Developmental origins of the conjoined twin mature embryo sacs in Smilax davidiana , with special notes on the formation of their embryos and endosperms

• Premise of the study: Smilax davidiana , a perennial deciduous vine in Smilacaceae, is widely distributed in East and South China. A recent investigation revealed the existence of twin embryo sacs in this species. This study describes their mature structure, analyzes their developmental origins, and discusses the formation of their embryos and endosperms. • Methods: Conventional paraffin section, light microscopy, and various staining techniques were used to document the course of megasporogenesis and mature embryo sac structure in pistillate flowers. • Key results: Two chalazal megaspores of the tetrad became functional, giving rise to twin embryo sacs. Two mature embryo sacs of the same size (equal‐conjoined twins); a dominant mature embryo sac and a subordinate one (unequal‐conjoined twins); or very rarely, fully fused twin mature embryo sacs sharing a common chalazal domain are formed. • Conclusion: A single embryo occurring in conjoined twins is the outcome of two eggs competing for sperm rather than a result of contention of two pre‐embryos (or young embryos) for nutrition. Conjoined twin embryo sacs could provide a new pathway to the formation of pentaploid endosperms.