Maternal small RNAs as gatekeepers of hybrid compatibility in plants and animals

Hybridization between species can enable introgression of advantageous traits, yet hybrid offspring often show reduced fitness, sterility, or developmental arrest due to postzygotic hybrid incompatibility. Understanding the molecular bases of these barriers could help overcome reproductive limits and enable the creation of currently inaccessible hybrids. In this review, we compare two small RNA-associated routes through which parental genomes can become incompatible: maternally biased sirenRNAs in the endosperm of flowering plants and animal PIWI-interacting RNAs (piRNAs), with emphasis on Drosophila hybrid dysgenesis and mammalian pachytene piRNA biology. We highlight conceptual parallels, including sequence-specific recognition, dosage-sensitive regulation, transposon or repeat control, and parent-of-origin effects. We also emphasize important differences: sirenRNA-associated incompatibility in plants emerges postfertilization during endosperm development, whereas piRNA-mediated incompatibilities in animals primarily affect germline genome defense and reproductive fertility.