Starvation for isoleucine inhibits chromosome, minichromosome and pBR322 DNA replication in a stringent strain of E. coli, but does not do so in a relaxed mutant. Starvation for other amino acids inhibits either chromosome and minichromosome replication in both strains. From these results we conclude that oriC and pBR322 replication are stringently regulated and that isoleucine seems not to be essential for the protein synthesis required at the initiation of oriC replication. Deprivation of isoleucine in a Rel− strain gives rise to amplification of minichromosome and pBR322 with a better yield of the latter plasmid than that following treatment with chloramphenicol.

Selection and mapping of mutations affecting cysteine synthesis in Aspergillus nidulans was carried out. A new locus, cysE, is described, the mutants of which are deficient in in vivo conversion of O-acetylserine to cysteine, a step mediated by cysteine synthase. Three loci (cysB, C and E) were thus found to control this step in vivo, apparently without affecting the enzyme activity in vitro. By scoring for propargylglycine sensitivity of cys mutants, chromosomal map positions were obtained for all five cysteine loci (A, B, C, D and E).

A new gene designated sG was identified in Aspergillus nidulans by mutation affecting the enzyme sulphite reductase and leading to a strong derepression of arylsulphatase and enzymes constituting the alternative pathway of cysteine synthesis. The results indicate that proper physiological functioning of this pathway is strongly dependent on full activity of the sulphate assimilation pathway.

The Rec-1 strain of Caenorhabditis elegans increases recombination frequency three-fold. In this paper, we have investigated the effect of Rec-1 on the intragenic recombination phenomena of crossing-over and gene conversion. These events were increased two- to three-fold as was X-chromosome non-disjunction. All of the recovered recombinants were independent events, indicating that Rec-1 does not act pre-meiotically. The pattern of recombination in the Rec-1 strain resembles a meiotic pattern more than a radiation expansion. We conclude from this result that the Rec-1 enhancement of recombination is not the result of an increased number of DNA lesions randomly distributed along the chromosome. The increased recombination frequency of Rec-1 was not accompanied by any detrimental effects on growth, progeny number or spontaneous mutation rate. In this regard, the results may have implications for models which propose either selective advantage or disadvantage accompanying increased recombination.

We have investigated the effect of the ribosome-targeted antibiotic kasugamycin (ksg) in Podospora anserina. While ksg inhibits both growth and sporulation, it has a stronger inhibitory effect on the sporulation process. It was previously reported that sporulation of Podospora could be impaired when ribosomes translate with a too high accuracy, and since ksg was demonstrated to increase the ribosomal accuracy in E. coli, we wondered whether it would act similarly in Podospora. As a first approach we have isolated two mutations at different loci, Ks1 and Ks2, that increase the resistance to ksg at the level of both growth and sporulation. Interestingly Ks1−1 also confers a decreased resistance to paromomycin, which is a mistranslation inducer. Characterization of Ks1−1 and Ks2−1 mutants suggests that they could be ribosomal mutants.

The hordein polypeptide controlled by the Hrd G (Hor 4) locus in Elgina and derived lines was purified by preparative isoelectric focusing. The amino acid composition was similar to those of the major B hordeins encoded by the Hor 2 locus. Genetic analysis confirmed that Hor 4 is located proximally to Hor 1 on the short arm of chromosome 5. It is speculated that Hor 4 arose by translocation of genes from Hor 2, possibly in an ancestor of Elgina.

A barley endosperm cDNA clone was used to study the polymorphism and chromosomal location of β-amylase genes in barley. Analysis of DNA from seven cultivars digested with three restriction endonucleases showed two types of pattern, one present in Sultan and the other in the remaining six cultivars. A copy-number reconstruction indicated the presence of about three gene copies per haploid genome. Analysis of the six available whole chromosome addition lines and selected telocentric chromosome additions of barley into wheat showed the location of genes on the short arm of chromosome 2 (probably one copy) and the long arm of chromosome 4 (probably two copies).

