A method is proposed for estimating effective population size (N) from data on linkage disequilibrium among neutral genes at several polymorphic loci or restriction sites. The efficiency of the method increases with larger sample size and more tightly linked genes; but for very tightly linked genes estimates of N are more dependent on long-term than on recent population history. Two sets of data are analysed as examples.

Staphylococcus aureus M4 has chromosomal resistance to streptomycin, plasmid-borne resistance to penicillin and tetracycline and probably chromosomal inducible erythromycin resistance. It also has constitutive erythromycin resistance which is unstable and linked to kanamycin and lincomycin resistance and the ability to produce a diffusible pigment. Variants have been isolated which have stable kanamycin, lincomycin and constitutive erythromycin resistance and these do not produce the diffusible pigment. In transduction experiments kanamycin, lincomycin and constitutive erythromycin resistance were always co-transduced together with streptomycin resistance. The transduction frequencies were approximately 100 times higher with the stable variants compared with the parent. The transductants, irrespective of the donor used, all had stable resistance and did not produce the diffusible pigment. Although transduction with UV-irradiated transducing lysates was characteristic of a plasmid, no corresponding plasmid DNA has been detected.

Segregation-distorting t-sperm show a specific increase in N-acetylglucosamine: galactosyltransferase activity over wild-type (+/+) due to a deficiency of a wild-type galactosyltransferase inhibitor (Shur & Bennett, 1979). Eight other enzymic activities are indistinguishable between +- and t-sperm suspensions. In this study, three additional points are analysed. First, galactosyltransferases are assayed on sperm homozygous for a semilethal haplotype (tW2/tW2), relative to heterozygous (+ /tw2) and wild-type (+/+) controls. tW2/tW2 assays circumvent the +-sperm inhibition of t-sperm galactosyltransferases that occurs in heterozygous + /t-assays and show that t-sperm are actually four times as active as wild-type. Second, sperm which are compound heterozygotes for two complementing lethal t-haplotypes (tlx/tly), have nearly twice the theoretical enzyme level of tlx/tly sperm. Thus, in either homozygous (tW2/tW2) or double heterozygous (tlx/tly) form t-haplotypes act synergistically on sperm galactosyltransferase activity.

Third, and most interesting, sperm bearing either recombinant, viable t-haplotypes (+/tv, tvx/tvy), or one of three dominant T/t-complex mutations, were assayed to determine which portions of the T/t-complex are responsible for elevated galactosyltransferase activity. Results show that sperm bearing recombinant, non-segregation-distorting, viable tv-haplotypes no longer show elevated transferase activity. Therefore, the elevated t1-sperm galactosyltransferase activity strictly correlates with the increased transmission frequency of t1-sperm. These studies strengthen further the hypothesis that sperm surface galactosyltransferases are involved in egg binding during fertilization, and that t1-sperm segregation-distortion results, at least in part, from increased galactosyltransferase activity.

The resolution of antibiotic-CsCl gradients enabled an examination of the satellite DNAs in the nuclear DNA of Drosophila simulans. Of the eight distinct satellite DNAs which were detected, four band at almost the same buoyant density in CsCl but can be resolved in netropsin sulphate-CsCl gradients. Each consists of a repeated sequence which, in five of the satellites, is shown to be arranged in tandem for long regions of the chromosomal DNA. One satellite (1·697 g/ml in CsCl) contains repeated sequences interspersed with other sequences. The satellite DNAs were compared with the satellite DNAs known to be present in the sibling species, D. melanogaster. The two species have different overall complements of satellite DNAs, but one satellite (1·672 g/ml) may be identical.

Eleven catabolic markers have been located on the chromosome of Pseudomonas aeruginosa PAO using FPS-mediated conjugation and G101 transduction. Most of these markers are located in the region 20–35 min, and the remainder in the region later than 60 min. Four chu genes concerned in the sequential degradation of choline to glycine are closely linked.

Yeast cells contain many copies of mitochondrial (mit) genomes. The question we tried to answer was how mit− mutations occurring in one genome as a result of mutagenic treatment might yield homoplasmic mutant cells. Three processes were considered. First, that these cells originate by segregation of mutant and standard alleles during cell division. Secondly, that they originate through intracellular selection, for which cell division is not required. Thirdly, that recombination involving the mutant and standard alleles is non-reciprocal and unidirectional mit+ → mit− so that the mutant allele is spread into the entire population of mitochondrial genomes within a cell, thus making it homoplasmic mit−. The results indicate that the first process, although efficiently producing homoplasmic cells from heteroplasmic zygotes (for review see Birky, 1978), seems to play only a minor, if any, role in producing homoplasmic mutant progenies from mutagenized cells. The most important is the second process, that is, intracellular selection occurring in cells which have one or a few genomes carrying mit− mutations, while the remaining genomes are irreversibly damaged. The third process, unidirectional mit+ → mit− conversion, does not seem to play any part.

