In Paramecium caudatum, syngen 3, cells become mature (i.e. capable of undergoing conjugation) by about 50 fissions after conjugation. Matingtype-instability, another mating characteristic, begins to occur between 80 and 120 fissions after conjugation in heterozygotes for the mating type locus (Myohara & Hiwatashi, 1975). Mating-type instability also occurs in dominant homozygotes, earlier than in heterozygotes. In two different early maturing mutants the homozygotes become mature 20 fissions earlier than in wild-type clones, and mating-type instabiliiy also begins earlier than in wild-type clones. The number of fissions from maturity to the occurrence of the mating type change does not differ.

Fifty-six accessions of Triticum monococcum and one accession each of T. beoticum and T. sinskajae were analysed for their storage protein compositions and breadmaking quality as determined by the SDS-sedimentation test. In total 30 different alleles at the Glu-A1m locus coding for high-molecular-weight glutenin subunits (HMW-GS), 25 alleles at the Gli-A1m locus coding for ω- and γ-gliadins and 45 alleles at the Gli-A2m locus controlling the synthesis of α/β-gliadins were detected. Most accessions contained one x-type and one y-type HMW-GS and two genotypes were null for both types of subunits. Two polypeptides within the mobility range of HMW-GS in SDS-PAGE were shown to be ω-type gliadins encoded by genes on the short arm of chromosome 1 A. T. sinskajae and several ‘monococcum’ accessions were shown to share the same alleles at Gli-A1m, Gli-A2m and Glu-A1m, confirming sinskajae as a subspecies of T. monococcum. The SDS-sedimentation volumes of most accessions were very low (11–35 ml), a few accessions showing mean sedimentation volumes as high as 90–93 ml. Through the comparison between biotypes occurring in some accessions of ‘monococcum’, good bread-making quality was found to be associated with the presence of alleles y, c and i at the Gli-A1m locus. All accessions were resistant to leaf rust and rich in protein (≥ 16·5%), and most of them showed resistance to powdery mildew.

Intergeneric recombination of chromosomal genes between strains of Klebsiella pneumoniae and Enterobacter aerogenes has been observed. In these experiments a male strain of K. pneumoniae acted as donor and E. aerogenes acted as recipient. Closely linked genes were co-transferred with very high frequency, and transfer of a large fragment carrying several separated genes was not uncommon. Subcultures of a hybrid which had received a large fragment carrying five markers were tested for genetic stability; no spontaneous segregants were found among 34000 colonies tested. These results suggest that the transferred fragment had been integrated, and indicate the close genetic relationship between K. pneumoniae and E. aerogenes. This is in agreement with their very similar biochemical properties.

The various lethal t-haplotypes are divided into several complementation groups defined by lethality of tx/ty heterozygotes, and only partial complementation is usually observed in heterozygotes between t complementation groups. Several effects of the t region have been found to be preferentially or exclusively detectable in males. This paper describes results showing that the viability of t6/tw5 heterozygotes is influenced by the sex of the embryo; female viability is only slightly reduced (viability ratio approximately 0·80), while male viability is far below normal (0·25).

A model of centric fusions between the X or Y chromosome and an autosome carrying a selected locus is studied. It is assumed that fusions are not associated with any fitness effects, and that all chromosomes disjoin regularly from their homologues. It is shown that a necessary condition for the fusion to be favoured is that there is a selectively maintained sex difference in allele frequencies at the selected locus. If this condition is satisfied, the initial rate of increase of a rare Y-autosome fusion is about three times that of an X-autosome fusion, with the same parameter values. Computer calculations of the final equilibrium states reached by populations containing such fusions were done.

A population genetic model with a single locus at which balancing selection acts and many linked loci at which neutral mutations can occur is analysed using the coalescent approach. The model incorporates geographic subdivision with migration, as well as mutation, recombination, and genetic drift of neutral variation. It is found that geographic subdivision can affect genetic variation even with high rates of migration, providing that selection is strong enough to maintain different allele frequencies at the selected locus. Published sequence data from the alcohol dehydrogenase locus of Drosophila melanogaster are found to fit the proposed model slightly better than a similar model without subdivision.

Two mutations at the niaD locus (structural gene for nitrate reductase apo-protein) are genotypically suppressible. Both mutations result in loss of nitrate reductase enzyme activity and cross reacting material and are non complementing, nonleaky and highly revertible. They have the properties of nonsense mutations. This implies that some of the allele specific suppressors, which act on these and alleles at several other loci, are nonsense suppressors.

The influence of linkage on the rates of response to continued directional selection of quantitative traits deriving from variation contributed by new mutations in finite populations is investigated. Mutant genes are assumed to have additive effects which are randomly sampled from a symmetric distribution, and to be randomly located on the chromosome. Results were obtained by Monte Carlo simulation.

The expected steady rate of response, when variability from new mutations is balanced by that lost by drift and selection, is reduced the tighter the linkage, but the reduction is small unless there are few, short chromosomes. For a given rate of new mutational variance per haploid chromosome set per generation, greater effects of linkage are obtained in large than in small populations, because more mutants segregate. The response and influence of linkage are essentially the same whether the new variance is due to many genes of small effect or few of large effect.

The variability of response between replicates and generations was investigated, and the contribution to this of new mutants or recombination of existing mutants compared. Usually most genetic variability was due to the occurrence of a new favourable mutant of large effect.

Chromosomal localization of a gene involved in sisomicin production of Micromonospora rosea n.sp. was suggested by the results of matings between antibiotic producer and non-producer auxotrophic mutants. The frequency of producers among prototrophic recombinants varied with the chromosomal markers used for selection. The sis allele of the parent which was able to overgrow its partner in mixed culture predominated amongst the recombinants.

Four white-spored allelic mutants of Ascobolus immersus were used to study the effect of mutant and cross specificity on the recombination pattern in intragenic crosses.

In the locus studied no correlation was found between the position of mutants on the map and their basic conversion frequencies. One of the mutants evidently caused an increase of conversion frequency of the two others. Crossing-over in intragenic recombination may be mutant-specific as revealed by using two mutants which give no recombinants when crossed with each other. The frequency of crossing-over was higher in crosses in repulsion than in coupling involving the same pairs of mutants. Polarization observed in two-point crosses was due in some instances predominantly to differences in basic conversion frequencies of the mutants used, and in others to the relative position of the mutated sites. Mutants located in the central part of the studied region were those which converted most frequently in two-point crosses. No reciprocal conversions were observed.

A number of recombinant asci resulting from two or more separate but highly correlated recombinational events within a gene were found.

We have shown in an earlier paper that there is routinely a large correlation between the heterozygosity of common (P > 0·05) alleles and the number of rare (P ≤ 0·05) alleles at allozyme loci in Drosophila. We postulated that this correlation might be due to a high rate of intragenic recombination. While these correlations are large enough to be significantly different from zero, their relation to the mean correlations expected under the neutrality models is unknown. This paper reports the findings of a computer analysis determining the correlation for neutral allele pools as specified by the infinite-allele and charge-state models.

In the analysis, mean correlations for a range of Neμ values and sample sizes of 100 and 1000 genes varied from a high of 0·284 to a low of – 0·780. For the particular values of Neμ relating to the heterozygosity of Drosophila allozymes in natural populations, tests of the empirical correlations to the means expected under the neutral models are made. Most empirical correlations are significantly different from the mean correlations under the neutrality models.