KanR seedlings. In subsequent generations, the segregation ratio in these families stabilized at approximately 1 KanR:3 KanS. Molecular analyses showed that the deficiency of KanR individuals reflected the complete absence of the introduced DNA. Reciprocal backcrosses to untransformed plants showed unequal transmission of the KanR trait through the gametes in these exceptional lines. In five cases, this was primarily a failure of transmission through the microgametophyte (pollen) and in the other two cases, primarily a failure of transmission through the megagametophyte (embryo sac or egg). The number of seeds per silique was reduced by 50% in the latter two lines. We conclude that our exceptional transformants contain T-DNA insertions that delete or disrupt genes essential for gametophytic growth and development.