This report examines several issues bearing upon intragenic recombination in higher eukaryotes. The fine structure data accumulated in our analysis of the genetic organization of the rosy locus in Drosophila melanogaster. Firstly, we confirm that a conversion event has a markedly less than 50% probability of resulting in flanking marker exchange, a finding consistent with more recent analyses of the available Saccharomyces data (e.g. Fogel et al. 1978). As reported earlier, co-conversion of recombinationally separable sites within the rosy locus occurs (McCarron, Gelbart & Chovnick, 1974). In this report, we demonstrate that the frequency of co-conversion is inversely proportional to the distance between co-converting sites. As in fungi, real conversion frequency differences are observed among rosy mutant alleles, and the data suggest that there may be a relationship between allele conversion frequency and map position. Unlike Neurospora and Saccharomyces, only one flanking marker exchange class is recovered from any given mutant heteroallele recombination experiment. In this respect, the Drosophila system resembles Aspergillus. As in Neurospora and Saccharomyces, rosy locus intragenic recombinants associated with flanking marker exchange exhibit interference with crossing over in adjacent regions, while no interference is seen among recombinants exhibiting parental flanking markers. Finally, experimental results are discussed which demonstrate the occurrence of postmeiotic segregation in Drosophila. These analogies between Drosophila and fungi provide further evidence in support of the notion that eukaryotes share common molecular mechanism(s) of meiotic recombination.

The recovery of two EMS induced mutations which are dominant suppressors of the lethality of cryptocephal in Drosophila melanogaster are described. One of these mutations Su(crc)1 is described in detail. It maps very close to cryptocephal at 54·7 on the second chromosome and its suppression of cryptocephal is temperature-sensitive. Temperature shift experiments show that the temperature-sensitive period is from before the pupariation until 12 h post pupariation. The temperature-sensitive period of Su(crc)1 is discussed in relation to the expression of l(2)crc, head eversion and the timing of pupal chitin synthesis.

Four new t haplotypes, tTu1 through tTu4, are described, three of them derived from the tw12tf haplotype and one (tTu4) from the tw2 haplotype. The tTu1 and tTu4 haplotypes cause taillessness in T/tTu1 or T/tTu4 heterozygotes, lack the lethality factor, weakly suppress recombination in the T−H−2 interval, and are transmitted to offspring from tTu/ + males at nearly Mendelian ratios. The tTu3 haplotype resembles tTu1 and tTu4 except for the fact that the T/tTu3 heterozygotes have normal-length tails. The tTu2 haplotype probably carries the lethal factor of tTu12tf, suppresses crossing-over in the T-H-2 and tf-H-2 intervals, and displays a slightly subnormal transmission ratio. In the compound heterozygote tTu1/tTu2, the male transmission ratio of the tTu1 chromosome is close to that of the original tTu12tf haplotype. A similar effect is observed in the tTu3/tTu2 heterozygote. This observation is interpreted as evidence for two regions within the t complex controlling the male transmission ratios. One of the regions is close to the tail-modifying region, the other is close to the lethality factor. Our findings parallel closely those made in the segregation distorter system in Drosophila.

Recombinant-inbred (RI) strains of mice derived from the cross of strains C57BL/6J and DBA/2J were used to study the inheritance of susceptibility to cortisone-induced cleft palate. Most of the RI strains could be classified as either resistant, like C57BL/6J, or susceptible, like DBA/2J, suggesting the segregation of a major locus. An association with the phosphoglucomutase-1 locus (Pgm-1) on Chromosome 5 was observed. There was no association with the H-2 locus on Chromosome 17 as had been reported in previous studies utilizing different strains. We conclude that susceptibility to cortisone-induced cleft palate may be determined by different loci depending on the strains studied.

The duodenal mucosa of normal (+/+) and heterozygous tortoiseshell (Moto / +) female mice was analysed by an ultrastructural histochemical copper localization technique. Copper was rarely detected along the duodenal microvilli of + / + female mice. However, copper was localized on the surface of the brush border of the duodenal mucosa and within pinocytotic vesicles at the base of the microvilli of Moto /+ female mice. The mottled defect may involve a defective intestinal uptake of copper, as well as a faulty copper transport mechanism.

Mouse mutant genes which result in defects similar to those of medical importance in man may be of value as models for the study of the defect concerned. We report here a new gene causing congenital cataract in the mouse, which may be useful in the understanding of cataract in man.

A further point of interest is that Kratochvilova & Ehling (1979) have recently developed a new method of measuring increased mutation rates in the mouse, by examining offspring of animals treated with mutagens for the presence of cataracts due to mutant genes. For the purposes of this test it is valuable to have information on the number and map position of loci which can mutate to give cataracts.