A genetic subtraction strategy for determination of the embryo sac transcriptome. (a) A branch of a coatlique (coa) showing undeveloped siliques. Arrows point to a small silique, which bears female sterile ovules inside the carpel (insert: wild-type Ler branch). (b) Morphology of a mature wild-type ovule bearing an embryo sac (ES) before anthesis. (c) A functional embryo sac is absent in coa (degenerated megaspores [DM]). Note that the ovule sporophyte is morphologically equivalent to that of the wild type. (d) Functional categories of genes identified by a microarray-based comparison of coa and sporocyteless (spl; based on data from Yu and coworkers ) with the wild type. The embryo sac expressed transcriptome is shown to the left. Embryo sac expressed genes were grouped as preferentially expressed in the embryo sac if they were not detected in previous sporophytic microarrays . The size of the specific transcriptome in each class is marked on each bar by a dark outline. Functional categories of genes that were identified as over-expressed in the sporophyte of coa and spl are shown to the right. Scale bars: 1 cm in panel a (2 cm in the insert of panel a), and 50 μm in panels b and c.