Initial Fertility (IF) strains of Streptomyces coelicolor are able to convert recipient strains (UF) to the IF condition by contact, without concomitant transfer of chromosomal markers. The conversion is prevented by the presence of acridine orange in the medium of the mixed culture. Acridine orange is also moderately effective in inducing the formation of UF variants from IF-treated strains. No effect of the drug is observed on UF variant formation from Normal Fertility (NF) strains nor on the behaviour of the fertility factor in NF × UF mixed cultures. The hypothesis is put forward that the fertility factor works as an episome in S. coelicolor, fixed to the chromosome in the NF strains, free in the IF strains and missing in the UF strains.

In this paper an exact treatment is given for the stochastic behaviour of the frequency of haploid genotypes in an infinite population when the absolute fitnesses of the two genotypes vary at random over generations. The main qualitative result from this treatment is that natural selection will favour that allele with the largest geometric mean fitness. A diffusion equation is derived whose solution is identical to the exact solution. The drift coefficient for this equation is of the form − μp(1 − p) + σ2(½ − p)p(l − p). This differs from the drift coefficient used in previous treatments of this problem and reduces the rate of quasi-fixation. Various waiting time problems are solved using this diffusion equation.

The shape of the mandible in. nine sublines of C57BL/Gr, seven other strains of ‘C57 ancestry’ and four unrelated strains was studied by multivariate techniques. The generalized distance function was used to classify individuals in the groups which they most closely resembled. The degree of misclassification depended on the pedigree relationship between strains and sublines. The generalized distance between pairs of subline centeroids was also highly correlated (r = 0·60) with the number of generations between them. A canonical variate analysis was used to reduce the dimensionality so that a graphical display of the relationships between strains and sublines could be made. The results agreed closely with the classification analysis. It was concluded that the shape of the mandible could be used for subline identification though the accuracy of this technique depends on how closely the sublines are related.

Brassica juncea Coss (2n = 36) is a naturally occurring allotetraploid. In the progenies of F1 hybrids of its constituent parents B. campestris sub.sp. oleifera var. toria × B. nigra and B. campestris sub.sp. japonica × B. nigra, the majority of the plants were parthenogenetic homozygous amphidiploids like B. juncea whereas a few differed in morphological and physiological characteristics. The latter, presumed to have arisen partheno-genetically, were suspected to be homozygous for chromosomes changed by allosyndetic recombination. One of these derived plants with curley leaves was crossed first with one of the homozygous amphidiploids and secondly with B. c. sub.sp. oleifera var. toria. Sporocytes of the two hybrids were studied at meiosis. The presence of one or two quadrivalents in the first hybrid and two trivalents in the second suggested that allosyndetic recombination had occurred in the 18-chromosome hybrid. The progeny was presumed to have arisen through pseudo-diploid parthenogenesis and this in association with allosyndetic recombination may provide a means of breeding higher yielding genotypes.

Amongst a collection of temperature sensitive (TS) mutants of Escherichia coli K-12, some have been found which can grow at the restrictive temperature (42 °C) if the osmotic pressure of the medium is raised by the addition of sodium chloride (1%) or sucrose (12·5%). These mutants are described as temperature sensitive osmotic remedial (TSOR) mutants. At the restrictive temperature they are not osmotically fragile, but do display decreased resistance to inhibitory agents such as deoxycholate, actino-mycin D and acridine orange; they also show release of the periplasmic enzyme ribonuclease. These results indicate a change in the cell's outer permeability barrier. The genes affected in six of the mutants have been located on the E. coli linkage map. The mutations, which occur at loci not previously described, have been named envM–envT to indicate their effect on the cell envelope.

Male Glossina morsitans were subjected to various doses of gamma radiation in air or nitrogen and mated to untreated females. The sex ratio of the F1 progeny was biased towards males, and this may be explained by the extra vulnerability to dominant lethal induction that the presence of the large X chromosome gives to female determining sperms. The mean fertility of the F1 progeny was subnormal. This was due to the induction of 50 % sterility in a large proportion of F1 individuals. Cytogenetic examination of the progeny of outcrosses of F1 individuals indicated that in most of the semi-steriles there were translocations involving the large autosomes or the Y chromosomes. Almost all the normally fertile F1 flies gave only cytogenetically normal progeny. The F1 sex-ratio distortion and semi-sterility would provide a ‘bonus’ in the application of the sterile male technique, which would amount to a 15–50% saving in the releases required to achieve a population control target compared with the requirement if the F1 was normal.

An approximate expression, is derived for the rate of change in frequency of an inversion introduced at a low initial frequency into a multi-locus system at equilibrium under recombination and selection. It is shown that this expression gives accurate predictions of the rate of progress of the inversion, even if the initial population is perturbed somewhat from equilibrium. Extensions to the cases where there are sex differences in recombination and selection are considered. An implication of the results is that selection pressure for newly arisen inversions depends on the existence of a stable equilibrium with linkage disequilibrium. The expected chance of survival of a new inversion in a large population is shown to be approximately one half the square root of the loss in fitness due to recombination.

Lifetime reproductive performance was studied in 50 sib-pairs of female mice, one member of each pair chromosomally XX and the other XO. Twenty-five pairs were irradiated with 25 rad X-rays at 10 days of age and 25 were unirradiated. In both the irradiated and control series the XO mice had a significantly shorter reproductive life than the XX ones, but unirradiated XO bred longer than irradiated XX. The median age of unirradiated XO at birth of last litter was 280 days and they had 6·5 ± 0·80 litters, whereas for XX the figures were 420days and 12·6 ± 0·74. The mean litter-size of XO mice was only about 55% and their lifetime productivity 34% of that of their XX sibs. Similarly, the lifetime productivity of irradiated XX females was only 31% of that of unirradiated XX. Histological studies showed that in the unirradiated XO mice reproduction ended through shortage of oocytes, and the resulting secondary ovarian changes were similar to those in irradiated mice. Thus, the differences between human and mouse XO types are to some extent reconciled. Both become sterile through death of oocytes, in humans before puberty and in mice after. This difference may be connected with different times from oogenesis to puberty in the two species.

An enhancement of cross-over frequencies previously reported by Newcombe & Threlkeld (1972) is shown to be due to two regions located on linkage group I, a few cross-over units to the right of the centromere. The distal region appears to be shorter than the proximal region, but equally effective in enhancing cross-over frequencies. The longer proximal region is readily divisable by cross-overs and probably spans several crossover units.

The properties of two transfer-deficient Flac mutants (in Escherichia coli K12) carrying mutations in the cistron traL are described. The traL product is needed for conjugational DNA transfer and for pilus formation, but not for surface exclusion; it is not plasmid-specific. traL maps between traA and traE; one of the traL mutations is apparently polar on traA. The properties of Tra+ revertants of this mutation suggest that the traL product is directly required for F-specific phage infection.