An HSR in chromosome 1 which is found in many feral populations of Mus musculus domesticus was shown in previous studies to consist of a high-copy long-range repeat cluster. One such cluster, MUT, showed distorted transmission ratios when introduced by female parents. MUT/ + offspring were preferentially recovered at the expense of + / + embryos in the progeny of ♀ MUT/+ x♂ +/+ but were found at the expected 1:1 ratio in reciprocal crosses. Preferential recovery of maternal MUT was due to lethality of postimplantation + /+ embryos. There was no distortion of the recovery ratio in MUT/+ x MUT/MUT progeny: maternal MUT and + clusters were present among live implants at a 1:1 ratio. Maternal and zygotic effects therefore contribute to the phenomenon. The mechanism of their interaction is unknown.

The effect of the Xce (x-chromosome controlling element) genotype on the randomness of X-chromosome inactivation in the mouse was studied by monitoring the expression of the X-linked locus pgk-1. The main aim was to test whether the Xce genotype modified the preferential expression of the maternally derived X-chromosome in the yolk sac endoderm. Quantitative electrophoresis of phosphoglycerate kinase (PGK-1) was used to study Pgk-1 expression in the foetus, yolk sac mesoderm and yolk sac endoderm at 13½ days post coitum. The Xcea/Xcec genotype caused non-random X-chromosome expression in the foetus and yolk sac mesoderm. However, there was no evidence that the Xce genotype moderates the preferential expression of the maternally derived X-chromosome in the yolk sac endoderm, as reported by Rastan & Cattanach (1983).

The transient distribution of allele frequencies in a finite population is derived under the assumption that there are k possible alleic states at a locus and mutation occurs in all directions. At steady state this distribution becomes identical with the distribution obtained by Wright, Kimura and Crow when k = ∞. The rate of approach to the steady state distribution is generally very slow, the asymptotic rate being 2v + 1/(2N), where v and N are the mutation rate and effective population size, respectively. Using this distribution it is shown that when population size is suddenly increased, the expected number of alleles increases more rapidly than the expected heterozygosity. Implications of the present study on testing hypotheses for the maintenance of genetic variability in populations are discussed.

Equations are presented relating the number of generations elapsed to the fractions of bacteria carrying one or both plasmids in a clone derived from a cell containing two incompatible plasmids.

The response to selection in any line depends on the size of the initial sample by which the line is founded. For a single locus with additive gene action the effects of number of founders on early rate of response and on final limits are studied in relation to selection intensity and number of parents in the selected line. The reduction in total response caused by a small number of founders is greatest for large populations under intense selection, especially when the desirable alleles are rare in the base population. If these alleles are at high frequencies it is possible that a line which has gone through a bottleneck may be more sensitive to a reduced population size during subsequent selection than a line which has not. Under some conditions replicate selection lines founded with small samples are likely to be less variable in response than lines founded with moderately large samples.

The cataract-producing mutant gene previously described as ‘Shrivelled’ appears to be a recurrence of the mutant described by Paget as cataracta congenita subcapsularis, and should henceforth be referred to as CatFr.

Haploid amoebae of Physarum polycephalum may form plasmodia sexually by ‘crossing’, which involves cellular and nuclear fusion, or asexually by ‘selfing’, which occurs without nuclear fusion. In most amoebal strains, selfing is seen in clonal cultures only at very low frequency. In the present study, we have shown that selfing occurs at a similarly low frequency in mixtures of crossing-incompatible amoebae, but is stimulated in crossing-compatible mixtures. In certain compatible mixtures involving mutant strains, where crossing is temperature-sensitive, selfing may be stimulated even at a temperature that largely or wholly abolishes crossing. The extent to which selfing is stimulated appears to be influenced by matB, a locus which is known to affect the frequency of amoebal fusion. We have failed to detect any filter-transmissible factor that might be responsible for the effects we have observed. We suggest a sequence of events that might bring about ‘stimulated selfing’ as a consequence of abortive crossing.

Nine diverse lines of cultivated barley (Hordeum vulgare) and 11 lines of its wild progenitor (H. spontaneum) were assayed for variation in their chloroplast DNA by digestion with ten restriction endonucleases. The cultivated lines exhibited a single cpDNA polymorphism, whereas the wild material exhibited five. The significantly lower level of diversity among the cultivated lines was unexpected because both cultivated and wild lines had been selected for comparable levels of diversity for nuclear encoded isozyme loci. These results suggest that the level of cytoplasmic diversity was markedly restricted during the domestication of cultivated barley.

The relationship between locus variability and genetic identity is analysed in pooled data from a collection of Drosophila and vertebrate allozyme surveys. For vertebrates the analysis provides evidence that loci with high expected heterozygosity diverge at a faster rate than loci with low heterozygosity in comparisons involving pairs of populations and pairs of species. This relationship is not observed in comparisons of Drosophila species pairs.

Understanding how genetic variability is maintained in natural populations is of both theoretical and practical interest. In particular, the subdivision of populations into demes linked by low levels of migration has been suggested to play an important role. But the maintenance of genetic variation in populations is also often linked to the maintenance of sexual reproduction: any force that acts to maintain sex should also act to maintain variation. One theory for the maintenance of sex, the Red Queen, states that sex and variation are maintained by antagonistic coevolutionary interactions – especially those between hosts and their harmful parasites – that give rise to negative frequency-dependent selection. In this paper I present a model to examine the relationships between population subdivision, negative frequency-dependent selection due to parasites, the maintenance of sex, and the preservation of alleles from fixation. The results show strong interactions between migration rates, negative frequency-dependent selection, and the maintenance of variability for sexual and asexual populations.

Several stocks of Paramecium aurelia that had never expressed an E serotype were examined for E-reacting material. The regular presence of the cross-reacting antigen G in these stocks ruled out the use of hapten inhibition and other standard immunological techniques. Consequently, a genetic approach was attempted in which hybrids of the E-deficient strains and non-E stocks were bred, and antigen isolated and tested for the existence of molecules with a specificity differing from that of the E-containing parent.

Gel diffusion analysis suggested that E antigen from a hybrid indeed could differ from the ‘normal’ E. One of these hybrid clones which behaved as though its E antigen may have possessed a determinant contributed by the E-deficient strain was used as an immunizing antigen. The antibodies elicited possessed specificity unique for the hybrid. Thus the possession of E-like determinants by E-deficient stocks was confirmed.