The quaking (qk) locus on mouse chromosome 17 has been defined by a single viable quaking allele. Three new alleles of quaking were selected after ENU mutagenesis by their failure to complement the quaking phenotype. The qkk2 allele was induced on wild-type chromatin and the qkkt1 and qkkt4 alleles were induced on t-chromatin. Each is a recessive embryonic lethal mutation. They fail to complement each other and are not complemented by the deletion, TtOrl. Homozygotes and hemizygotes die at 8–9·5 days gestation, but not at a single precise time. Because the classical quaking mutation complements the lethality of these new alleles, but they fail to complement its quaking phenotype (myelination defect), we conclude that the quaking+ function is required for embryonic survival as well as for myelination.

DNA reassociation kinetics were used to determine the amounts and organization of repetitive and unique DNA in four mosquito species: Anopheles quadrimaculatus (Say), Culex pipiens (L.), Aedes albopictus (Skuse) and Ae. triseriatus (Say). Intraspecific variation in repetitive DNA amounts was examined in two geographic strains of Ae. albopictus fom Calcutta, India and the island of Mauritius. Repetitive and unique sequences in An. quadrimaculatus were distributed in a pattern of long period interspersion. Repetitive DNA in all other mosquito species exhibited a pattern of short period interspersion. The amounts of fold-back, middle repetitive, and highly repetitive sequences increased with genome size. The amount of foldback DNA increased at a much slower rate than the middle and highly repetitive sequences. Intraspecific variation in genome size in Ae. albopictus was due primarily to the amounts of highly repetitive DNA. S1 nuclease digestion of repetitive DNA in all species revealed a positive correlation between genome size and the proportion of the repetitive DNA consisting of short repeats. The amounts of long and short repeats increased with genome size but short repeats increased at a higher rate. The repetitive DNA of the Mauritius strain contained approximately 15% more short repeats than the Calcutta strain. These findings suggest that genome evolution in mosquitoes has resulted from changes in both the amounts and organization of repetitive elements.

In marsupials, X chromosome inactivation is paternal and incomplete. The tissue-specific pattern of inactivation of X-linked loci (G6PD, PGK, GLA) has been attributed to a piecemeal inactivation of different regions of the X. We here propose an alternative hypothesis, in which inactivation of the marsupial X is a chromosome-wide event, but is differentially regulated in different tissues. This hypothesis was suggested by the relationship between the positions and activity of genes on the kangaroo paternal X. In the absence of an HPRT polymorphism, we have used somatic cell hybridization to assess the activity of the paternal HPRT allele in lymphocytes and fibroblasts. The absence of the paternal X, and of the paternal forms of G6PD or PGK, from 33 cell hybrids made by fusing HPRT-deficient rodent cells with lymphocytes or fibroblasts of heterozygous females, suggests that the HPRT gene on the paternal X is inactive in both tissues and therefore not selectable. Since HPRT is located medially on the Xq near GLA, which shares the same characteristics of activity, we suggest that the locus-specific and tissue-specific patterns of activity result from a differential spread of inactivation from a single control locus, located near HPRT and GLA, outwards in both directions to G6PD and PGK. The nucleolus organizer region on the short arm does not seem to be part of the inactivated unit.

XY and XX doublesex D. melanogaster, expressing variable intersexual phenotypes, were compared for their pheromone levels (e.g. 7, 11-heptacosadiene, the main excitatory pheromone of females, and vaccenyl acetate, an inhibitory compound produced only by males). Despite the intersexual phenotype and the presence of female traits, the pheromone patterns of the homozygous dsx mutants, XY as well as XX, were similar to those of heterozygous males. Female-specific dienes were never found in significant amounts in such flies, which often showed significant amounts of the male-specific acetate and triggered very reduced levels of male courtship wing vibration.

The chromosomal region surrounding the ed, dp and cl genes has been studied cytogenetically (24–26 on 2L chromosome). It contains three Minutes and a haplo-sterile function. For isolation of deficiencies and mutations these haplo-insufficient functions were covered by an insertional translocation of 24D4-25F2 into the X chromosome, or by tandem duplications. 112 lethal and visible mutations induced by EMS and X-rays have been localized by deficiency mapping to 20 subregions. They specify 42 loci in a 48 band interval consistent with the notion that most of the bands encode a single lethal function. The dp, DTS, tkv and suppressor/enhancer loci for position-effect variegation were studied in detail. A dominant suppressor function was localized within the structural part of the dp complex. New non-conditional lethals have been isolated for the DTS locus. Complementation analysis with the previously identified dominant heat-sensitive alleles places the site for heat sensitivity in the middle of the locus. Two haplo-abnormal enhancers of position-effect variegation were localized in the region 25F2–26A1. A triplo-abnormal suppressor function maps to 26B2–5; 26B9. The dose-dependent functions of these loci were studied by the use of deficiencies and duplications.

