Forty-one suppressors obtained after NG and EMS mutagenesis of two ‘polar’ mutants of segment ‘29’ in Podospora anserina were genetically analysed. Three classes of suppressor could be distinguished on spectrum pattern criteria. One representative suppressor of each class was demonstrated to be non-gene specific. The class I suppressor was dominant and only suppressed polar mutants in segment ‘29’ and non-ICR-induced ones in genes where polarity cannot be determined. Class II and III suppressors were partially dominant and they suppressed polar, non-polar and even ICR-induced mutants. The difference between classes II and III seems to be only quantitative. According to whether class II and III suppressors are considered strongly or weakly allele-specific, two hypotheses are considered. First, tRNAs could be involved in all three classes of suppression: class I would be nonsense-specific and classes II and III would be nonsense-missense suppressors. Secondly, tRNA could be involved only in class I suppression, while ribosomal ambiguity could be responsible for class II and III suppression.

Spontaneous reversions from instability to stability were studied in a mutant homothallic strain of yeast. The mutant strain was characterized by persistent lethal sectoring attributable to the presence of a recessive locus uns1. It was found that (1) the meiotic reversion rate is about 140 times higher than the mitotic reversion rate, (2) reversions are much more common in the homozygote uns1 uns1 than in the heterozygote UNS1 unsl, (3) revertant segregants and lethal segregants tend to occur together, and (4) meiotic revertants tend to occur as twin revertants. These results indicate that reversion from unstable to stable results from genetic recombination and that, in meiosis, this recombination involves an unequal exchange at the first division.

Simultaneous selection by independent culling levels of two correlated traits in all four combinations of directions was investigated with Tribolium in a replicated experiment extending over nine generations. In addition to the two primary traits, 13-day larval weight and pupal weight, four secondary traits (pupation time, adult emergence time, adult weight and larval number) were observed.

The observed responses for both selected and unselected traits agreed with theoretical expectations after the latter were adjusted for changes which occurred in genetic and phenotypic parameters. Phenotypio variances for the selected traits were correlated positively with population means, yet genetic variances and heritabilities declined in all selected populations. No change was detected in the genetic correlation between selected traits even though the divergent two-trait selection was designed especially to ‘break’ the positive correlation of + 0·55 ± 0·12 present in the base population.

Striking changes in growth and developmental patterns resulting from the divergent selection were discussed in terms of metamorphic limits and ‘stabilizing’ genetic correlations.

Intergeneric recombination of chromosomal genes between strains of Klebsiella pneumoniae and Enterobacter aerogenes has been observed. In these experiments a male strain of K. pneumoniae acted as donor and E. aerogenes acted as recipient. Closely linked genes were co-transferred with very high frequency, and transfer of a large fragment carrying several separated genes was not uncommon. Subcultures of a hybrid which had received a large fragment carrying five markers were tested for genetic stability; no spontaneous segregants were found among 34000 colonies tested. These results suggest that the transferred fragment had been integrated, and indicate the close genetic relationship between K. pneumoniae and E. aerogenes. This is in agreement with their very similar biochemical properties.

Doublets of Oxytricha fallax possess two complete sets of ciliature. The doublet phenotype is inherited through sexual and asexual reproduction as a cortically determined trait. The trait is also inherited through cystment, independent of cyst size. Prior work shows an absence of all visible cortical organelles, except cell membranes, in the cyst; thus the visible structures are not themselves determinative.

Results from excision experiments performed on encysting doublets indicate that the determinative difference between doublets and singlets is the presence of one or two ‘determinative regions’ located on the ventral surfaces of the organisms which serve as sites for the initiation of ciliary primordium development. Doublets possess two such areas, but singlets possess only one.

Cells carrying traJ− mutants of F are transfer-deficient and are good recipients in conjugation (Achtman, Willets & Clark, 1972). In addition, the traJ gene product is involved in pilus- and in plasmid-specificity (Willetts, 1971). J-independent mutants were isolated as revertants of a traJ− mutant; they still carry the traJ− mutation but also carry at least one other mutation which results in transfer in the absence of the traJ gene product. J-independent transfer of these mutants is not inhibited by the R 100 repressor. Various models are presented which can account for the properties of traJ− mutants and of these J-independent revertants.

A derivative line of the homothallic Colonia strain of Physarum polycephalum has been isolated which produces plasmodia with high efficiency within clones of amoebae. Using the synergistic effect of ultraviolet light and caffeine, mutants of this line have been isolated which fail to undergo the developmental transition between haploid amoebae and diploid plasmodia (apt mutants). They are isolated by selecting for amoebae which fail to produce plasmodia within clones. Complementation tests of four mutants have shown that they are mutants of four different loci and they are recessive to wild-type. A further analysis of one mutant reveals that the apt-1 locus is unlinked to three other known markers. Crosses of this mutant with heterothallic strains yield progeny which are homothallic indicating that the lesion is not a revertant from a homothallic to a heterothallic mating-type. The use of this system in isolating developmental mutants is discussed.

A defined medium for Physarum polycephalum plasmodia has been devised containing glutamic acid, glycine, methionine, biotin, thiamine, glucose, salts and haematin, which supports good growth on agar plates of many different strains. Tests on this medium have revealed a requirement for valine in some plasmodia formed by homothallic progeny (mthapt-l+) from the cross a (mt1apt-l+) × APT1 (mthapt-l−). The valine requirement was also inherited among heterothallic progeny of this cross and its segregation was followed in several heterothallic crosses. To explain the results it is proposed that valine synthesis requires the presence of dominant alleles at either of two unlinked loci and that only plasmodia homozygous for recessive alleles at both loci are valine dependent. In some crosses studied only one pair of alleles is segregating and valine requirement thus provides a useful genetic marker, and the first reported nutritional marker in P. polycephalum. The value of crosses with apt− mutants for both the detection and analysis of plasmodial markers is demonstrated and discussed.

The origin of a number of R-factors used for genetic studies in Pseudomonas aeruginosa is described.