A technique based upon auxotrophic mutants and recessive suppressor genes has been used to select for somatic recombinants in the dikaryon of Coprinus lagopus. Both haploid and disomic nuclei with a reassortment of chromosomes from the two component nuclei of the dikaryon were found. The presence of the disomics indicates that the recombination process is one of fusion of nuclei with a gradual haploidization by loss of chromosomes at mitotic divisions. An extensive test for crossing-over in the A chromosome failed to give an unambiguous crossover recombinant, but it produced a stock with a normal A chromosome and a fragment of the chromosome.

A spontaneous mutation ‘tich’ (gene symbol tch) appeared as a recessive mutation in inbred mice of strain A. TL. Homozygotes are rather dumpy mice of approximately normal weight but with short limbs and tail. Skeletal measurements on backcross siblings show that the mandible bones are almost normal but long bones and some parts of the pelvic and pectoral girdles are short. Although tich resembles brachypodism phenotypically it is not linked to agouti, and does not match the description of any other skeletal mutation. There was some evidence for weak linkage with albinism on chromosome 7. The mutation has reappeared amongst the A. TL mice of a UK commercial breeder and may have been accepted as the norm for A. TL amongst some European users of this mouse.

This paper shows that a number of models of the maintenance of polymorphism in a heterogeneous environment, including those of Levene and Dempster, can be derived from a simple assumption about the way in which the numbers and kinds of individuals emerging from a niche depend on the number of eggs laid in it. It is shown that for such models, unless selective advantages per locus are large, protected polymorphism requires that the relative niche sizes lie in a narrow range. This lack of robustness applies also to models of stable polymorphism proposed by Clarke and by Stewart & Levin. Excluding models relaying on habitat selection or restricted migration, the only models which may escape this criticism are diploid models with partial dominance with respect to fitness, such as one proposed by Gillespie, in which in all niches the fitness of heterozygotes is higher than the arithmetic mean of the homozygotes.

M and M′ strains of Drosophila melanogaster in the P-M system of hybrid dysgenesis were compared in two series of tests, with the following results. (1) The singed-weak hypermutability regulation test showed that M′ strains had lower P excision rates than M strains, suggesting that P-elements repression must occur in M′ strains although it is not detectable by gonadal dysgenesis assays. (2) The evolution of mixed P + M and mixed P + M′ populations was compared, using a strong P strain. The P + M cultures invariably evolved in a few generations into strong P cultures, while the P + M′ cultures evolved into P-type cultures with reduced P-factor potentials. However, after 30 generations of culture, both these types of mixed cultures had similar P copy numbers, suggesting that regulation of copy number had occurred in them.

Nine (41%) of the 22 enzymic and non-enzymic loci examined in a strain of mice divergently selected for six-week body weight (six lines selected in each direction and six controls) were found to be polymorphic. The degree of polymorphism varied between the replicates from a maximum of 38% to a minimum of 14% with an average individual heterozygous at 7·7% of its loci. There was no obvious association between any of the isozyme variants and body size. The frequency distribution among the 18 lines was adequately accounted for by random genetic drift. However, an association was observed between body size and the Hbb locus; the Hbbs allele was found to be fixed in all of the six Large lines. An examination of the variance of gene frequencies at this locus excluded random genetic drift as an explanation for the fixation.

Six wheat/Agropyron intermedium addition lines are described on the basis of their phenotype and biochemical markers. An assessment of homoeology of each addition chromosome is made. Chromosome morphology, plant phenotype, isozyme and protein studies are compared with similar data for other wheat/alien addition lines and other members of the Triticeae. These comparisons give consistent results and it is concluded that addition lines L1, L2, L3, L4, L5 and L7 carry Agropyron chromosomes of homoeologous groups 7, 3, 1, 4, 5 and 6 respectively. This agrees with previously published work with one exception: the L5 chromosome belongs to homoeologous group 5 and not group 2 as proposed by Figueiras et al. (1986).

Eleven mutants of R-factor R57 have been isolated which show constitutive expression of resistance to tetracycline (Tc). These derepressed (Tdr) mutants all gave a much greater resistance to Tc and to its analogue, minocycline, than could be obtained by optimal induction of cells carrying the wild-type (T+) determinant. Cells carrying each of the Tdr mutants together with T+ of either R6-S or of a plasmid found in Escherichia coli mi19 showed inducible Tc resistance, indicating that the Tdr mutants were all recessive, i.e. of repressor-negative type. Tdr1 was not recessive to the T-determinant of RP1, suggesting that the repressor gene products of the T-determinants in R57 and RP1 have different specificities.

The variation from spontaneous mutations for 6-week body weight in the mouse was estimated by selection from a cross of two inbred sublines, C57BL/6 and C57BL/10, separated about 50 years previously from the same inbred line. Selection was practised high and low for 12 generations from the F2, followed by one generation of relaxation. The lines diverged by approximately 1·7 g or 0·7 sd. The additive genetic variance was estimated in the F2 by restricted maximum likelihood and from the selection response, and from this variance the mutational heritability hM2 was estimated using the number of generations since divergence. Estimates of hM2 range from 0·08 to 0·10% depending on the method of analysis. These estimates are similar to those found for other species, but lower than other estimates for the mouse. It is concluded that substantial natural and, perhaps, artificial selection operated during the maintenance of the sublines.

