Pollen wall structure in Anigozanthos viridis (Haemodoraceae)

The exine of <em>Anigozanthos viridis</em> Endl. pollen is divided into two zones by a plane which has been referred to as a"commissural line"; in our results we refer to it as a junction plane - a more apt description. Following young stages there is a pronounced reduction in thickness of the inner exine, probably due to stretching of the exine without the addition of new material as development progresses. We describe the fibrillar zone under the exine in microspores as oncus-like. Since this onciform zone precedes microspore mitosis and intine formation the fibrillar zone is divided into two layers in mature pollen an onciform part and the intine proper - it is not a two layered intine.

sidecar pollen, an Arabidopsis thaliana male gametophytic mutant with aberrant cell divisions during pollen development

During pollen development each product of meiosis undergoes a stereotypical pattern of cell divisions to give rise to a three-celled gametophyte, the pollen grain. First an asymmetric mitosis generates a larger vegetative cell and a smaller generative cell, then the generative cell undergoes a second mitosis to give rise to two sperm cells. It is unknown how this pattern of cell divisions is controlled. We have identified an Arabidopsis gene, SIDECAR POLLEN, which is required for the normal cell division pattern during pollen development. In the genetic background of the NoO ecotype, sidecar pollen heterozygotes have about 45% wild-type pollen, 48% aborted pollen and 7% pollen with an extra cell. Homozygous sidecar pollen plants have about 20% wild-type pollen, 53% aborted pollen and 27% extra-celled pollen. Similar ratios of sidecar pollen phenotypes are seen in the Columbia ecotype but sidecar pollen is a gametophytic lethal in the Landsberg erecta ecotype. Thus this allele of sidecar pollen shows differential gametophytic penetrance and variable expressivity in different genetic backgrounds. The extra cell has the cell identity of a vegetative cell and is produced prior to any asymmetric microspore mitosis. Pollen tetrad analysis directly demonstrates that SIDECAR POLLEN is indeed expressed in male gametophytes. To our knowledge, scp is the first male gametophytic mutation to be described in Arabidopsis.

Microtubular and actin filament configurations during microspore and pollen development in Brassica napus cv. Topas

The structures of the microtubular and microfilamental cytoskeletons were investigated during the development of microspores and pollen grains of Brassica napus L. cv. Topas. Microfilaments were observed directly with rhodamine–phalloidin and microtubules with FITC by indirect immunofluorescent staining and transmission electron microscopy. We observed microtubules in all developmental stages and noted several changes in the configuration of the microtubular cytoskeleton during microspore development, microspore mitosis, and pollen development. A preprophase band before microspore mitosis was not observed. The arrest of the microspore nucleus in an eccentric position is likely caused by microtubules as is the shape of the phragmoplast at microspore mitosis. Despite the application of various staining methods, i.e., labelling of fixed and unfixed fresh and cryosectioned microspores and pollen with rhodamine–phalloidin, microfilaments could not be observed in all developmental stages. Prominent microfilamental arrays were observed during cytokinesis of microspore mitosis and during the free generative cell stage. They mark the stages with different configurations. Key words: Brassica napus, immunolabelling, cytoskeleton, microspore and pollen development.

