After liquid-holding treatment in saline, cultures of yeast exposed to u.v. light showed an increased resistance to a second exposure to u.v. irradiation. This increase in resistance during a second series of u.v. exposures was correlated with an increase in the induction of intragenic recombinants. In contrast, no increase in the frequency of the intergenic recombinants or mutations to prototrophy could be detected during the second u.v. dose range. The results obtained could not be explained by the induction of meiosis during liquid holding or by changes in the timing of cell division after u.v. exposure.

A model of u.v. repair in yeast is postulated in which liquid-holding treatment results in changes in the proportions of lesions repaired by excision repair and recombination repair (respectively).

The effect of optimizing selection, mutation and drift on a metric character determined by a large number of loci with equal effects without dominance was investigated theoretically. Conditions for a stable equilibrium under selection and mutation, in the absence of drift, have been obtained, and hence the amount of genetic variability which can be maintained by mutation has been determined. An approximate expression for the average amount of genetic variability to be expected in the presence of drift in a population of finite size has also been obtained and evaluated.

In Saccharomyces lactis, a class of mutants isolated for low β-glucosidase activity are reduced in activity for β-galactosidase as well. Genetic studies indicate that their properties are the result of a single mutation in a nuclear gene. In diploide containing a wild-type and mutant β-galactosidase allele, the mutant phenotype is partially dominant. The two enzymes can be separated physically and under appropriate conditions are induced independently in wild-type strains.

The probability of fixation of a mutant gene in a finite population was investigated by taking into account the effect of random fluctuation of selection intensity. It was shown that not only the product of the effective population number and average selection coefficient (Nes) is important, but also the ratio of the mean and the variance of selection coefficient (s/ Vs) has an important influence on fixation probability. In particular, when this ratio is small, a mutant gene, even if selected against, becomes fixed in the population like a selectively neutral mutant. In general, when random fluctuation of selection intensity is not negligible it may be convenient to use ‘effective selection coefficient’. The bearing of the present findings on the neutral mutation-random drift theory of evolution and variation at the molecular level was discussed.

Pseudomonas aeruginosa carries polar pili which act as receptors for RNA-containing bacteriophages. In order to confirm, that these pili were not involved in the transfer of the sex factor FP 2, eleven bacterial strains, both FP 2+ and FP 2−, were examined in the electron microscope for the presence of pili and tested for sensitivity to the RNA phage PP7. Pili were found on all strains save one which was resistant to phage PP7. It was also found by electron microscopy that about 25 times more pili per cell were present after PP7 adsorption than before it. This result is discussed with reference to the pilus retraction theory, providing further evidence that some kinds of pili retract instead of acting as simple tubes for the transfer of genetic material. The strong supporting evidence provided by the infective processes of male-specific coliphages is discussed and compared to current knowledge of P. aeruginosa RNA phages.

It was also found that pili were present on the host strain for the P. aeruginosa filamentous phage Pf. Although similar in appearance to RNA phage pili, these differed in that they did not adsorb phage PP7. However, it seemed likely that they were receptors for Pf. A structural comparison is made between P. aeruginosa pili and Escherichia coli F-pili. It is possible that P. aeruginosa pili could be coded for by a plasmid other than FP 2.

Two new sublines of the C57BL/Gr strain of mice have been studied which were derived from earlier sublines in which no genetic variance could be demonstrated. The incidence of some 31 minor skeletal variants was examined which could thus go up or down. As new subline differences have arisen with about the same frequency and mean magnitude of effect as in the past, there is no doubt that, at least in the C57BL strain, subline differentiation is a continuing process. Its high frequency (about 0·01 changes per character at risk per generation) is difficult to reconcile with spontaneous mutation rates of single genes in the mouse. The possibility must thus be considered that some other self-perpetuating processes or entities of some degree of stability are handed down in the lines of descent in which they have arisen, or perhaps become unmasked. There is no reason to suppose that this type of event is confined to inbred strains (in which it can be demonstrated fairly easily); it presumably occurs similarly in mixed stocks in which it would scarcely be detectable.

Saline loading caused an increase in glomerular filtration rate in RAP mice but not in CBA mice. On the basis of progeny testing of F 2 hybrids, backcrosses to CBA, and inbred lines derived from backcrosses to RAP, it was concluded that the difference between the strains RAP and CBA was probably largely accounted for by a single gene locus. The use of this gene in physiological investigations of the control of glomerular filtration rate is suggested.

The ‘neutral mutation theory’ holds that most amino acid substitutions in evolution are selectively neutral. The known pattern of variation in human haemoglobins can only be made consistent with this theory if the human species has passed through a bottleneck of numbers in the recent past. If this theory is true, estimates of the necessary size and duration of this bottleneck can be made. A theory is developed which leads to an estimate of Yg, n, the number of alleles present in a population which arise between g and n generations ago, and hence to the estimate

where u is the neutral mutation rate and Ne the effective population size, for the probability that a population contains no such alleles. Using data on haemoglobins, this gives an approximate upper limit to the time elapsed since the bottleneck in human numbers. Either such a bottleneck occurred, or the neutral mutation theory is false; data on other proteins will enable a choice between these possibilities to be made.

A mutant with the properties of a recipient has been isolated from the P. aeruginosa donor strain PAT (FP 2 + ) following treatment with the acridine-mustard ICR-191. While this strain displays the properties expected of a female or recipient in a number of genetic tests, the FP 2 determined property of mercury resistance is retained by the strain, suggesting that it may carry the FP2 factor in a mutated form. Treatment of the donor strain PAT (FP2 + ) with acridine-mustard has produced mutant male strains with the ability to form recombinants with other male strains at frequencies similar to that obtained in FP2+ × FP2 − matings. This characteristic has been shown to be due to a mutation in the FP2 factor which is dominant to the wild-type function. The isolation of stable male strains carrying both the mutant and wild type forms of the sex factor suggests that more than one copy of the FP 2 factor occurs in P. aeruginosa strain PAT donors.