The murine mutation, Cat Fraser (CatFr), causes dominantly inherited ocular cataracts. Lenses of adult mice bearing this mutation contain reduced amounts of all seven γ-crystallin proteins and their corresponding transcripts. Levels of other lens proteins and transcripts appear normal and no extra-ocular effects of the mutation have been observed. The selective effect of this mutation on the γ-crystallins is consistent with the possibility that the site at which it occurs is involved in the coordinated regulation of the family of genes which encodes them. We have shown that several restriction fragment length polymorphisms in the γ-crystallin genes segregate independently of the CatFr mutation. Therefore, despite its selective effect on the expression of the γ-crystallin genes, the mutation is not linked to them. This observation rules out the possibility that the mutation is in a cis-acting regulatory site.

Three new t-haplotypes, tp 4, tp 12 and tp 14, were isolated from M. musculus male mice captured in Central and East Bohemia, Czechoslovakia, about 400 km from the zone of hybridization between M. musculus and M. domesticus species. Complementation tests have shown that all three new t-haplotypes belong to tw 73 group. When compared with 5 t-haplotypes from M. domesticus they displayed the same pattern of BamHI restriction fragments with H-2 class I genes, and they also shared the t-specific 5·2 kb TaqI fragment of the alpha globin pseudogene. However, they differed from M. domesticus t-haplotypes at the D17Leh443 locus since they all displayed a 10·5 kb MspI fragment, labelled by the Tu443 probe, not found in wild type-chromosomes or in M. domesticus t-haplotypes. A hypothesis is proposed that t-haplotypes in M. domesticus originated by a single successful introgression from a parental species during speciation.

The ribosomal RNAs produced by the multigene families on the X and Y chromosomes of Drosophila melanogaster are very similar despite the apparent evolutionary isolation of the X and Y chromosomal rDNA. X–Y exchange through the rDNA is one mechanism that may promote co-evolution of the two gene clusters by transferring Y rDNA copies to the X chromosome. This hypothesis predicts that the proximal rDNA of X chromosomes will be Y-like. Consequently, rDNA variants found only on the X chromosome (such as those interrupted by type 1 insertions) should be significantly clustered in the distal X nucleolus organizer. Proximal and distal portions of the X chromosome nucleolus organizer were separated by recombination between the inverted chromosomes In(1)scv2 (breakpoint in the centre of the rDNA) and In(1)sc4Lsc8R (no rDNA). Molecular analyses of the resulting stocks demonstrated that rRNA genes containing type 1 insertions were predominantly located on the chromosome carrying the distal portion of the X rDNA, thus confirming a prediction of the X–Y exchange hypothesis for the co-evolution of X and Y chromosomal rDNA. Distal clustering is not predicted by the alternative hypotheses of selection or gene conversion.

With 12 alleles at the Est-α locus of Drosophila virilis, 44 genotypes of females heterozygous for a pair of them were constructed, and about 4 × 104 of their F1 progenies per genotype were electrophoretically examined. Eighty-three variants different from both of their parental alleles were obtained from a total of 2·0 × 106 progenies. Statistical analysis suggested most of their variants to be recombinants. And no null or inactive allele, of which frequency is several percent in natural populations, appeared in 11·6 × 104 progenies from females heterozygous for active alleles. These results suggest that the high variation of the Est-α locus in this species is generated by recombinants from female heterozygotes.

It has been proposed that isoenzymes functioning within cell organelles (chloroplasts, mitochondria) are genetically less variable than their cytoplasmic counterparts, as a result either of constraints imposed by the need to cross organelle membranes or from the different and specialized nature of organelle metabolism. However, some recent findings concerning chloroplast and cytoplasmic isozyme variability are not consistent with this thesis. We have analyzed a number of surveys of electrophoretically detectable enzyme variation in vertebrates, and show that for each of the four tested enzymes (malate dehydrogenase, isocitrate dehydrogenase, malic enzyme, and aspartate aminotransferase), the mitochondrial isozymes are less variable than their corresponding cytosplasmic forms. The mean heterozygosities across the four enzymes are 0·083 and 0·038 for the cytoplasmic and mitochondrial forms respectively. We conclude that mitochondrial isozymes are indeed subject to greater constraints than cytoplasmic forms and have fewer sites able to accept neutral or slightly deleterious mutations. It is also noted that of the enzymes analyzed, that with the smallest subunit molecular weight (MDH) has the least variable cytoplasmic and mitochondrial isozymes, whereas the enzyme with the largest subunits (ME) has the most variable isozymes.

