PATERNAL TRANSMISSION OF A SEED SIZE-REDUCTION GENE VARIES WITH AGE OF A PRIMARY TRANSFORMANT AND SEED SET IS ALSO INFLUENCED BY GENE-EXPRESSION IN MATERNAL TISSUES

We have constructed chimaeric genes that are expressed in embryo and e ndosperm compartments of the seed, induce dominant seed lethality and have potential to reduce seed size in 75% of seeds within a fruit such as Citrus [7]. The genes are not entirely seed-specific as a proporti on of primary test tobacco transformants containing their gene were fu lly male-sterile [7]. Here we investigated why a proportion of apparen tly male-fertile transgenic plants showed segregation distortion from the 75% seed lethality expected for a single dominant gene. Reciprocal crosses were conducted between pollen fertile, primary tobacco transf ormants containing various copies of the CG1-400-RNase gene [7] and wi ld-type tobacco plants to examine the transmission of the gene through maternal and paternal gametes and also the effects of gene dosage in embryo and endosperm compartments on seed viability and phenotype. Pol len viability, seed set and seed phenotype were examined over a 16 mon th period to assess stability of gene expression in primary transforma nts because woody, fruit crops containing these genes will be vegetati vely propagated from primary transformants. In male-fertile transforma nts, the gene was observed to be expressed to varying degrees post-mei otically in pollen over the time period examined resulting in lethalit y of transgenic pollen and reduced paternal transmission. A variable, low-level maternal expression component was also detected that resulte d in seed lethality and influenced morphological variation in the seed lethal phenotype. The maternal and paternal expression components cau sed seed lethality to range from 50 to 75%. This study indicates the n eed to select for transformants with stable pollen transmission and hi gh seed expression and raises questions in relation to possible enviro nmental and epistatic effects on gene expression in primary, hemizygou s transformants over long growth periods.