Development of megaspores, microspores, and male and female gametophytes in Daphne tangutica Maxim an economically valuable shrub

Daphne tangutica Maxim is a perennial evergreen shrub with significant ornamental and economic value, but its low fruit set rate restricts its development and utilization. To improve breeding and conservation strategies, there is an urgent need to clarify its gametophyte development process and the causes of abortion. Using wild populations as materials, paraffin sectioning and microscopic observation were employed to systematically conduct cytological observations and descriptions of megasporogenesis and microsporogenesis, male gametophyte development, the type of anther wall development, and embryo sac formation, among others. The microspores of D. tangutica are tetrahedral in shape, and mature pollen is mainly of the bicellular type; the anther wall shows basic-type development. The megaspore develops into a Polygonum-type and ultimately forms a Polygonum-type embryo sac, indicating that the female gametophyte functions are basically intact. During the late stage of pollen development, residual material appears in the tapetum (forming continuous or semi-continuous band-like structures along the inner wall of the anther), accompanied by partial anther abortion and the production of non-viable pollen, resulting in a reduced number of fertile pollen grains. Environmental stresses at high altitude, such as low temperatures, strong winds, a short growing season, and restricted pollinator activity, may exacerbate the occurrence of developmental asynchrony and pollen abortion. This study systematically elucidates for the first time the developmental processes of the megaspores and male and female gametophytes of D. tangutica , revealing that pollen abortion associated with asynchronous male development and delayed tapetum degradation is one of the main factors leading to low fruit set, whereas the female gametophyte is generally normal. The research findings provide a theoretical basis for embryological studies of Thymelaeaceae plants and offer support for determining appropriate pollination timing, improving propagation and breeding schemes, and further investigating the mechanisms of abortion.