Conjugating pairs of syngen 1 exposed to 0·1 M-CaCl2 during the time of macronuclear development often abort the new macronuclei and retain the old macronucleus and its associated phenotypes. The new micro-nucleus is retained, however, so that macro–micro heterocaryons can be constructed. Because the macronucleus is from a sexually mature strain, the heterocaryon is capable of mating immediately, without passing a period of sexual immaturity. Macronuclear abortion may prove a useful short-cut in genetic analysis.

The two clones produced by the two members of a conjugating pair are normally of the same mating type, about 50% of such synclones being mating type I, 50% type II. This ratio suggests (1) that mating type is determined by a pair of alleles at one locus or by a pair of mating type determining chromosomes, and (2) that one mating type is homozygous, the other heterozygous or hemizygous. Mating type ratios are unaffected by temperature or parental age. Exceptions are of three types: (1) mates that produce one clone of type I, the other of type II, (2) mates which fuse permanently and yield a clone pure for either mating I or mating type II, and (3) conjugation of three mates one of which is regularly non-viable, the two survivors producing either two clones of type I, two clones of type II or one of each type. The bases of these exceptional results remain obscure.

The effects of two different deletions of the tryptophan operon on the cotransduction linkage of the nearby cysB and pyrF markers were studied using three sets of donor lysates, each produced by a different HT mutant P22 phage strain. Each trp operon deletion (present in both donor and recipient to preserve homology) caused changes in the cotransduction frequencies. This indicated that the HT mutant phage encapsulating mechanism, whose ability to discriminate phage DNA from host-cell DNA is absent or diminished, could still distinguish among nucleotide sequences in selecting bacterial chromosome sites at which to initiate transducing particle formation. The three HT mutant phage strains each produced different sets of cotransduction linkage values, indicating that this aspect of substrate specificity was altered differently and uniquely by each HT mutation.

The association of α and β2 subunits of tryptophan synthetase from Escherichia coli and Salmonella typhimurium in homologous and heterologous combinations was studied by sucrose density gradient centri-fugation. Under conditions allowing for optimal association of subunits derived from the same source, subunit association in the mixture E. coli α–S. typhimurium β2 was weaker than normal while in the reciprocal combination of S. typhimurium α–E. coli β2 it was tighter than normal.

These observations suggest that a certain degree of binding between the α and β2 subunits of tryptophan synthetase could have had a selective advantage during the evolutionary divergence of the species of Entero-bacteriaceae, so that a mutation leading to the substitution of an amino acid involved in α–β2 association in one of the subunits could have been compensated by a mutation in the complementary one.

Temperature-sensitive mutants in genes 12 and 18 of phage P22 show recessive DNA-negative phenotypes, but dominant integration-negative phenotypes. In contrast, am mutants in genes 12 and 18 are recessive for both DNA synthesis and lysogeny. The data are interpreted to indicate that the active forms of these two gene products are multimers.

The DNA contents of nuclei during gametogenesis and embryogenesis in Ascaris lumbricoides were measured by Feulgen-microspectrophoto-metry. The variability in the mean value for the haploid amount of DNA in sperm from different males processed at different times was not significant when sperm cell samples were taken from the same region of the seminal vesicle. As the sperm mature, the extent of uptake of Feulgen dye decreases nonsystematically. A similar phenomenon occurs during embryogenesis, and as noted by others, primary oocytes in the terminal portion of the oviduct become Feulgen-negative. Feulgen-positive primary oocytes maintain a 4C DNA value without significant variation. Notwithstanding the differences in Feulgen-DNA values in certain types of nuclei, our evidence supports the view that in Ascaris lumbricoides the amount of intraspecific DNA has a constant value between individual organisms and from one generation to the next. About 34% of the DNA of the zygote is lost through chromatin diminution at the third embryonic cleavage. This quantity represents 0·23 pg DNA per haploid equivalent.