The conjugation regions of IncF plasmids are closely related in that they share extensive DNA homology, and that they specify related pili. Variations between individual conjugation gene products of different IncF plasmids have, however, been noted. We have extended these observations by carrying out a systematic survey of twelve such plasmids, to examine the numbers and the groupings of the plasmid-specific alleles of several genes required for conjugation and its control.

Using vector plasmids carrying cloned origins of transfer (oriT), four different specificities were recognized, and these were correlated with the specificities of the genes with products that may act at this site (traM, traY and traZ). The traY gene is the first gene of the major transfer operon, and is therefore located close to the site at which the traJ protein acts to induce expression of the operon: correspondingly, correlation was observed between the oriT/traMYZ and traJ specificities in most of the plasmids. In turn, traJ is negatively regulated by the finO and finP products acting in concert: the finO product was relatively non-specific, but six finP alleles were identified, again with specificities correlated with those of traJ. Our explanation for this unexpectedly large number of finP alleles derives from the concept that the finP product is an RNA molecule rather than a protein. Although the conjugative pili encoded by IncF plasmids are closely related, they confer different efficiencies of plating of the various F-specific bacteriophages. We distinguished four groups on this basis, presumably resulting from differences in the primary amino-acid sequences of the pilin proteins. These groups could be related to the surface exclusion system specificities, consistent with the hypothesis that surface exclusion acts at least in part by preventing interaction between the pilus and the recipient cell surface.

From these data, information about the evolutionary relationships between the twelve IncF plasmids can be deduced.

Despite recent advances in our understanding of chromatin ultrastructure, little is known of the arrangement of chromosomes during interphase, the portion of the cell cycle associated with somatic gene transcription. An experimental procedure is described which has allowed the determination of the nature of the relative arrangement during interphase of chromosomes in a specific diploid cell type of Drosophila, the salivary gland anlage of the 10–14-h-old embryo. At this stage of development the salivary gland cells have ceased mitotic divisions. Embryos of 10–14 h in age were irradiated with 12000 rads of gamma radiation and then allowed to develop into third instar larvae. The polytene chromosomes of these larvae were examined for radiation-induced interchanges. From the distribution of observed interchanges, three major features of interphase chromosome arrangement were inferred. (1) Each euchromatic chromosomal arm occupies a specific domain within the interphase nucleus which does not appreciably overlap with those of other arms. (2) Within these chromosomal domains DNA folding is very extensive. (3) The heterochromatic regions of each chromosomal arm are sequestered from the euchromatic regions. An additional point of interest concerns the nature of the interchanges observed. No reciprocal interchanges were observed – all appeared to be partial exchanges, possibly subchromatid interchanges involving only one DNA strand from each of the two exchange sites.

Genetic and cytological analyses of house-flies collected from 12 pig-breeding farms throughout the British Isles demonstrated that the non-standard sex determination mechanism prevailing in South-East England, involving a dominant female determinant (F) and virtual homozygosity for a male determinant on the X chromosome (Xm, both males and females morphologically XX), was not typical of the country as a whole. Instead there was a gradual decrease in the frequency of F, Xm and a rarer male determinant M III, and a concomitant increase in the standard male determining Y chromosome, on moving north, east and west of this region. Only the Scottish and probably the Irish populations were fully standard (XX females XY males), although one from the East Anglian coast in which non-standard determinants were rare was predominantly of this type. Populations from intermediate areas possessed complex multifactorial mechanisms in which Y, F Xm and M III coexisted. It is hypothesized that this radial cline in sex determinants, like the latitudinal cline known from mainland Europe, represents a transient polymorphism caused by the recent and continuing invasion of non-standard determinants into originally standard populations. The cause(s) of this apparently rapid evolutionary change, however, remain unclear.

Male mice from congenic lines carry Y-chromosomes derived from two pairs of inbred strains (CBA/FaCamSt and C57BL/FaSt; PHL/St and PHL-YH/St) on various genetic backgrounds were compared. Serum testosterone levels, and the response of target organs in castrated animals to graded doses of exogenous testerone propionate were measured. These comparisons produced evidence for two Y-chromosomal loci influencing androgen metabolism. One of these affects serum testosterone levels, with variant alleles on the Y-chromosomes derived from the PHL and PHL-YH strains. The other locus influences the response to testosterone of target organs, most significantly seminal vesicle, and variant alleles are found in the CBA and C57 strains. The effects of both loci are modulated by the genetic background. The relationship of these loci to other Y-chromosomal loci in the mouse is briefly discussed.

