Cytological observations in abortion formation and morphological character of pollen in seedless chestnut rose (Rosa sterilis)

Seedless chestnut rose (Rosa sterilis S. D. Shi), a promising fruit with a high content of vitamin C, characterizes in bearing seedless fruit due to the severe male sterility, which is scarce among the Rosa species. To better understand the period and cause of the complete male sterility of seedless chestnut rose, the pollen mother cell meiosis, microspore development, callose dynamics, and pollen morphological characteristics of this plant species were investigated in comparison with those of with congener, chestnut rose (Rosa roxburghii Tratt). Cytologically, seedless chestnut rose showed abnormal behaviors of chromosomes during meiosis I, including the formation of chromosome bridges, lagging chromosomes, fragmented chromosomes, asymmetric chromosomal arrangement, etc. which were further ascribed to the structure mutation of chromosomes. During the formation of tetrads, anomalous tetrads were also observed. In the early mononuclear stage, the callose disintegration was delayed in comparison with that of chestnut rose, partially leading to the underdevelopment of pollen grains. Subsequently nucleus appeared to disintegrate during the binuclear stage, giving rise to the termination of microspore development. Microstructural and ultrastructural observations of microspore formation and development demonstrated that the pollen abortion of seedless chestnut rose started in meiosis of the pollen mother cell, followed by anomalies of the tetrad, afterwards, with the delayed disintegration of callose in the early mononucleate stage, as well as by nucleolysis in the binucleate stage, led to the malformation of pollen grain. Therefore, abnormal behavior of meiotic chromosomes was the predominant cause for complete male sterility of seedless chestnut rose, which may be helpful for the further elucidation of the mechanism of male sterility in the genus Rosa the formation of male sterility in the genus Rosa.