Fig. 2. The paternal MSI1 allele is expressed in the embryo and endosperm. (A) Schematic of the PCR assay used to amplify specifically either the MSI1 or MSI1^* allele. Primer combination s1-as1 amplifies only the MSI1 allele (as shown beneath, left), whereas primer combination s2-as1 amplifies only the MSI1^* allele (beneath, right). (B) Time-course analysis of maternal and paternal MSI1 expression. Reciprocal crosses of wild-type (MSI1) and MSI1^* Arabidopsis plants were performed, and expression of maternal and paternal alleles was analyzed by RT-PCR in siliques derived from these crosses. The upper panel shows results from primer combination s1-as1, the lower panel from primer combination s2-as1. (C) Seeds derived from a cross of MSI and MSI1^* plants were dissected 6 days after pollination (DAP). Embryos and endosperm plus seed coat fractions were analyzed for expression of maternal (MSI1) and paternal MSI1^* alleles by RT-PCR. ACTIN provided a positive control.