Cortical morphogenesis in Paramecium aurelia: mutants affecting cell shape

Ciliary force-responsive striated fibers promote basal body connections and cortical interactions

Multi-ciliary arrays promote fluid flow and cellular motility using the polarized and coordinated beating of hundreds of motile cilia. Tetrahymena basal bodies (BBs) nucleate and position cilia, whereby BB-associated striated fibers (SFs) promote BB anchorage and orientation into ciliary rows. Mutants that shorten SFs cause disoriented BBs. In contrast to the cytotaxis model, we show that disoriented BBs with short SFs can regain normal orientation if SF length is restored. In addition, SFs adopt unique lengths by their shrinkage and growth to establish and maintain BB connections and cortical interactions in a ciliary force-dependent mechanism. Tetrahymena SFs comprise at least eight uniquely localizing proteins belonging to the SF-assemblin family. Loss of different proteins that localize to the SF base disrupts either SF steady-state length or ciliary force-induced SF elongation. Thus, the dynamic regulation of SFs promotes BB connections and cortical interactions to organize ciliary arrays.

Loss of a Fragile Chromosome Region leads to the Screwy Phenotype in Paramecium tetraurelia

A conspicuous cell-shape phenotype known as “screwy” was reported to result from mutations at two or three uncharacterized loci in the ciliate Paramecium tetraurelia. Here, we describe a new screwy mutation, Spinning Top, which appeared spontaneously in the cross of an unrelated mutant with reference strain 51. The macronuclear (MAC) genome of the Spinning Top mutant is shown to lack a ~28.5-kb segment containing 18 genes at the end of one chromosome, which appears to result from a collinear deletion in the micronuclear (MIC) genome. We tested several candidate genes from the deleted locus by dsRNA-induced silencing in wild-type cells, and identified a single gene responsible for the phenotype. This gene, named Spade, encodes a 566-aa glutamine-rich protein with a C₂HC zinc finger. Its silencing leads to a fast phenotype switch during vegetative growth, but cells recover a wild-type phenotype only 5–6 divisions after silencing is stopped. We analyzed 5 independently-obtained mutant alleles of the Sc1 locus, and concluded that all of them also lack the Spade gene and a number of neighboring genes in the MAC and MIC genomes. Mapping of the MAC deletion breakpoints revealed two different positions among the 5 alleles, both of which differ from the Spinning Top breakpoint. These results suggest that this MIC chromosome region is intrinsically unstable in strain 51.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Genetic analysis of morphogenetic processes in Paramecium. I. A mutation affecting trichocyst formation and nuclear division

Mutation tam38 of Paramecium tetraurelia is a nuclear recessive mutation with a pleiotropic effect on both trichocyst morphogenesis and nuclear processes. The analysis of the defective nuclear processes (micronuclear and macronuclear divisions, nuclear reorganization at autogamy) shows that these defects result from an abnormal localization of the nuclei. Phenocopies of tam38 abnormalities can be obtained by vinblastine treatment of wild-type cells at late stages of division. Taking into account the similarity between tam38 and a series of other mutations which also prevent trichocyst attachment to the cell surface and disturb nuclear divisions, the following interpretation is proposed: the absence of attached trichocyst induces structural changes in the plasma membrane or in the cortical region which disturb the normal cortical control of the localization of nuclei.

Form and pattern in ciliated protozoa: analysis of a genic mutant with altered cell shape in Tetrahymena pyriformis, Syngen 1

A single cell isolated from the sexual progeny of mutagenized parents gave rise to a clone of cells with an abnormal, conical shape. Breeding analysis revealed that this shape results from the action of a single recessive gene, co (conical). Homozygous mutant cells are shorter and wider than wild type cells, and have their widest point at a more posterior position. Nonetheless, cortical parameters such as number of ciliary rows, number of ciliary units within these rows, and positions of contractile vacuole pores remain essentially unchanged in conical cells, suggesting a considerable degree of mutual independence of pattern and form. Shape changes prior to cell division bring about some convergence in form of dividing conical and wild type cells. However, in conical cells the new oral apparatus and fission line form well posterior to the cell equator, so the opisthes are invariably smaller than proters. Macronuclei nonetheless undergo constriction at the normal central location, and the characteristic inequality in the DNA content of the two macronuclear division products is not increased by the conical condition. Generation times are, on the average, nearly the same in the two wild type daughter cells and in conical proters, while the small conical opisthes have generation times averaging one-third longer. This prolongation explains why population doubling times are always somewhat longer in cultures of conical cells than in parallel cultures of wild type cells. The unusually long generation times of conical opisthes allows for the compensation of initial size differences. Observations on shape changes in conjugating cells of various genotypic combinations suggest that the wild type gene product is not freely exchangeable across the conjugation bridge. The implications of the conical phenotype for problems of cellular patterning and positioning of organelle systems are considered in the discussion.