Mitochondrial malic enzyme (MOD-2) was found to be missing in partially complementing genotypes between lethal deletion alleles at the albino locus in Chromosome 7 of the mouse. Since such partial complementers survive to adulthood, the absence of normal mitochondrial malic enzyme is compatible with life; however, the sterility of both females and males may be correlated with this enzyme deficiency. Of the six radiation-induced lethal albino mutations, five includedeletions for the Mod-2 locus mapping 1 centimorgan distally from c. Our data indicate that the maximum genetic distance occupied by any of these deletions is 6 centimorgans.

The origin of triploid (3N) 5½-day blastocysts in rabbits is inferred from the segregation of sex chromosomes and of an autosomal M-marker whose properties are described. 39 triploids and no tetraploids were scored among 1454 chromosomally scored blastocysts. A delay of 8 h between an ovulatory injection and subsequent insemination raised the estimated normal incidence of 0·59% triploid blastocysts to 3·13%. The increase is ascribed primarily to digyny (17 blastocysts), and to diandry probably mediated by dispermy (1 blastocyst). The triploid components of the two 2N/3N mosaics and the one 3N/6N were digynic. Neither superovulation nor insemination of excessive numbers of spermatozoa could be shown to give rise to triploid embryos. The diandric triploid was X Y Y, the first of this constitution apparently reported in the rabbit. There was some evidence that X X Y triploid blastocysts up to 5½-day gestation are more viable than X X X. In the 2N/3N mosaics each component had been entered by one spermatozoon, and the dliploid component could not have been merely a contamination by dliploid maternal somatic cells. In 2N/4N, 2N/4N/8N and 3N/6N mosaics, each polyploid component showed an exact doubling of the marker chromosome constitution of a component of lower ploidy; their origin is ascribed to doubling or redoubling of chromosome number in isolated embryonic cells. With earlier data included, 49·08 (± s.e. 1·96)% of 652 diploid blastocysts were X Y. 460 non-experimental weaned rabbits were all cliploid.

The ‘maxy’ technique was used for scoring the frequencies of whole body and fractional mutations in the progeny of males of Drosophila melanogaster that had been irradiated with 2000 R, irradiation being followed or preceded by treatment with caffeine or acriflavine. Three 2-day broods were used, and males irradiated without subsidiary chemical treatment were used as controls. Both chemicals whether given as post- or pre-treatment strongly reduced the frequencies of fractional mutants in all broods. The frequency of whole-body mutants was unaffected in the first two broods and suffered a moderate reduction in the third. These results are discussed in relation to the hypothesis that repair processes regulate the development of a primary lesion into either a whole-body or a fractional mutant and that the functioning of the repair enzymes depends on the state of maturity of the male germ cell.

Initial experiments demonstrated that the plasmid R6K cannot be transferred to or maintained readily in the E. coli DNA polymerase I deficient strain JG138 polA1. Results with E. coli MM386 polA12 (R6K), which has a temperature sensitive polymerase I enzyme, showed cell division becomes abnormal when the polymerase I enzyme of the host bacteria is inactivated at the restrictive temperature. Under conditions of polymerase I deficiency, R6K replication, as measured by monitoring R-factor-mediated β-lactamase activity, also becomes abnormal with the loss of multiple R6K copies per cell and the apparent maintenance of a single R-factor copy per cell.

We have examined karyotypes of five species from five genera of cyprinid fishes endemic to the central and southeastern United States: Campostoma anomalum, Hybognathus hayi, Hybopsis aestivalis, Phenacobius mirabilis, and Pimephales vigilax. All five have a diploid chromosome number of 50. Variation in chromosome arm number among the five species is slight, and may be due to measurement error or technique difficulties. The karyotypes of 40 North American cyprinids are now known. All but five species have 50 (diploid) chromosomes. Variation in chromosome arm number also appears minimal; one-armed chromosomes (centromeres subterminal to terminal) normally comprise only a small fraction of the karyotype, and each species has roughly the same number of chromosomes with median and submedian centromeres. The conservatism of gross chromosomal evolution among these fishes is not in accord with recent hypotheses which suggest that progressive evolution of organisms may depend to a large degree on gene rearrangement brought about by gross chromosomal restructuring. Cyprinids are a highly speciose group in North America, and there is relatively strong morphological differentiation among species. The present data suggest that gross chromosomal restructuring may play only a minor role in the speciation and evolution of these fishes.

