Distribution of Microtubular Cytoskeletons and Organelle Nucleoids During Microsporogenesis in a 2n Pollen Producer of Hybrid Populus

The distribution of microtubular cytoskeletons and organelle nucleoids during microsporogenesis in Populus simonii Carr. x P. nigra L. ‘Tongliao’ was studied by indirect immunofluorescence and 4’,6-diamidino-2- phenylindole (DAPI) staining. Our purpose was to reveal the relationship between changes in microtubule and chromosome behavior and cytokinesis and to explain the mechanism of 2n pollen production. We observed stable frequencies of 2n pollen production, ranging from 1.56% to 2.29%, between 2006 and 2008. Moreover, conjoined pollen grains were also observed. Meiotic abnormalities, including univalents, lagging chromosomes and micronuclei, were observed during microsporogenesis. Triads and dyads were also detected in meiotic products. Parallel spindles in metaphase II were unable to fuse, owing to the existence of an organelle band. Regularly, in telophase II, primary phragmoplasts were organized between sister nuclei, and secondary phragmoplasts formed between non-sister nuclei. Cell plates were initiated by centrifugal expansion of phragmoplasts and cytoplasmic infurrowing started at the junctions between the microsporocyte wall and the expanded phragmoplasts. However, a secondary phragmoplast was absent in some microsporocytes. These observations suggest that the occurrence of 2n pollen may result from the partial failure of cytokinesis caused by the absence of secondary phragmoplasts in this hybrid.