Salmonella typhimurium phage P22 can recombine with a serologically and morphologically unrelated Salmonella phage Fels 2 to yield the hybrid phage F22 at a frequency of about 10−12. F22 has inherited the entire late genes (protein coat and tail structural genes) of Fels 2 but carries some P22 early genes. P22 genes in the F22 phage were indentified by marker rescue of various P22 mutants. The F22 genome carries the x-erf-c-18-12 segment of the P22 genome. A number of P22 amber mutants were also tested to support these data. The F22 region homologous to P22 was mapped by scoring the ratio of P22 backcross recombinant types in lysates of F22 lysogens superinfected with P22c2ts12. The ratio of the distances between these markers and the ends of the homologous region was determined. Furthermore a new F22 hybrid designated F22dis, containing both P22 immunity regions (c and Im), was isolated at a frequency of about 10−4 by superinfecting F22 lysogens with P22. F22dis phage has lost some Fels 2 gene(s) which have been replaced by the P22 segment containing the Im region, resulting in formation of a defective hybrid phage.

The presence of a gene designated bncA which produces binucleate and trinucleate conidia in A. nidulans alters the instability of disomics, diploids, and strains with chromosome duplication. In disomics, the gene bncA increases instability. In duplicate and diploid strains, the bncA gene reduces instability by acting as a partial stabilizer. In the strain with chromosome duplication, the bncA gene produces increased percentages of bi- and trinucleate conidia, a fact that may be interpreted to be due to the larger conidial volume of this strain or to the combined effect of bncA and of the strain, which normally already exhibits a small amount of binucleate conidia.

Populations of Drosophila melanogaster were subjected to selection for differing oviposition preference under allopatric and sympatric conditions. Flies were presented with the choice of a potato-based medium and a medium containing sugar and killed yeast on which to lay their eggs. Some gene flow was possible under sympatric conditions. In the allopatric lines selection was successful in rapidly generating an increased preference for sugar, and in the sympatric lines divergent oviposition preferences were generated in two cases out of four. A significant degree of reproductive isolation between one pair of allopatric lines was generated after eighteen months of selection.

We analyse the progression of linkage disequilibrium produced by random genetic drift in populations subject to cyclical fluctuations in size. Our model is applied to natural populations of Drosophila which show an annual demographic cycle of bottleneck (finite size) and demographic burst (size supposed to be infinite). In these populations, linkage disequilibrium stabilizes in such a way that, at equilibrium, the expected square of the correlation of gene frequencies E(r2) shows a stable cycle from year to year. If two loci are tightly linked, E(r2) barely varies during the annual cycle. Its values remain close to the value expected in a population of the same but constant effective size. If two loci are loosely linked, fluctuations in E(r2) are large. The maximum value, reached at the end of the bottleneck, is 10 to 100 times greater than the value obtained at the end of the burst. Our results show that the interpretation of observed linkage disequilibrium, by means of statistical tests, requires an accurate knowledge of population demography.

In Drosophila melanogaster a large number of the genes coding for 18S and 28S rRNA are interrupted in the 28S region by insertions of two types. Ribosomal insertion transcripts were compared in wild-type and bobbed strains. We found that the level of insertion transcripts increased in bobbed mutants after deletion of 50% of INS− genes, and inversely decreased in revertants when more than 50% of wild-type levels. Among type II insertion transcripts we found a predominant 3·5 kb RNA, precisely of the most frequent insertion size. No primary insertion transcript has been found, although it could be undetected if very fast splicing leads to mature 28S occurs.

Linkage crosses were used to determine the gene order of aminoacylase-1, beta-galactosidase, and transferrin on mouse Chromosome 9. The most likely gene order, which was more than 107 times more likely than the next most likely order, was transferrin, aminoacylase-1, beta-galactosidase. These results demonstrate that aminoacylase-1 and beta-galactosidase mark a chromosomal segment that has been conserved since divergence of lineages leading to man and mouse. The length of this segment is about 11 cM and the estimate of the true length of the conserved segment marked by these two genes is 33 cM. A second aminoacylase allozyme, which requires MgCl2, is described.

In mice homozygous for the asebia mutation (abJ/abJ) the sebaceous glands were small and the sebaceous cells abnormal, the fully grown hair follicles were twice as long as those in wild-type (+ / +) mice, the catagen and telogen follicles were abnormal and, because the active phase of the hair cycle was longer than in + / + mice, the initiation of the second hair cycle was delayed. The abnormalities in the sebaceous glands and in catagen and telogen follicles were also present in abJ/abJ embryonic skin grown on a nude host but anagen of the second hair cycle commenced at about the same time in abJ / abJ and + / + grafts. When recombinants incorporating mutant or wild-type epidermis and dermis were grown on a nude host, the abnormalities in the sebaceous glands and the catagen and telogen follicles were only observed in the recombinants incorporating abJ / abJ epidermis. It was concluded that mutant activity in the epidermis was responsible for the abnormalities in the sebaceous glands and in catagen and telogen follicles and that mutant activity at some site distant from the skin was responsible for the abnormalities in the timing of the hair cycles.

When DNA sequence data on various kinds of homologous genes sampled from two related species are available, there is a way to infer the effective size of their ancestral species, which is a simple consequence of gene genealogical considerations. This method, when applied to the common ancestral species of human and rat, human and mouse, human and bovine, or rodents and bovine estimates their effective sizes all to be of the order of 107, supporting the view that these species indeed shared, around 75 million years ago, a common ancestral species from which they are descended. The effective size thus estimated would imply that the ancestral species was abundant enough to have ample opportunity for adaptive radiation. The extent of silent polymorphism in that species might have been very large, possibly comparable to the number of silent substitutions accumulated in a gene after the mammalian divergence. Some causes that may alter these results and require a more elaborated statistical analysis are discussed.

A cloned Y-specific sequence (Bishop et al. 1985) was used as a diagnostic probe to distinguish between Mus musculus domesticus and Mus musculus musculusY-chromosomes. Analysis of the RFLPs obtained with genomic DNA isolated from wild mice caught along the contact zone between M. m. domesticus and M. m. musculus in Bulgaria and Denmark showed that the Y-chromosome flow between the two semi-species is very limited. The degree of Y-chromosome penetration was compared with that of seven diagnostic autosomal loci and the mitochondrial DNA. Breeding experiments showed that the lack of Y-chromosome introgression from one semispecies to the other was not due to a major hybrid breakdown. The results suggest that the disruption of differentiated co-adapted gene systems could play a role in limiting Y-introgression.

The autosomal recessive gene, dysgenetic lens (dyl) in the mouse has been mapped on chromosome 4. Two- and three-point crosses involving b (brown) and Mup-1 (Major urinary protein-1) indicate the following gene order: dyl–b–Mup-1. The approximate distance between dyl and b is 12 and between dyl and Mup-1 is 20 cM.