Hybrid seed failure may serve as a powerful and fast-evolving species barrier in flowering plants. Based on both evolutionary theory and empirical observations, the endosperm has been implicated as the nexus of developmental failures leading to hybrid embryo abortion.
Work by Roth et al. on wild tomatoes (Solanum section Lycopersicon, Solanaceae) provides evidence that hybrid seed failure is almost universal between two of three studied species pairs, while the third yields mixtures of inviable and viable hybrid seeds. They identified endosperm proliferation defects at the early globular embryo stage as a plausible cause of seed abortion. Their observations suggest similarities in hybrid endosperm malfunction across developmental types (nuclear and cellular-type endosperm), and may pave the way for future work integrating morphological and molecular footprints of hybrid seed failure.