[Cortical cytoskeletal ring in prophase II leads to correction of abnormalities of the first meiotic division and to meiotic restitution of pollen mother cell nucleus].

The deviation of prophase cytoskeletal ring formation was determined during meiotic division in 50% of pollen mother cells (PMCs) in maize haploid No 1498 (Zea mays). At prophase in both meiotic divisions the cytoskeletal ring is formed in cortical region of cytoplasm instead of perinuclear. Sometimes formation of both perinuclear and cortical rings is observed in the same cell. It has been shown that in multinucleate PMCs the cortical ring leads to the consolidation of chromosomes into common spindle and to meiotic restitution.

Asynchronous meiosis in an interspecific hybrid of Brachiaria ruziziensis and B. brizantha.

Male meiosis is generally synchronous in higher plants. The regulation of the cell cycle is still not well understood, and a powerful tool for gaining an understanding of this regulation is the development of mutations that affect cell-cycle synchrony. We report here asynchronous microsporogenesis in an interspecific hybrid between two important tropical grasses. In young spikelets of the interspecific hybrid 49.10% of anther meiocytes entered meiosis, exhibiting typical phases of the first and second divisions, while the other 50.90% showed distinctive features of early prophase. In older spikelets, anthers containing mature pollen grains also displayed meiocytes still undergoing meiosis. At this time, the latter cells were enclosed by the exine wall. Despite asynchrony, all cells completed meiosis. Old anthers contained only pollen grains that appeared to be in the same stage of development. Pollen fertility was estimated to be 52.76% in dehiscent anthers. An independent genetic control for meiosis synchrony and meiotic stages is suggested.

Expression of flap endonuclease-1 during meiosis in a basidiomycete, Coprinus cinereus.

In the basidiomycete Coprinus cinereus (C. cinereus), which shows a highly synchronous meiotic cell cycle, the meiotic prophase I cells demonstrate flap endonuclease-1 activity. To investigate its role during meiosis, we isolated a C. cinereus cDNA homolog of flap endonuclease-1 (CcFEN-1), 1377bp in length with the open reading frame (ORF) encoding a predicted molecular mass of 51 kDa. At amino-acid residues Glu276-Pro345, a specific inserted sequence composed of 70 amino acids rich in polar forms was found to exist, without sequence identity to other eukaryotic FEN-1 or the polar amino acid rich sequences found in C. cinereus PCNA and C. cinereus DNA ligase IV, although the lengths and percentages of polar amino acids were similar. Northern hybridization analysis indicated CcFEN-1 to be expressed not only in the pre-meiotic S phase but also in meiotic prophase I. The roles of CcFEN-1 during meiosis are discussed.

Nuclear ploidy is contingent on the microtubular cycle responsible for plant cytokinesis.

Division of the plant cell relies on the preprophase band of microtubules (PPB)-phragmoplast system. Cells of onion (Allium cepa L.) root meristems were rendered binucleate by preventing the consolidation of cell plate formation in telophase with 5 mM caffeine. These binucleates developed either a single PPB around one of their two nuclei or two PPBs, one per nucleus, in the prophase of the ensuing mitosis. Prophase cells developing one single PPB were shorter in length (42.3 +/- 4.1 microm) than those developing 2 PPBs (49.8 +/- 4.1 microm), and interphase duration was inversely related to cell length. Cells whose length was less than or equal to 42 microm, i.e., which had not even reached the mean size of the small binucleates in prophase, were followed throughout mitosis. In metaphase, they always assembled two mitotic spindles (one per nucleus). However, the cells that had assembled a single PPB also developed a single phragmoplast in telophase, leading to polyploidization. As these meristematic cells were not wide enough to accommodate the midzones of both mitotic spindles in any single plane transversal to the cell elongation axis, the spindles tilted until their midzones formed a continuum where the single common phragmoplast assembled. Its position was thereby uncoupled from that of the preceding PPB. Subsequently, the chromosomes from two different half-spindles were included, by a common nuclear envelope, in a single tetraploid nucleus. Finally, the cytokinetic plate segregated the two tetraploid nuclei formed at each side of the phragmoplast into two independent sister cells.

Loosening of a preprophase band of microtubules in onion (Allium cepa L.) root tip cells by kinase inhibitors.

Effects of kinase inhibitors on the preprophase band of microtubules in onion (Allium cepa L.) root tip cells were examined. Bundled microtubules in preprophase bands were dispersed on the cell cortex when onion seedlings were incubated with 2.5-5.0 mM 6-dimethylaminopurine. Fifteen min was enough for the bundled microtubules to disappear. Although many preprophase bands remained when the seedlings were incubated with 60 microM staurosporin, these preprophase band microtubules were loosened and the width of the band became broad. These results sugget that some kinases are involved in the microtubule bundling in the preprophase band development.

Characterization of two structurally related Xenopus laevis protein tyrosine phosphatases with homology to lipid-binding proteins.

We have chosen Xenopus laevis as a model system to study how protein tyrosine phosphatases (PTPases) function in growth and development. As an initial step, we have previously isolated in a polymerase chain reaction (PCR)-based protocol cDNA fragments which correspond to sequences within the catalytic domains of PTPases (Yang, Q., and Tonks, N. K. (1993) Adv. Protein Phosphatases 7, 359-372). Two of these PCR products, designated X1 and X10, have now been used to screen a X. laevis ovary cDNA library to obtain complete coding sequences for two distinct PTPases. The X1 cDNA encodes a protein (PTPX1) of 693 amino acids (approximately 79 kDa); the X10 cDNA encodes a protein of 597 amino acids (approximately 69 kDa). Both PTPX1 and PTPX10 lack potential membrane spanning sequences and therefore can be classified as non-transmembrane/cytoplasmic PTPases. While the overall structure of these PTPases are similar, sharing segments of 95% amino acid identity, they differ in that PTPX1 contains a unique 97-amino acid insert between the N-terminal segment and C-terminal catalytic domain. The absence of complete identity between PTPX1 and PTPX10 suggests that these two sequences are the products of separate genes and not the result of alternative splicing. This conclusion is confirmed by PCR analysis of Xenopus genomic DNA. Both PTPases share sequence identities in their N-terminal segments with two lipid-binding proteins, cellular retinaldehyde-binding protein and SEC14p, a phospholipid transferase. In addition, the unique insert sequence of PTPX1 shares identity with PSSA, a protein involved in phosphatidylserine biosynthesis. Sequence comparison suggests that PTPX10 is the Xenopus homolog of the human PTPase Meg-02, while PTPX1 is a structurally related yet distinct PTPase. Intrinsic PTPase activity of PTPX1 and PTPX10 was demonstrated in lysates of Sf9 cells infected with recombinant baculoviruses encoding either enzyme. PTPX1 can be recovered in both soluble and membrane fractions from Xenopus oocytes with the membrane form exhibiting approximately 4-fold higher activity than the soluble form.