Anatomical Insights into Megasporogenesis, Microsporogenesis, and Gametophyte Development in Michelia alba: Causes of Reproductive Failure

Background Michelia alba DC. (Magnoliaceae), economically significant tree species in Southeast Asia, extremely low natural fruit production due to reproductive failure. This study aims to characterize the anatomical causes of this phenomenon by investigating megasporogenesis, microsporogenesis, and gametophyte development. Results Anatomical analysis revealed that microspore tetrads were arranged tetrahedrally, isobilaterally, or in T-shapes and that mature pollen was bicellular. Ovules were anatropous, forming linearly arranged megaspore tetrads; only the chalazal megaspore developed into a functional Polygonum-type embryo sac (7 cells, 8 nuclei). Critical abnormalities were identified: 1. Male gametophyte failure (>90% abortion): Premature tapetum degeneration and abnormal anther wall contraction caused microspore adhesion and pollen collapse. 2. Female gametophyte failure (>90% arrest): Most embryo sacs are arrested before maturity lacking essential cells (egg cells, synergids). Only 2 of the 200 examined sections contained mature embryo sacs. Conclusions Concurrent defects in male and female gametophyte development critically impair gametophyte functionality, explaining the low fruit set in M. alba. This study provides the first cytological elucidation of reproductive failure in this species and establishes a foundation for the conservation and breeding of endangered Magnoliaceae.