In this study, five separate alleles at the agouti locus in house mice were tested for potential effects on a battery of 13 minor skeletal variants. Six genotypes (aa, ata, atat, Aa, Avya, and Aya) were compared on a standard congenic background (C57BL/6). In log-linear analyses, three of the 13 characters showed significant genotype differences (another three were close to significance), and genotypes also exhibited overall significance in a multivariate randomization test. Both multidimensional scaling and clustering showed an association of aa with ata, Aa with Avya and Aya, and a general separation of atat from the other genotypes. Genotype differences averaged 0·63 in probit standard deviations, 0·09 when assessed by the mean measure of divergence. Since the general magnitude of effects of these major genes was quite similar to those previously estimated for presumptive polygenes from subline divergence studies, it was concluded that major genes may often act as polygenes and make important contributions to the variation in minor skeletal variants.

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).

Male mice carrying one complete t haplotype transmit it to virtually all offspring. This implies that either the sperm carrying the t haplotype have an advantage in fertilization, or that the sperm not carrying a t haplotype are dysfunctional. To distinguish between these possibilities, epididymal sperm from tw32/ + and congenic + / + males were artificially inseminated into the uterus of T / + females, and the transmission ratio determined from the phenotype of the offspring. After artificial insemination (AI) of sperm from tw32/ + males, the mean ratio was 0·95 (3 experiments, 5 litters), demonstrating that the transmission ratio remains high after AI into the uterus. After AI of a mixture of equal numbers of motile sperm from tw32/ + and congenic + / + males, the mean ratio was 0·22 (6 experiments, 8 litters). These results suggest that sperm carrying a t haplotype from t/ + males are not superior to sperm from + / + males, and therefore imply that sperm not carrying a t haplotype from t/ + males are dysfunctional.

Seven strains of guinea pig (Cavia porcellus) with different degrees of inbreeding were investigated for genetic variability by starch gel electrophoresis. Variants for six out of 31 loci (Gpd-1, Aat-1, Ak-2, Pgm-1, Ada and Gpi) are reported. The data suggest that the strains all come from a small founding population and therefore are genetically more homogenous than the visible characteristics imply. Differences between isoenzyme data and other genetic characters are described and possible underlying genetic mechanisms are discussed.

Forty-one third chromosomes extracted from a natural population of Drosophila melanogaster were assessed for net fitness and for the quantitative characters viability, net fertility, female productivity, male weight, abdominal bristle number, and sternopleural bristle number. Net homozygous and heterozygous fitness of the third chromosomes was estimated by competition against a marked balancer third chromosome. Average fitness of the homozygous lines relative to wild-type heterozygotes was 0·13, indicating substantial inbreeding depression for net fitness. All significant correlations of quantitative characters with fitness and with each other were high and positive. Homozygous fitness is strongly correlated with net fertility, viability, and female productivity, moderately associated with male weight, and not significantly associated with bristle traits. The combination of metric traits which best predicts homozygous fitness is the simple multiple of viability and female productivity. Heterozygous fitness is not correlated with homozygous fitness; furthermore, the relative contribution of metric traits to fitness in a heterozygous population is likely to be different from that deduced from homozygous lines. These observations are consistent with a model of genetic variation for fitness in natural populations caused by segregation of rare deleterious recessive alleles.

An analysis based on a model that is different from the traditional Fisher's model for quantitative characters under assortative mating reveals that the genotypic offspring–midparent regression can be affected by assortative mating of parents. It is demonstrated that the prediction that mating parents assortatively introduces only a negligible bias in the estimated coefficient of linear offspring-midparent regression is limited to Fisher's model and cannot be generalized.

Recombination is hard to understand in darwinian terms when the process is identified with the production of crossover chromosomes. As an alternative explanation I propose instead that biased conversion is the primary function of meiotic recombination. In particular I show that a conversion process directed against the most common type of genetic damage can substantially reduce the mutational load, even if the conversion force is weak and if the conversion process occasionally creates new mutations.