The allozyme polymorphism of Diadromus pulchellus has been analysed at 22 loci, the expression of one (Est-4) being male-limited. Six loci were found polymorphic for two (Ldh-1, Ldh-2, Acp-2, Pgi and Sdh) or three (Est-3) codominant alleles. Allele frequencies are similar in males and females at four loci in a laboratory population but there are important heterozygote deficiencies in females at three of them. The possibilities of negative heterosis or assortative mating to explain this deficiency are discussed.

Experiments were carried out to investigate genetic relationships within a population of Coriolus versicolor (a polyporaceous basidiomyceto causing white rot of wood) present in a birch stump. The population consisted of individual dikaryons occupying longitudinally continuous columns of decay separated from one another by narrow, dark, relatively undecayed interaction zones. These dikaryons were shown to be genetically homogeneous throughout their respective decay columns by dedikaryotization procedures. They were mutually antagonistic when paired in culture, but monokaryons derived from their fruit-bodies were interfertile. Experiments using synthesized dikaryons indicated that antagonism is inevitable between genetically distinct mycelia, but that the intensity of interaction diminishes with increased relatedness. Results of pairings between synthesized dikaryons and monokaryons varied according to the relatedness of the isolates. Antagonism invariably occurred when the monokaryon contained a nucleus differing from both nuclei in the dikaryon, but this did not necessarily prevent dikaryotization. Often in this situation dikaryotized sectors developed in the monokaryon visibly separated by zones of antagonism (‘tracks’). Where the monokaryon contained the same nucleus as one of the components of the dikaryon, antagonism usually occurred initially, but normally the colonies eventually fused.

A new type of genetic control of gene conversion is described from the Pasadena strains of the fungus Ascobolus immersus. It is characterized by cis/trans position effects and incomplete dominance. The P, K and 91 factors segregated from each other like Mendelian alleles and controlled the conversion frequencies and patterns of four nearby, closely-linked white ascospore colour mutations, although they did not usually coconvert with these w sites. Mutations of different origin responded similarly to the same control factors.

These control factors greatly affected the total conversion frequencies, the relative frequencies of the different detected conversion classes and various other conversion parameters. The detailed results are consistent either with P, K and 91 affecting both the frequency of hybrid DNA formation and the correction processes for removing mispaired bases, or if they do not affect the correction processes directly, then they must have large effects on the frequency of asymmetrical hybrid DNA formation, which must usually be much more common than symmetric hybrid DNA, and there must be both an inequality in the frequency with which the two homologous chromosomes (in these crosses, + bearing and w bearing) invade each other, and in the frequency with which the two strands of each chromatid invade the homologue in asymmetric hybrid DNA formation.

Sperm from mice heterozygous (t/+) for many t-lethal haplotypes show segregation distortion. More offspring inherit the t-haplotype than the normal wild haplotype. In contrast males heterozygous (tx/tv) for two t-lethal haplotypes are sterile although they produce normal-looking sperm. The experiments were undertaken to show whether sperm from tx/tv mice are the same as t-fertile sperm from t/+. Female albino mice were persuaded to mate first with coloured sterile tx/tv heterozygotes and then with normal white +/+ males. With the exception of one litter (from a t-heterozygous male which proved to have been misidentified) all the progeny were derived from the mating with the normal father. We conclude that t-bearing sperm from t-lethal (tx/tv) heterozygotes are inviable, and different from t-sperm in t/+ heterozygotes.

The herbicides Bentazone and Phenmedipharm kill the leaves of intact tobacco plants but do not affect callus cultures. Tolerant mutants were isolated by treating leaves of previously γ-irradiated haploid plants with herbicide then excising and culturing the green herbicide-resistant cell clones on the otherwise yellowed leaves. Among plants subsequently regenerated were a total of ten stable independently isolated mutants. Sexual crosses show these ten represent four Bentazone and two Phenmedipharm loci; all mutants were recessive to wild type.

The proportions of the two isozyme bands of the X-linked form of phosphoglycerate kinase (PGK-1) were compared in 16 tissues from four groups of adult heterozygous females. Little evidence was found for differences in expression of the two isozymes among tissues but there was a marked difference among the four groups of mice. The proportion of the PGK-1B enzyme was consistently lower in PGK-1AB heterozygous daughters of C3H/HeHa females than in corresponding heterozygotes with a C57BL/6Ha, DBA/2Ha or JBT/Jd mother. This difference was also observed in foetuses on the fourteenth day of gestation irrespective of whether the C3H/HeHa X chromosome was derived from the mother or the father. Sequential sampling of blood from the same heterozygous females provided no evidence for genetically determined cell selection in the adult erythropoietic tissue. The observed differences probably reflect variation at an X-chromosome controlling element locus among inbred strains of mice, similar to that described by Cattanach & Williams (1972) using X-linked morphological markers, although this has yet to be tested.