Chemostat cultures in which the limiting nutrient was lactose have been used to study the relative growth rate of Escherichia coli in relation to the enzyme activity of β-galactosidase. A novel genetic procedure was employed in order to obtain amino acid substitutions within the lacZ-encoded β-galactosidase that result in differences in enzyme activity too small to be detected by ordinary mutant screens. The cryptic substitutions were obtained as spontaneous revertants of nonsense mutations within the lacZ gene, and the enzymes differing from wild type were identified by means of polyacrylamide gel electrophoresis or thermal denaturation studies. The relation between enzyme activity and growth rate of these and other mutants supports a model of intermediary metabolism in which the flux of substrate through a metabolic pathway is represented by a concave function of the activity of any enzyme in the pathway. The consequence is that small differences in enzyme activity from wild type result in even smaller changes in fitness.

All major recombination pathways in the yeast Saccharomyces cerevisiae require the RAD52 gene product. We have examined the effect of the rad52-1 mutation on spontaneous mitotic recombination between heteroalleles, and found that prototrophs are produced at frequencies significantly above reversion. This residual recombination occurs at a relatively uniform level at all of the loci examined. To help understand the role that RAD52 plays in mitotic recombination, we examined recombination between all pairwise combinations of six mutant alleles of the LYS2 gene. The rad52-1 mutation decreased the variation in amount of recombination between the various pairwise combinations as well as lowering the overall frequency of recombination. The reduced variation results in a different pattern of recombination in rad52-1 cells than in wild type. One interpretation of these results is that the RAD52 gene product, directly or indirectly, plays a role in the formation or the resolution of mismatches in heteroduplex DNA.

Ditelocentric accessions of the Chinese Spring cultivar of Triticum aestivum were analysed electrophoretically for peptidase (PEPT) and amino peptidase (AMP = LAP) activity. Isozymic activity was missing in the accessions CSDT6AS and CSDT6BS when stained for PEPT. This was taken as evidence that the structural genes encoding these isozymes are located on the long arms of chromosomes 6A and 6B. These genes have been named Pept-A1 and Pept-B1, respectively. Isozymic activity was missing in the CSDT6BL accession when stained for AMP. This result reconfirms the previously published location of the gene Amp-B1 on the short arm of chromosome 6B and demonstrates that the two sets of loci are clearly different.

In a series of Lacerta vivipara samples from southern France's central mountains, the distribution of genotypes at the mannose phosphate isomerase (MPI) locus has only been accountable as due to sex linkage in female heterogamety, provided the W chromosome carries only one of the two observed alleles in the populations sampled. This is in perfect accordance with cytogenetical data. Reasons are presented that seem to limit acceptable hypotheses for the recent origin of this sex-linked polymorphism to three: a founder effect, a point mutation in Z chromosomes after crossover suppression, or hitch-hiking with the translocation involved.

Genetic variation in 40 domestic rabbits (Oryctolagus cuniculus) from eight different strains was investigated by horizontal starch gel electrophoresis. Twenty nine enzyme systems were examined in different tissues, 10 isoenzymes were found to be polymorphic. Indices of genetic variation show values comparable to those found in most other mammalian species. Thus the unusually high values reported previously by other authors may be due to a limited and not randomly chosen set of enzymes studied.

DK/Lm is a new inbred mouse strain with over 20 generations of brother–sister mating. The genotype of the DK/Lm mouse at the black-brown locus is b/b and heterozygosity at the agouti locus (Ay/a) is maintained. DK/Lm-Ay/a mice become sable in phenotype at the first moult, whereas C57BL/6J-Ay/a mice do not. The sable phenotype is defined as that of a mouse whose basic colour is yellow (phaeomelanic) but whose dorsal aspect is more or less darkened by the presence of nonyellow (eumelanic) pigment. At about 6 months of age the DK/Lm mouse gradually reverts to yellow in phenotype.

Mice of the two strains are compared. Observations are discussed and related to hypotheses regarding gene action at the a and e loci. The new strain is a useful experimental model for study of the relationship between gene action at the agouti locus and the important pleiotropic effects influenced by this locus.

The effects of population size and selection intensity, which are in the breeder's control, are investigated for ranges of values of quantities outside his control, namely the number, initial distribution of frequencies and effects of genes influencing the trait. Two alleles are assumed to be initially segregating at each locus, with no linkage, dominance or epistasis. The effects are assumed to follow a gamma distribution, using a wide range of its two parameters which specify both mean gene effect or selective value and the shape of the distribution, or the ratio of Wright's effective number to actual number of genes. The initial gene frequencies (q) are assumed to be either 0·5 at all loci, uniformly distributed over the range 0–1, or to have a U-shaped distribution, proportional to [q(1 − q)]−1 such as derives from neutral mutation, with gene effect and frequency distributions independent. The mean and variance of selection response and limits, in the absence of new mutation, are derived.

The shape of the distribution of effects is not usually important even up to the selection limit. With appropriate parametrization, the influence of the initial frequency distribution is small over a wide range of parameters. For reasonable choices of parameters, the effects of changing population size from those typically used in animal breeding programmes are likely to be small, but not negligible. For the initial U-shaped frequency distribution, further increases in population size are always expected to give a greater limit, regardless of present value, but not for the other distributions.

Genet. Res., Camb. (1986), 47, pp. 93–102

Phosphatase regulation in Aspergillus nidulans: responses to nutritional starvation

By Mark X. Caddick, Alan G. Brownlee and Herbert N. Arst

An error occurred in the heading of Table 3 on page 98. The Table should read as follows: