Aggregation chimaeras were constructed by fusing embryos homozygous for sash (Wsh) with fuzzy, leaden beige (fz, ln, bg) homozygotes to investigate the site of action of the beige and leaden loci. The genotype of the hair follicle was identified by fuzzy alleles (+ +fz or fzfz). All melanocytes were derived from the fuzzy leaden beige population, as sash homozygotes do not produce functional melanocytes. Reciprocal recombinant epidermal/ /dermal skin grafts were constructed from 14-day embryonic skin of homozygous fzln bg and either albino (aa cc) or pink-eyed dilution (pp) embryos to test for any dermal expression of leaden or beige, since the epidermal and dermal genotype of the chimaeric hair follicles could differ.

Patches of fuzzy and non-fuzzy hairs were distributed throughout the coats of two of the three chimaeras obtained. The pigmented regions were blue grey, typical of the leaden beige interactive phenotype. Large abnormal beige granules were found in fuzzy and non-fuzzy hairs. Melanocytes in both classes of growing follicles were nucleopetal, typical of leaden. Similarly, the results of the 14-day skin grafts showed that the beige and leaden loci are melanocyte-autonomous.

The chimaeras showed a pigment distribution resembling the heterozygous sash phenotype. Thus the 1:1 gene dosage of sash: wild type in heterozygotes and chimaeras has an overall effect on pigment pattern that overrides the predicted random distribution of the melanocyte precursors.

We have studied variability maintained in a quantitative trait by the balance between symmetric mutation and direct stabilizing selection with a fluctuating optimum. Using a simulational computer model, we have found that wide fluctuations, such that the range of the optimum changes exceeds the width of the fitness curve, increase the trait variance, often by two or three orders of magnitude, over its value under constant selection. This happens because such fluctuations cause frequent allele substitutions at the loci that control the trait. At any particular moment the variance is increased mostly due to one or several loci where more than one allele is currently common. The data on fluctuating selection in nature are reviewed

A complete genetic map has been established for the P22 clear plaque forming mutations cir4-l, cir5-l and cir6-l. These are located within or closely linked to the immI region of P22 and represent a new class of clear plaque forming mutants located outside and rather distant from the immC region. They were mapped with respect to the markers mnt, vy and ant of the immI region and to genes 16 and 9 which span it. The three cir mutations complement each other and – with one possible exception – the cl, c2 and c3 mutations of the immC region. P22 cir6-l – like P22 cir5-l (Harvey et al. 1981) – is suppressed by the ant−am19 allele, whereas P22 cir4-l is not. The results are discussed in terms of the regulation of early ant expression.

The mutational load of a multigene family with uniform members was studied by computer simulations. Two models of selection, truncation and exponential fitness, were examined, by using a simple model of gene conversion. It was found that the load is much smaller than the Haldane–Muller prediction under the truncation selection, and that it becomes approximately equal to the value calculated by the formula, nv(1 − q)/(m − nq), where n is the copy number, v is the rate of detrimental mutation per gene copy, m is the truncation point in terms of the number of detrimental genes eliminated, and q is the equilibrium frequency of detrimental mutation. However the equilibrium frequency cannot be analytically obtained. For the exponential fitness model, the load is close to the Haldane–Muller value. When there is no gene conversion, the load becomes larger than the cases with conversion both for the truncation and the exponential fitness models. Thus, gene conversion or other mechanisms that are responsible for contraction–expansion of mutants on chromosomes helps eliminating deleterious mutations occurring in multigene families.