Fifty-seven mouse strains were examined for genetic variation at 21 esterase loci. Three new alleles were found: Es-6d in strain A/WySna, Es-lle in FTC/CpbU and Es-18c in two WLL/BrA sublines. At most loci there was a single allele found in over 80% of strains, with one or two rare alleles. However, the Es-1, 3, 10, 13, 25 and 27 loci were much more polymorphic. Although several loci were linked on chomosomes 3, 8 and 9, linkage disequilibrium was only found between Es-5 and Es-11 (chromosome 8) and Es-26 and Es-27 (chromosome 3). There was also significant disequilibrium between Es-1 and 3, Es-1 and 10, and Es-3 and 10, which are on different chromosomes, suggesting that the 57 strains are not a random sample of inbred mouse strains. Fifty-four strains were closely related, with the Es-7b, –17a, –18a, –23c set of alleles, which are typical of Mus musculus domesticus. The three exceptional strains were MOL3 (Mus musculus molossinus), WLL/BrA (English–Norwegian origin) and TA2 (Chinese origin). There were 10 groups of strains which were identical at all loci. Sublines of the same strain were usually identical. Sometimes more distantly related strains, such as CBA/Bi, C3H/He, SM and DBA/Li, were identical, and in a few cases strains with no known common ancestry such as C58 and MAS were identical. Attempts to discriminate between a subset of 22 American and 15 European strains were unsuccessful, suggesting that the European strains add only in a quantitative manner to the gene pool of ‘laboratory mice’, whereas wild-derived strains such as MOL3 are genetically quite distinct from other laboratory mice.

To analyze abnormal fertilization in a hereditary mosaic strain (mo/mo) of Bombyx mori, the percentages of diploidy mosaic, polyploidy mosaic and polyploid eggs in a batch were estimated by using egg colour mutants (pe re). Among 48 890 eggs from crosses of pe + / + re, mo/mo females with pe re/pe re males, 9409 abnormal eggs were obtained; 4472 of them were diploidy mosaics (red-white eggs), 4038 were polyploids (black eggs) and 899 were polyploidy mosaics (566 black-white, 256 black-red and 77 black-white-red eggs). The total number of diploidy mosaic eggs was approximately equal to that of polyploid eggs. A significant correlation was detected between the diploidy mosaic and polyploid egg ratios within a batch. This suggests that diploidy mosaics are produced by double fertilization in which two genetically non-identical egg nuclei are fertilized in turn by a sperm, and polyploids are formed by the fertilization of a diploid, non-disjunctive egg nucleus gamete by a single sperm. Our results also indicated the presence of common factors modifying both mosaic and polyploid frequency. The concordance of the observed ratio of polyploidy mosaic eggs (1·84%) with the expected value (diploidy mosaic ratio × polyploidy ratio × 2 = 1·83%) suggests that the formation of mosaics occurs independently of the formation of polyploids in this abnormal fertilization process. We point out that it is necessary to modify Goldschmidt & Katsuki's general model to explain abnormal fertilization, and we propose several possible models.

A viable anaemic mouse mutant arose in the stock of a mouse fancier and has been characterized haematologically and genetically. Anaemic animals were less viable than normal animals (especially from 0 to 2 weeks of age) and had lower haemoglobin levels, percentage packed-cell volumes, higher red cell counts and lower mean cell volume than normal animals. Peripheral blood films showed a wide range of abnormal cells and extreme microcytosis. Linkage studies showed the mutant to be linked to the chromosome 15 markers Na Ca and bt; recombination with Ca was 1·37 ± 0·68 % for females and 10·5 ± 7·41 % for males. This position is similar to the microcytic anaemia, mk, mutant, and crosses between the viable anaemia mutation and mk/mk homozygotes showed the two to be allelic. Viable anaemia is therefore a second allele at the mk locus mkvan; new data give its position on chromosome 15.