Possible causes of extensive gametophyte abortion in Michelia alba

Michelia alba (Magnoliaceae) is an economically important ornamental tree species characterized by extremely low natural seed set. To clarify the cytological basis of this reproductive failure, we conducted a comprehensive anatomical investigation of microsporogenesis, megasporogenesis, and male and female gametophyte development. Under normal developmental patterns, microspore tetrads were tetrahedral, isobilateral, or T-shaped and developed into bicellular pollen grains, whereas the female gametophyte followed a typical monosporic Polygonum-type pathway. However, severe abnormalities were consistently observed in both reproductive systems. >90% of pollen grains exhibited developmental arrest or structural collapse, frequently associated with premature tapetal disorganization and early contraction of the anther wall. Concurrently, over 90% of ovules failed to produce normal embryo sacs, with most embryo sacs arresting before or shortly after the stage of the functional megaspore; among >200 serially sectioned ovules, only two normal mature embryo sacs were detected. These observations provide clear anatomical evidence that extensive gametophyte abortion is a major factor of reproductive failure in M. alba , with female gametophyte arrest appearing to be an important factor limiting reproductive success. The study establishes a cytological foundation for understanding reproductive instability in Magnoliaceae and provides structural insights relevant to germplasm conservation and breeding.