A 1·5 kilobasepair repeated DNA sequence is duplicated in direct orientation so as to flank the suppressor of forked gene in the euchromatin–heterochromatin transition region on the X chromosome of Drosophila melanogaster. These two copies are almost identical, but DNA blotting, analysis of cloned sequences and database searches show that elsewhere in the genome, homologous sequences are poorly conserved. They are often associated with other repeats, suggesting that they may belong to a scrambled and clustered middle repetitive DNA family. The sequences do not appear to be related to transposable elements and their location in different strains is conserved. In situ hybridization to metaphase chromosomes shows that homologous sequences are concentrated in the pericentric regions of the autosomes and the X chromosome. The sequences are not significantly under-represented in DNA from polytene tissue and must lie in the replicated regions of polytene chromosomes. The almost perfect conservation of the two repeats around suppressor of forked in D. melanogaster suggests they arose by duplication or gene conversion. Suppression of recombination in this chromosomal region presumably allows this unusual organization to be stably maintained. In the X-ray induced allele, suppressor of forked-L26, the sequence between the repeats, including the gene, and one copy of the repeat have been deleted.

Three methods of artificial induction of autogamy in Paramecium caudatum were described: (1) treatment with KCl+papain, (2) treatment with KCl and then with KCl + papain and (3) ordinary mating reaction and then treatment with papain. As expected, one-to-one segregation ratios were obtained in the progeny from the parents heterozygous for the two loci: mating type and lactate dehydrogenase. A high rate of autogamy is induced by method (1), but its use is restricted to only a few clones. Autogamy is also induced at a high rate by method (2), by which the induction is more stable. Autogamy is induced at a lower rate by method (3), but this method can be widely applied to every species of Paramecium which has complementary mating types. Some exautogamous progeny become completely sterile through successive autogamy. The cause of this sterility is discussed.

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.

Previous reports involving mammalian systems, particularly mice, have demonstrated the existence of cis- and trans-acting modifiers of transgene methylation. These modifiers are thought to be important in dominance modification, genome imprinting and cellular expression mosaicism. Their potential role in the penetrance and severity of many complex human diseases could be of even greater significance. In the present investigation we demonstrate that modifiers that act in a similar fashion to those identified in mice also exist in a non-mammalian vertebrate, the zebrafish Danio rerio. We also provide evidence that the transgene methylation pattern may be influenced by the sex of the individual and environmental modulators such as temperature and sodium butyrate. These data support the theory that this type of dominance modification is mechanistically similar to position effect variegation in Drosophila. Furthermore, these data suggest evolutionary conservation of the modifiers, at least within vertebrates, and imply that they and their actions are important in normal vertebrate development.

The gene conversion parameters which affect allele frequencies in populations are defined, and their ranges and typical values are given for several genera of fungi, where meiotic octads and tetrads provide the best information on conversion. Both gene conversion and disparity in direction of conversion are common. Data from Ascobolus immersus show that conversion properties are largely stable with time, but can be changed environmentally and by genetic conversion control factors. Equations are given for the interactions of selection, mutation and gene conversion in determining equilibrium frequencies. Numerical examples, using typical values of conversion parameters from the fungal data, show that for alleles which are selectively neutral or have very low selection coefficients, conversion will often have very large effects on their equilibrium frequencies and may lead to fixation. Where selection coefficients are higher, conversion has major effects on the frequencies of deleterious recessive alleles, but lesser effects on deleterious dominant alleles: a critical comparison is that of s with 2y. The available estimates for conversion parameters (at least in fungi) are of a magnitude to make gene conversion an important factor in evolution.

A Robertsonian translocation in the mouse between the X chromosome and chromosome 2 is described. The male and female carriers of the Rb(X.2)2Ad were fertile. A homozygous/hemizygous line was maintained. The influence of the X-autosomal Robertsonian translocation on anaphase I non-disjunction in male mice was studied by chromosome counts in cells at metaphase II of meoisis and by assessment of aneuploid progeny. The results conclusively show that the inclusion of Rb2Ad in the male genome induces non-disjunction at the first meoitic division. In second metaphase cells the frequency of sex-chromosomal aneuploidy was 10·8%, and secondary spermatocytes containing two or no sex chromosome were equally frequent. The Rb2Ad males sired 3·9% sex-chromosome aneuploid progeny. The difference in aneuploidy frequencies in the germ cells and among the progeny suggests that the viability of XO and XXY individuals is reduced. The pairing configurations of chromosomes 2, Rb2Ad and Y were studied during meiotic prophase by light and electron microscopy. Trivalent pairing was seen in all well spread nuclei. Complete pairing of the acrocentric autosome 2 with the corresponding segment of the Rb2Ad chromosome was only seen in 3·2% of the cells analysed in the electron microscope. The pairing between the X and the Y chromosome in the Rb2Ad males corresponded to that in males with normal karyotype. Reasons for sex-chromosomal non-disjunction despite the normal pairing pattern between the sex chromosomes may be seen in the terminal chiasma location coupled with the asynchronous separation of the sex chromosomes and the autosomes. The Rb2Ad chromosome can be useful for studies of X inactivation, as a marker for parental derivation of the X chromosome and for mapping loci by in situ hybridization.

