A technique has been developed for achieving synchrony of division by selection for immediate pre- and post-fission cells of Paramecium tetraurelia. Cell populations are fed granular tantalum, followed by density-gradient centrifugation. Dividing cells, which do not feed, are separated from feeding interphase cells. The selected cells are division-synchronous for at least two generations when resuspended in fresh culture fluid. This technique provides a rapid means of obtaining large numbers of synchronously dividing paramecia for cell-cycle studies and has general application for selective enrichment of non-feeding sub-populations of cells.

Lines of Triticum aestivum Chinese Spring (2n = 6x = 42) which were ditelocentric or doubly ditelocentric, in turn, for the 14 chromosomes of the A and B genomes were pollinated by Triticum urartu (2n = 14). The behaviour of the marked telocentric chromosomes was scored in the 14 distinct hybrids obtained from these pollinations. In 6 of the hybrids in which different A genome chromosomes were marked by telocentrics there were from 50 to 80% of the pollen mother cells in which the telocentrics were paired. In the seven hybrids in which different B genome chromosomes were marked the telocentrics were never paired. It was concluded that the genome of T. urartu matched very closely the A genome of hexaploid wheat and that it did not correspond, as had been proposed by Johnson, to the B genome. The pairing behaviour of the 14 T. aestivum × T. urartu hybrids was compared with earlier results obtained from hybrids between T. aestivum and T. boeoticum. It was proposed that the higher trivalent frequencies seen in the T. boeoticum hybrids could be due to homoeologous pairing and that the genotype of T. boeoticum has the capacity partly to suppress the activity of the Ph locus of chromosome 5B of wheat, as a result of which homoeologous pairing is normally prevented.

The killing action by mu toxin, which is contained in the cytoplasm of stock 540, Paramecium primaurelia, was demonstrated against the various stocks of paramecia by means of microinjection. Most of the toxin is present in the soluble fraction of the host cytoplasm. The toxin was precipitated by ammonium sulphate at 50–80% saturation, and was almost completely inactivated by incubation at 60 °C for 30 min. Pre-autogamous paramecia were more sensitive than post-autogamous ones to the toxin. Paramecia which bear endosymbionts were generally resistant to the mate-killer toxin.

When T6/+ female mice were mated to non-translocation-bearing males, the relative viability of the embryos at 13·5–14·5 days gestation was about 39%. About 36% of the oocytes ovulated by T6/+ females were aneuploid, as a result of non-disjunction at meiosis, the majority having either 19 or 21 chromosomes. However, aneuploidy only accounts for a proportion of the embryonic loss in T6/+ × +/+ matings, as many of the embryos with 41 chromosomes survive postnatally. The present findings indicate that approximately 50% of the oocytes ovulated with the normal haploid number of chromosomes (n = 20) were genetically unbalanced as a result of adjacent segregation, and that a high proportion of the resultant embryos die in the early postimplantation period. In the present study non-translocation-bearing mice which were genotypically similar to the T6/+ females acted as controls.

We have no serious disagreement with the mathematical arguments presented by Ohta & Kimura (1975). We do, however, have reservations about their conclusions that ‘the hitch-hiking effect is generally unimportant as a mechanism for reducing heterozygosity’, and still prefer our own approach (Maynard Smith & Haigh, 1974).

Oral morphogenesis in stock 51 of Paramecium tetraurelia was investigated using the techniques of Chatton-Lwoff and protargol silver impregnation. During the stomatogenesis accompanying divisional morphogenesis a new oral anlage field and endoral kinety are formed and persist throughout the interfission period in both the proter and the opisthe. This previously overlooked fact is important for understanding the developmental origins and significance of the endoral kinety and the oral anlage field. Previously, the kinetosomes constituting the oral anlage field were thought to be formed just prior to the onset of stomatogenesis, being in some way derived from the kinetosomes of the endoral kinety. The demonstration of a permanent anlage field as an integral component of the oral assemblage suggests that the earliest stages of stomatogenesis might best be viewed as temporally controlled surfacing and/or ciliation of pre-existing kinetosomes rather than their de novo synthesis. The endoral kinety would thus have no contributory role in the formation of the anlage field used in the immediately ensuing stomatogenesis.

Longitudinal microtubules appear in Paramecium tetraurelia when cells are ready to divide (ca. interfission age 0·9). They arise in the longitudinal cortical ridges between kineties and form, in each ridge, an incomplete circlet of 15–18 microtubules, disappearing 10–15 min after separation of the fission products (ca. interfission age 0·03–0·04). Vinblastine, colchicine, colcemid, mercaptoethanol, and cold treatments all result in loss of these microtubules, rounding up of the cells, and some suppression of cell division. The tubules are thought to play a role in cellular elongation, morphogenesis, and separation.

Cytoplasmic petite mutants, spontaneous and induced, show various patterns of ability υ. inability to utilize the sugars galactose, α-methylglucoside and maltose, depending on the strain from which they were isolated. Petites recombine and segregate their different sugar deficiencies among vegetative diploid progeny when crossed, indicating mitochondrial control. Crosses between petites and wild-type further indicate that mitochondrial factors may be interacting with nuclear factors in a complex regulatory system.

