Ronald E. Dengler

Ultrastructure of macrocyst formation in the cellular slime mold, Dictyostelium mucoroides: Extensive phagocytosis of amoebae by a specialized cell☆

Macrocyst differentiation in Dictyostelium mucoroides was carried out in shaking flasks. Observations of the development of macrocysts were made by light and electron microscopy. Macrocyst development begins with the clumping of stationary phase amoebae. Subsequently, the clumps become subdivided into smaller masses, each surrounded by a fibrillar sheath. At the center of each mass there arises a cytophagic cell which proceeds to engulf in turn all the cells in the mass. The engulfed cells (endocytes) undergo drastic changes in their fine structure and eventually are transformed into granules. During the early stages of engulfment the cytophagic cell has a single large nucleus, but by the time all the cells have been converted to endocytes the cytophagic cell has become multinucleate. The multinucleate condition persists in the granular stage. The thick cellulose wall surrounding each macrocyst is produced by the cytophagic cell soon after it has engulfed all the cells in the mass and before the granular stage.

Mutations in Arabidopsis condensin genes disrupt embryogenesis, meristem organization and segregation of homologous chromosomes during meiosis.

Proper chromatin condensation and sister chromatid resolution are essential for the maintenance of chromosomal integrity during cell division, and is in part mediated by a conserved multisubunit apparatus termed the condensin complex. The core subunits of the complex are members of the SMC2 (Structural Maintenance of Chromosomes) and SMC4 gene families. We have cloned an Arabidopsis gene, AtCAP-E1, which is a functional ortholog of the yeast SMC2 gene. A second, highly homologous SMC2 gene, AtCAPE-2, was identified by the Arabidopsis genome project. SMC2 gene expression in Arabidopsis was correlated with the mitotic activity of tissues, with high level expression observed in meristematic cells. The two genes are differentially expressed with AtCAP-E1 accounting for more than 85% of the total SMC2 transcript pool. The titan3 mutant is the result of a T-DNA insertion into AtCAP-E1, but other than subtle endosperm defects, titan3 is viable and fecund. We identified a T-DNA insertion mutant of AtCAP-E2, which showed no obvious mutant phenotype, indicating that the two genes are functionally redundant. Genetic crosses were employed to examine the consequences of reduced SMC2 levels. Both male and female gametogenesis were compromised in double mutant spores. Embryo lethality was observed for both double homozygous and AtCAP-E1-/-, AtCAP-E2+/- plants; arrest occurred at or before the globular stage and was associated with altered planes of cell division in both the suspensor and the embryo. Down regulation of both genes by antisense technology, as well as in AtCAP-E1+/-, AtCAP-E2-/- plants results in meristem disorganization and fasciation. Our data are consistent with the interpretation that threshold levels of SMC2 proteins are required for normal development and that AtCAP-E2 may have a higher affinity for its target than AtCAP-E1.