PARENT-OF-ORIGIN EFFECTS ON SEED DEVELOPMENT IN ARABIDOPSIS-THALIANA

Many flowering plants are polyploid, but crosses between individuals o f different ploidies produce seeds that develop abnormally and usually abort. Often, seeds from interploidy crosses develop differently depe nding on whether the mother or father contributes more chromosome sets , suggesting that maternal and paternal genomes are not functionally e quivalent. Here we present the first cytological investigation of seed development following interploidy crosses in Arabidopsis thaliana. We find that crosses between diploid and tetraploid plants in either dir ection, resulting in double the normal dose of maternal or paternal ge nomes in the seed, produce viable seeds containing triploid embryos. H owever, development of the seed and in particular the endosperm is abn ormal, with maternal and paternal genomic excess producing complementa ry phenotypes. A double dose of maternal genomes with respect to pater nal contribution inhibits endosperm development and ultimately produce s a smaller embryo. In contrast, a double dose of paternal genomes pro motes growth of the endosperm and embryo. Reciprocal crosses between d iploids and hexaploids, resulting in a triple dose of maternal or pate rnal genomes, produce seeds that begin development with similar but mo re extreme phenotypes than those with a double dose, but these invaria bly abort. One explanation of our observations is that seeds with mate rnal or paternal excess contain different doses of maternally or pater nally expressed imprinted loci affecting endosperm development.