Male gamete development and early tapetal degeneration in cytoplasmic mate-sterile pepper investigated by meiotic, anatomical and ultrastructural analyses

Cytoplasmic male-sterile (CMS) pepper (Capsicum annuum L.) is a useful germplasm for heterosis breeding, however, the process of male gamete development and the sterility mechanism is unclear. In the present study, the developmental sequence in CMS pepper and the causative factors responsible for pollen abortion were investigated through combined meiotic analysis, paraffin section observations, and transmission electron microscopy. Results from meiotic analysis showed that meiosis in a male-sterile inbred line, CMS 21A, was normal, and uni-nucleate pollen could be released from the tetrads of both CMS 21A and the fertile maintainer line 21B. However, only the uni-nucleate pollen from CMS 21B could develop into functional gametes, while the uni-nucleate pollen from the 21A went through a process of rupture, and the cellular components were released into the anther sac. Observations of paraffin sections viewed with the light microscope showed that the tapetum of CMS 21A at the uni-nucleate stage swelled abnormally and was pressed against pollen grains of the locule. Further observations with transmission electron microscopy revealed that the mitochondria in CMS 21A tapetum were highly vacuolated. and there was no accumulation of sporopollenin on the surface of CMS 21A pollen, suggesting a close relationship between early degeneration of tapetum and CMS.