Destruction by laser microbeam of oral and ventral cirral primordia for the future opisthe of 1/2 of a pre-fission doublet results in the permanent loss of ability to form oral and ventral cirral primordia on the irradiated half. This observation provides further evidence for the postulated ‘determinative region’ present on each ventral surface of these organisms. The opisthe of the irradiated doublet retains both original rows of marginal cirri, which are located mid-dorsally with respect to the remaining ventral surface. These two rows of marginal cirri are reproduced faithfully in all future opisthes, but are lost in most proter lines. The pattern of loss in proters involves migration of the extra rows of marginal cirri to the cell's right margin followed by aberrant morphogenesis and subsequent resorption. These results show that the reproduction of marginal cirri is at least partially dependent upon the presence of marginal cirri and their inherent structural characteristics as well as upon their position on the cell surface.

Models of the macronucleus in Paramecium tetraurelia which assume known levels of ploidy and random segregation of subunits smaller than a haploid set at both fission and macronuclear regeneration (MR) are not consistent with the hypothesis that senescence is due to aneuploid imbalance. Either senescence has some other basis or there is a mechanism for regular distribution of subunits at MR. Random segregation models for fission and MR are consistent with most data on survival and heterozygote stability, but if the ploidy level is 860 they fail to account for the data of Nyberg (this volume). Since the ploidy level may be higher than 860, models of random segregation cannot be ruled out for Paramecium. Models of the macronucleus in hypotrichs which assume randomly segregating chromosome fragments are consistent with data on survival and on stability of heterozygous genotypes at fission.

The organization of the genetic material in the macronucleus of ciliates has been the subject of considerable controversy. Two of the four models of macronuclear structure predict assortment of the alternative phenotypes of heterozygotes during vegetative growth. Early studies of the phenotypic behaviour of heterozygotes after macronuclear regeneration (Sonneborn, 1947) had supported a diploid subunit model. The availability of quantitative predictions of the rate of assortment for the haploid and chromosomal models (Preer, this volume) and the existence of two alleles controlling a quantitative trait, copper tolerance, in Paramecium tetraurelia, has provided an opportunity to test these models. The median tolerance limits to copper of unselected sublines were measured as a function of age. There was no increase in the variance among sublines, as the haploid and chromosomal models predict. Quantitative evaluation shows that subdiploid models with a kinetic complexity of 860 or less are not compatible with the results. This experiment was not sensitive enough, however, to exclude subdiploid models if the kinetic complexity is 2000 or greater. Selection on heterozygotes also failed to provide evidence in favour of assortment. All the results are consistent with and support the diploid subunit model of the Paramecium macronucleus.

Genes or sequences of DNA present in multiple copies per cell include entire genomes of mitochondria and chloroplasts, nuclear ribosomal RNA genes, and highly repetitive sequences in heterochromatin. All copies are nearly identical, in spite of mutational pressure and weak selection. A zygote containing mitochondrial or chloroplast genophores of two different genotypes quickly produces progeny pure for one genotype or another (vegetative segregation). Evidence from yeast and Chlamy-domonas suggests that organelle genophores undergo repeated rounds of random mating and recombination. When two molecules carrying different alleles at a locus recombine, gene conversion can result in the cell becoming pure for one allele. Stochastic matching and conversion (SMAC) has been studied by computer simulations which suggest that it will tend to eliminate new mutations in yeast mitochondrial DNA and speed up vegetative segregation. We have verified previous suggestions that gene conversion, occurring during unequal mitotic sister-strand crossing-over, provides an efficient mechanism for maintaining the homogeneity of repeated sequences in eukaryotic chromosomes.

Mating types I and II of Blepharisma japonicum v. intermedium excrete gamones 1 (blepharmone J) and 2 (blepharismone) respectively. The gamone of one type transforms cells of the other type so that they can conjugate with each other. We found that three other species, B. americanum, B. musculus and B. stoltei, have two types of cells homologous to those in B. japonicum; one (type II) excretes a factor which has the same activity as gamone 2 of B. japonicum, the other (type I) responds to this gamone by cell union. Type I cells of these species also excrete a gamone which induces pairs in type II cells of particular strains. Complementarity for mating is observed in some combinations of the two types.

These results indicate that each of the four species has at least one pair of complementary mating types, I and II, with the gamones of the type II's being the same molecule, blepharismone, while gamones of type I's are species- or syngen-specific blepharmone. These generic and specific gamones can be utilized to clarify existing taxonomic and evolutionary questions in the genus Blepharisma.

The macronuclei of Stentor coeruleus were isolated on a discontinuous sucrose gradient and their DNA was purified by conventional methods. The GC content was 32 mole%. The DNA banded as a single peak on analytical ultracentrifugation at 1·691 g/cm3. The molecular weight of the DNA was 5 × 106 to 4 × 107 daltons. Genome size determined by DNA–DNA reassociation kinetics was 6 × 1010 daltons. The macronuclear genome was mostly simple, about 85% being made of non-repetitive sequences.

Activated cysts of Didinium nasutum continue excystment after treatment with actinomycin D or puromycin. Cysts treated at developmental stages 19–20 emerge holociliated and lacking the seizing organ. No effect of the antibiotics at any other stages was found, suggesting that it is at stage 19–20 that the initiation of normal ciliation and of mouth formation occurs. Treatment with other metabolic inhibitors either blocks excystment completely or has no effect on normal development depending upon dosage, suggesting that the action of the antibiotics is specific.