Cytological abnormalities and pollen abortion in interspecific hybrids of Nicotiana

Interspecific cross breeding introduces superior agronomic traits into cultivated species; however, problematic pollen sterility occurs in the hybrids. Our previous study obtained interspecific hybrids from the cross between a cytoplasmic male sterility line of Nicotiana tabacum and Nicotiana alata, and some of the hybrids were pollen sterile. Here, we conducted an in-depth cellular study to understand the cytological mechanism of pollen abortion in these hybrids (F1-D sterile) compared with pollen development in the fertile hybrids (F1-S sterile) from the same cross. The ultrastructure observation showed that the membrane of microspore in F1-D sterile hybrid was deficient at all represented developmental stages. Chromosome behavior during meiosis was studied by carbol fuchsin staining, which indicated that cytomixis, chromosome leakage and asymmetric callose wall deposition occurred with high frequency in the microsporocyte of F1-D sterile. The results of the ultrastructure and 6-diamidino-2-phenylindole (DAPI) analyses also showed that the cytoplasm and nucleus were unstable and extruded from F1-D sterile microspore during the developmental process, leading to mature pollen grains that were vacuous and collapsed in the aperture region. In addition, delayed tapetum degradation was also detected in the anther of F1-D sterile, and might be associated with irregular sporopollenin deposition in the aperture region of F1-D sterile pollen. Genetic unbalance and cytomembrane deficiency might both be responsible for the instability of the chromosome, nuclear and cytoplasm, and resulted in pollen abortion in F1-D sterile hybrids, and irregular tapetum degradation might also be related with pollen sterility.