1. Antigenic polymorphism between individuals of the subspecies Peromyscus maniculatus gracilis is demonstrated.

2. Absorption analysis of heteroimmune rabbit sera using erythrocytes of individual mice revealed two unitary complete (saline) agglutinins, designated as anti-A and anti-B, reactive to erythrocytic antigens in this species.

Isoimmunization apparently demonstrated these same antigenic differences.

3. The two antigenic characters (A and B) defined by the agglutinins are inherited as if simply related to allelic factors designated PmA and PmB respectively.

4. These two antigens are found in representatives of both the subspecies P. m. gracilis and P. m. bairdii, and, in addition, the A antigen was observed in a laboratory stock of P. polionotus. Tested individuals of the species P. leucopus and Mus musculus, however, possessed neither antigen, and samples of human blood also lacked both antigens.

5. Species-specific agglutinins distinguishing individuals of the species P. leucopus and P. maniculatus, and population-specific agglutinins which distinguished between individuals of the subspecies P. m. bairdii and P. m. gracilis, have been observed.

A selection experiment for sternopleural bristles in Drosophila melanogaster was undertaken to measure the effect of suppressing crossing-over on chromosomes II and III using the inversions Curly and Moiré marked with a dominant gene, which severely reduce crossing-over. In one set of lines selected wild-type males were mated to selected females, heterozygous for Cy and Mé, and in a parallel set selected males carrying the inversions were mated to selected wild-type females. Because there is no crossing-over in the males in this species, crossing-over is much reduced in the first set and is at its usual level in the second. The effect of the selection was measured on flies which did not carry the inversions. The suppression of crossing-over reduced the advance at the limit by 28 ± 8% for selection upwards and by 22 ± 7% for selection downwards. The segregation ratios of the inversions were observed throughout the experiment. At the end, the proportion of wild-type flies emerging was not different in the two sets of lines. The results are consistent with an assumption of initial linkage equilibrium between loci affecting sternopleural bristles in the base population.

DK/Lm is a new inbred mouse strain with over 20 generations of brother–sister mating. The genotype of the DK/Lm mouse at the black-brown locus is b/b and heterozygosity at the agouti locus (Ay/a) is maintained. DK/Lm-Ay/a mice become sable in phenotype at the first moult, whereas C57BL/6J-Ay/a mice do not. The sable phenotype is defined as that of a mouse whose basic colour is yellow (phaeomelanic) but whose dorsal aspect is more or less darkened by the presence of nonyellow (eumelanic) pigment. At about 6 months of age the DK/Lm mouse gradually reverts to yellow in phenotype.

Mice of the two strains are compared. Observations are discussed and related to hypotheses regarding gene action at the a and e loci. The new strain is a useful experimental model for study of the relationship between gene action at the agouti locus and the important pleiotropic effects influenced by this locus.

Klebsiella aerogenes strain V9A carries determinants AK and TK, giving resistance to ampicillin and tetracycline, and a plasmid FKlac, but no active sex factor. F− and I-type sex factors were able to transfer TK from V9A to Escherichia coli K12 and between strains of K12, and TK behaved as a separate plasmid with its own replicon. AK could not be transferred, except possibly by a sex factor carrying its own A determinant, but the evidence for such transfer was inconclusive. It is suggested that AK is either a chromosomal gene or is in a plasmid with a cell attachment in Klebsiella not represented in E. coli K12. AK produces a β-lactamase.