The sex specific bacteriophage PR4 has been found to plate on P. aeruginosa strains harbouring the TOL catabolic plasmid or the plasmid pND2 derived from TOL. Based on this, attempts were made to place TOL into a Pseudomonas plasmid incompatibility group and by showing that pND2 is incompatible with the R plasmid R2, TOL has been placed into the P-9 group. The NAH catabolic plasmid has been reported to be incompatible with TOL, pND2 and a variety of other plasmids derived from TOL. Thus, these plasmids also would appear to belong to the P-9 incompatibility group.

Linkage disequilibria between pairs of 8 polymorphic enzyme loci (αGpdh, Mdh, Adh, Est-6, Pgm, Odh, Est-C and Acph) in some 100 natural population samples of Drosophila melanogaster were examined. The estimates of linkage disequilibrium were made from zygotic frequencies. The magnitude of linkage disequilibria are small and similar to those in previous reports. Variation in linkage disequilibrium among related subpopulations was analysed by analysis of variance of the correlation coefficients. Despite the small absolute value of linkage disequilibrium there is a suggestion of a correlation among related subpopulations. The magnitude of linkage disequilibrium was observed to be positively correlated with linkage. Two cage populations were observed to demonstrate large amounts of linkage disequilibrium between closely linked loci in contrast to the situation in natural populations. This is attributable to the finite sizes of these cage populations.

The question of Drosophila laterality has been investigated in two species and in several strains by attempting to modify degrees of laterality (wing folding and directional choices in mazes) via selection and by ascertaining individual courtship behaviour preferences (circling, foreleg tapping, and wing extending). The results of courtship studies in D. paulistorum strains and in D. melanogaster indicate that, although individual members of a strain may be entirely right or left dominant, the majority of individuals are ambidextrous. As a group, there was no preference for right or left behaviours, and laterality was not a factor in success of courtship. The absence of lateral preference in courtship is consistent with the failure to demonstrate any genetic component for D. melanogaster left and right wing folding and maze behaviours.

The sterility which is associated with male recombination induced by 31.1 MRF was studied genetically and cytologically. In all crosses it was found that female sterility mainly involves failure of the heterozygous females to lay eggs because their ovaries are atrophic. Under the optical microscope, the atrophic ovaries were seen to contain only germaria in their ovarioles. It was also found that in some cases 31.1 MRF affects only one of the two ovaries of the same female. This observation suggests that defective development of atrophic ovaries is not due to influences from the rest of the body but should be attributed to the inability of the germ cells to differentiate. Moreover, various stocks as well as homologous chromosomes were found to react differently to 31.1 MRF with respect to female sterility. In their effect on male sterility it was observed that some strains behave as neutral and others as reactive when mated with 31.1/Cy L4 males.

Evidence is presented that a strain of Pseudomonas putida harbours a catabolic plasmid which encodes for the degradation of p-cresol through the protocatechuate ortho cleavage pathway. This plasmid can transfer giving approximately 10−3 transconjugants per donor cell, can be cured with mitomycin C, belongs to the P-9 plasmid incompatibility group and can be transduced with the bacteriophage pf16.

Mutations in the rex gene of phage λ affect lysogeny. λrex− phages have an increased probability of forming abortive lysogens instead of stable lysogens. In addition, established lysogens produce elevated levels of cured cells during anaerobic but not aerobic growth. It is suggested that the function of the rex gene is related to excision or repressor function.

The method of heritability estimation proposed by Abplanalp (‘linear heritability’ estimation) is reviewed and the formulation simplified. It is equivalent to the regression of performance of sibs of an individual, itself excluded, on that of the individual. The method is found to be slightly biased in small samples but, in terms of sampling variance, can make efficient use of the data.

Systematic crosses between various strains of Drosophila melanogaster lead in some cases to partly sterile F1 females (SF females). Two main classes of strain, inducer and reactive, have been denned on the basis of this sterility, which shows very specific physiological features. SF females arise only when reactive females are crossed with inducer males. In contrast, F1 females (RSF) produced by the reciprocal cross between inducer females and reactive males have normal fertility. All wild populations tested are of the inducer category, laboratory strains are either inducer or reactive. Sterility is the result of interaction between two genetic factors denoted I and R, respectively responsible for the inducer and reactive conditions and whose unusual genetic behaviour has been described in other papers. The present paper reports experiments showing that the I–R interaction is also responsible for high levels of X nondisjunction and of mutation in the SF female germ-line. The analogy with the P-M system of Kidwell, Kidwell & Sved (1977b), is discussed as are also the implications of the existence of the I-R system for spontaneous mutation research in D. melanogaster.

The accumulation of deleterious mutations in a finite diploid selfing population is investigated. It is shown that the conditions for accumulation are very similar to those for the accumulation of mutations in an asexual population by ‘Muller's ratchet’. The ratchet is likely to operate in both types of population if there is a large class of slightly deleterious mutations.

Fertilized eggs were transferred reciprocally between large and small mothers, to study maternal effects on body weight in mice selected for large and small size, respectively. Prenatal maternal effects were not important in our material, but postnatal maternal effects were detectable. The postnatal effects accrued mostly from the inadequacy of small mothers for large offspring; small offspring were largely unaffected by the type of mother. Genetic maternal effects were only of limited importance; maternal influences of environmental origin arose from variation in litter size.

A new locus for choline-requirement has been identified in Aspergillus nidulans and designated choC3. It has been located to linkage group VIII at a position 31·9 ± 2·2 map units to the left of the sD locus.

It is proposed that this locus codes either for the enzyme that catalyses the transmethylation of phosphatidyl monomethylaminoethanol to phosphatidyl dimethylaminoethanol or for a molecule essential for the synthesis or function of this enzyme.

Non-nodulating mutants of Rhizobium trifolii were found in cultures incubated at 35 °C for 7 days. From among ten strains incubated at the elevated temperature six became non-nodulating. The frequency of Nod− mutants in R. trifolii cultures varied from 1 to 75% depending on the strain tested. It is suggested that gene(s) controlling nodulation are located on a large plasmid.