DEVELOPMENTAL CYTOLOGY OF 13 GENETIC MALE STERILE LOCI IN MAIZE

Thirteen nonallelic genetic-male sterile loci of maize (Zea mays L.) were investigated cytologically to determine the microsporogenesis breakdown characteristics for each mutant. These male-sterile mutants included ms1, ms2, ms5, ms6, ms7, ms8, ms9, ms10, ms11, ms12, ms13, ms14, and ms17 in A632 and 0h43 inbred backgrounds. Male-sterile mutants ms8 and ms9 resulted in abnormal microspore (pollen) mother cells that exhibited nearly normal nuclear development but abnormal cellular development. These mutants had the earliest effect on microsporogenesis. Male-sterile mutants ms5, ms11, and ms14 had the latest effect on microsporogenesis in that microspores developed until the microspore mitosis stage. Other male-sterile mutants seemed to have similar expressions when compared with each other. Mutants ms2 and ms7 both lacked significant microspore wall formation at the time of microspore collapse. Mutants ms10 and ms13 were similar in that the microspore wall developed to approximately one-half the normal thickness before microspore collapse. A unique feature of ms1 was the occurrence of an abnormally thickened microspore wall. Almost complete microspore wall development occurred in ms12 plants despite nuclear degradation. Mutant ms6 was cytologically and genetically similar to polymitotic (po). Mutant ms17 had variable expression that most notably affected spindle formation. These observations may be useful in utilizing genetic male sterility in maize hybrid seed production schemes.

ON THE MECHANISM OF CHROMATIN LOSS INDUCED BY THE B CHROMOSOME OF MAIZE

ABSTRACT Knobbed regions of the regular maize complement frequently are eliminated at the second microspore division in spores which have two or more B chromosomes. Evidence is presented that no or little loss occurs in spores with one B and that the rate is not increased in spores with more than two B's.—The B chromosomes from an unrelated strain proved as effective in inducing loss as did the B's of the original high loss stock.—Chromatin loss induced by B's is restricted to knobbed A chromosomes and occurs only at the second microspore division. Knobbed chromosomes 3, 5, and 9 have been tested and all interact with B's to give loss. Chromosomes with large knobs are more frequently broken than are those with smaller knobs and knobless chromosomes show negligible loss.—Although knobs and B's are essential for chromatin elimination, modifying genes can markedly affect the rate of loss.——Two knobbed heterologous chromosomes undergo simultaneous loss more frequently than expected from independent events. The data indicate that joint loss occurs in competent cells and that preferential assortment of the two deficient chromosomes to specific poles is unlikely.—B chromosomes and deficient chromosomes assort independently at the second microspore anaphase.—Genetic data from crosses with marker genes in both arms of chromosome 3 show that breakage of the postulated dicentric bridge does not occur solely at the centric region since a variety of deficient chromosomes were recovered.—Nondisjunction of B chromosomes and elimination of knobbed chromatin take place during the second microspore mitosis. The argument is advanced that the two phenomena result from faulty replication of heterochromatic segments. The position of the nonreplicating segment in the two kinds of chromosomes determines whether nondisjunction or breakage takes place.—Finally, it is suggested that all of the reported effects of the B chromosome can be accounted for if the B is a parasitic entity having no genetic function other than controlling the replication of its proximal heterochromatic knob and increasing the ability of B-containing sperm cells to compete successfully for fertilization of the egg.

Resolution of Channels in the Exine by Translocation of Colloidal Iron

The morphology of the exine of many pollen grains, at the time of flowering, is such that one can suppose that transport of substances through the exine occurred during pollen development. Holes or channels, microscopic to submicroscopic, are described for a large number of grains. An inner part of the exine of Epilobium angustifolium L. and E. montanum L., which may be referred to as the endexine, has irregularly shaped channels early in pollen development although by microspore mitosis there is no indication of such channeling in chemically fixed material. The nucleus in microspores used in the experiment reported here was in prophase of microspore mitosis and the endexine, while lamellated in untreated grains, did not contain irregularly shaped channels. Untreated material from the same part of the inflorescence as iron treated stamens was examined following fixation with 0.1M glutaraldehyde in cacodylate-HCl buffer at pH 6.9 (315 milliosmoles) for 24 hrs, 4% formaldehyde in phosphate buffer at pH 7.2 (1,300 milliosmoles) for 12 hrs, 1% glutaraldehyde mixed with 0.1% osmium tetroxide for 20 min, osmium tetroxide in deionized water for 2 hrs and 1% glutaraldehyde mixed with 4% formaldehyde in 0.1M cacodylate-HCl buffer at pH 6.9 for two hrs.