Relationship between phenotype and gene expression resulting from interploidy crosses and the fis1 mutation. (A) Composite seed showing the major phenotypic characteristics of maternal and paternal excess crosses. Cellularization/differentiation of endosperm proceeds from the micropylar ('maternal') to chalazal ('paternal') pole, occurring precociously in maternal excess and late or never in paternal excess. Chz = chalazal endosperm, PE = peripheral endosperm. Transcriptional profiling shows that fis1X2x seeds are strongly similar to extreme paternal excess in gene expression as well as phenotype. (B) PHE1 and other MADS-box transcription factors that encode interacting proteins are upregulated in paternal excess/fis1 and down or unchanged in maternal excess. Therefore these are strong candidates for key regulators of the interploidy cross phenotypes.