By means of rye chromosome CR from Imperial added to Chinese Spring wheat the alien substitutions 4A/CR, 4A/CRS, 4B/CR, 4D/CR, 4D/CRS, 7A/CRL, 7B/CR, 7B/CRL, 7D/CR and 7D/CRL were isolated. Substitutions 4A/CRS and 4D/CRS had a better fertility and vegetative vigour than the corresponding substitutions with the complete chromosome CR. In substitutions the entire chromosome CR did not compensate for the absence of wheat chromosomes of group 7, whereas the substitutions 7A/CRL, 7B/CRL and 7D/CRL were more fertile than the corresponding nullisomics 7A, 7B and 7D. It was shown that the short arm of CR is homoeologous to wheat telosomes 4Aα, 4BL and 4DS, whereas most of the long arm of CR is homoeologous to the wheat arms 7AS, 7BS and 7DS. Rye chromosome CR is designated 4R. Only three substitutions with rye chromosome DR were produced. The fertility of substitutions 7A/DR and 7B/DR were considerably lower than that of nulli-7A and nulli-7B. The 4B/DR substitution was sterile. Rye chromosome DR is believed to be a double interchanged chromosome in comparison to the corresponding Secale montanum chromosomes. Rye chromosome DR is designated 7R. On the basis of substitution ability of CR and DR and several homoeologous gene loci on these chromosomes an evolutionary scheme for the derivation of the Secale cereale genome from Secale montanum is outlined.

The survival of the plasmid RP 1–1 in E. coli depends on the presence of a functional polA gene. Unlike other plasmids that have this requirement, the replication of RP 1–1 is inhibited by chloramphenicol. RP 1–1 cannot be inherited by E. coli recA mutants, and inactivation of the recA gene product in a recAts mutant leads to destruction of the plasmid. RP 1–1 cannot be inherited by recB or recC E. coli unless the strains also carry a suppressor of these genes, such as sbcA or sbcB. The pattern of replication of RP 1–1 in E. coli K12 and its mutants suggests that the survival of this plasmid in this species is the outcome of a balance between DNA polymerase I-specified replication and exonuclease destruction.

The genetic basis of variation in rate of seedling growth, and development has been examined in the Australian commercial population of Phalaris tuberosa. A model of additive genetic maternal effects has been used, with seed weight of the female parent as an index of maternal ability. Rate of leaf appearance, rate of tillering and growth per tiller are all genetically variable in the population, with estimated heritabilities of 0·36, 0·23 and 0·34 respectively on an individual seedling basis. Total seedling growth has a lower heritability (0·17), due to a negative genetic correlation between tiller production and growth per tiller ( − 0·46). These two components have also been shown to be subject to qualitatively different seed size maternal effects. Genetic differences in seed size in the female parent have been found to influence growth per tiller, while environmental differences in seed size affect primarily the rates of leaf appearance and tiller production.

We studied the amount of Mu(G+) and Mu(G−) phages in different Mu lysates prepared either upon induction or upon infection of E. coli and Erwinia strains. We also looked at the level of expression of the modification function (mom) by Mu(G−) phages, both after induction and after infection of E. coli and Erwinia. The expression of mom seems to be regulated in the same manner in E. coli and in the strain of Erwinia carotovora tested. The proportion of both types of Mu(G+) and Mu(G−) phages in induced lysates is very variable and we found growth conditions favouring the production of Mu(G−) particles. This should extend the use of Mu as a genetic tool and as a generalized transducing phage to many enterobacteria.

Recombination between chloramphenicol-sensitive (Cms) mutants of Rl, and R100, has been demonstrated in Escherichia coli K12 rec+; it occurs at reduced frequency in recB and recC, and is not detectable in reeA, indicating that R factor recombination depends on host functions. Some mutants of R1 also recombine with an R100 mutant in a similar way. recA cells carrying an R1 and an R100 Cms mutant (hetero-R state) have a low level of chloramphenicol-resistance, and form a chloramphenicol acetyl transferase that has lower specific activity than enzyme from hosts carrying wild-type or recombinant factors. These results suggest the occurrence of interallelic complementation between mutant R factors.

A tricomplex heterozygote has a synthetic chromosome complement consisting of four pairs of arms of chromosomes 2 and 3 in the form of a compound of two homologous arms (a homocompound) and of three compounds of two nonhomologous arms (heterocompounds), each being homologous to an arm of different compounds. In meiosis, pairing of homologous arms results in the formation of a single and a multiple configuration that are structural equivalents of a univalent and a trivalent. Data are presented indicating that, in a given complement, the pattern of the distribution of three heterocompounds at division I is the same in males and in females. The distribution depends on the arrangement of the 2nd and the 3rd chromosome centromeres in the trivalent. In configurations presumed to be homocentric (all three chromosomes having homologous centromeres), the distribution was random or nearly random while, in configurations presumed to be heterocentric, the distribution appears non-random, with one of the segregation alternatives being roughly twice as frequent as either one of the two other alternatives that were more or less equal in frequency. The results could be explained in terms of the 3rd chromosome centromere being ‘strong’ in directing the two 2nd chromosome centromeres to the opposite pole at division I, an explanation implying a functional differentiation of the two autosomal centromeres or adjacent sequences. Data are also presented showing that in females the distribution of the homocompound is non-random with respect to the distribution of the heterocompounds; the homocompound was recovered preferentially together with the single one of the three heterocompounds. This is inconsistent with the prediction based on the theory assuming an existence of two independent pairing pools.

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.

A total of 357 house mice (Mus domesticus) from 83 localities uniformly distributed throughout Switzerland were screened for the presence of a homogenously staining region (HSR) on chromosome 1. Altogether 47 mice from 11 localities were HSR/ + or HSR/HSR. One sample of 11 individuals all had an HSR/HSR karyotype. Almost all mice with the variant were collected from the Rhone valley (HSR frequency: 61%) and Val Bregaglia (HSR frequency: 81%). For samples from most of thearea of Switzerland, the HSR was absent. There was no strong association between the geographic distribution of the HSR and the areas of occurrence of metacentrics. However, at Chiggiogna the HSR was found on Rb (1·3). Possible explanations for the HSR polymorphism are discussed.