03495naa a2200265 a 450000100080000000500110000800800410001910000250006024501360008526000090022150000190023052026900024965000110293965000250295065000270297565000250300265000350302765000110306265000130307365300160308670000220310270000200312470000130314477300720315713270222023-07-20       2004    bl uuuu     u00u1 u    #d1 aMENDES-BONATO, A. B.  aAbnormal pollen mitoses (PM I and PM II) in an interspecific hybrid of Brachiaria ruziziensis and Brachiaria decumbens (Gramineae).  c2004  aResearch note.  aMicrogametogenesis was analysed in an interspecific hybrid resulting from a cross between an artificially tetraploidized sexual accession of Brachiaria ruziziensis (2n = 4x = 36) and the apomictic cultivar (cv. Basilisk) of B. decumbens (2n = 4x = 36). Although each microspore initiated its differentiation by pollen mitosis, polarization of the nucleus was not observed in 28.85% of the microspores, and the typical hemispherical cell plate was not detected as well. The usual asymmetry was not seen at first pollen mitosis (PM I), and microspores therefore lacked differentiation between the vegetative and the generative cell. After telophase, each cell entered PM II, but this was not always followed by cytokinesis. Tripolar spindle and restitutional nuclei were observed among microspores that lacked the first cytokinesis. Pollen from such abnormal divisions were sterile and larger than those resulting from the normal ones. Male gametogenesis in flowering plants depends on a determinative asymmetry in the cell division at pollen mitosis I (PM I), which gives rise to a larger vegetative cell and a smaller generative cell. To achieve this asymmetry, the microspore undergoes several unique cellular events, including the establishment of cell polarity through nuclear migration, development of an asymmetric mitotic spindle, and an expected process of cytokinesis to form a hemispherical cell plate. Control of gametophytic cytokinesis is, therefore, a critical process in pollen cell fate determination which results in the asymmetric distribution of cellular components that presumably include cell fate determinants (Twell et al. 1998; Park and Twell 2001). During cell division in somatic cells, a pre-prophase band of microtubules marks the future division plane and the exact site of cytokinesis, and the cell plate arises from the phragmoplast and grows toward the cell wall (Sylvester 2000). On the other hand, in gametophytic cytokinesis at PM I the pre-prophase band is absent and a unique hemispherical cell plate is formed and surrounds the generative nucleus (Terasaka and Niitsu 1990). Asymmetric cytoplasm cleavage is ensured by a curved profile of phragmoplast microtubules that appear to guide the centrifugal growth of the cell plate from its margin (Brown and Lemmon 1991). Mutations altering cell division asymmetry during pollen mitosis have been reported in Arabidopsis thaliana (Park et al. 1998) and B. decumbens (Junqueira Filho et al. 2003). This paper reports a new occurrence of symmetry in pollen mitoses (PM I and PM II) in a hybrid involving B. decumbens as the male genitor. Absence of cytokinesis following PM I and PM II was also observed.  aPollen  aBrachiaria Decumbens  aBrachiaria Ruziziensis  aGramínea Forrageira  aMelhoramento Genético Vegetal  aMitose  aPastagem  aBraquiária1 aPAGLIARINI, M. S.1 aVALLE, C. B. do1 aJANK, L.  tJournal of Genetics, Bangaloregv. 83, n. 3, p. 279-283, Dec. 2004.