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.

Sperm bearing complete t-haplotypes are preferentially transmitted during fertilization from heterozygous +/t males, often in excess of 95% relative to their +-bearing meiotic partner. Sperm from t-bearing males have an approximate two- to fourfold increase in β1, 4-galactosyltransferase (GalTase) activity, a cell surface protein that mediates sperm binding to the egg zona pellucida. The elevated GalTase activity strictly correlates with the preferential transmission of t-sperm from +/t males, since eight other enzymes show normal levels of activity on t-sperm. Furthermore, sperm bearing proximal partial t-haplotypes, which are no longer favoured during fertilization, have normal levels of GalTase activity. Nevertheless, it has been unclear whether the elevated sperm GalTase activity on t-sperm is due to specific loci in the distal segment of the T/t-complex, or rather, is an indirect consequence of the abnormal sperm function characteristic of +/t and tx/ty males. In this study, it is shown that the elevated sperm GalTase activity is due specifically to factors that reside within the distal segment of the T/t complex, which also contains Tcd-2, the strongest of the distorter loci. Since the structural locus for GalTase is located on mouse chromosome 4, these results also show that T/t-complex alleles on chromosome 17 are regulatory in nature and affect the expression of sperm surface components critical for normal fertilization. Models are presented to explain how elevated GalTase activity could contribute to sperm transmission distortion.

Over 400 temperature-sensitive mutants of Ustilago maydis have been tested for DNA synthesis at the restrictive temperature of 32°C by measuring 14C adenine incorporation into DNA and RNA. Five mutants were defective in DNA synthesis but none was completely blocked. One mutant, tsd-1, which is unlinked to the others, forms long uninucleate filaments at 32°C which die exponentially after 4h temperature treatment. The phenotype is comparable to that of thymine-starved bacteria, but it is possible that rather than being specifically defective in DNA synthesis the mutant is blocked in nuclear division.

A total of eighty-seven revertants were induced by ultra-violet light in an am3 strain. All of these revertants appear to be the result of mutation at sites in or close to the am locus. Fourteen of the eighty-seven revertants were partial revertants in that under some conditions of assay they possessed low glutamate dehydrogenase activity compared with the wild-type although their growth rate was similar to that of the wild-type. Enzyme extracts of thirteen of the partial revertants were assayed for glutamate dehydrogenase in various ways in order to establish qualitative distinctions between different kinds of mutant enzyme. On the basis of these tests six different groups were established, of which one contained six revertants, one three and the others one. All except one of the mutant enzyme types showed a marked activation when incubated with α-oxoglutarate plus NADPH2, and all of these had Michaelis constants for ammonium ion much higher than is found for the wild-type enzyme. The remaining group of three revertants gave, at first, no enzyme activity in any of the assay systems. Two of these (the third was not tested) were shown to produce an enzyme variety which becomes quite inactive in phosphate buffer at pH 8·0 but can be fully activated by the addition of ethylenediamine tetra-acetic acid. Forced heterocaryons between each of six partial revertants and eleven am mutants were made and the resultant sixty-six heterocaryons assayed for glutamate dehydrogenase activity. The partial revertants differed among themselves in their complementation characteristics. Some complemented with none of the am mutants, some with am1 only, and some with am1 or with am7. The complementation tests confirmed the differences established by the enzyme studies. The data presented here, together with previous work, demonstrate that ultra-violet light induced mutation in an am strain can result in at least eight types of revertant differing from each other in respect of the glutamate dehydrogenase variety which each can produce.

Electrophoretic surveys for nine species of Drosophila have been summarized in terms of the relative contribution to heterozygosity of each of ten gene frequency classes, the mean frequency of heterozygotes within subpopulations, and the degree of genetic divergence between subpopulations. It has been shown that the neutral model proposed by Kimura, and modified by Ohta to include the accumulation of slightly disadvantageous mutations, is capable of explaining all features of the data. The consistent difference between group I and group II enzymes can be explained by a difference in the average intensity of selection against mutational variants in the two groups. A highly significant difference between the temperate and tropical species in the distribution of heterozygosity appears to be due to the smaller effective breeding population sizes in the case of the temperate species.

Mitochondrial chloramphenicol and oligomycin resistance mutations were used to investigate mitochondrial inheritance in A. nidulans. Mitochondrial RFLPs could not be used to distinguish between paternal and maternal mitochondria because none were detected in the 54 isolates investigated. Several thousand ascospores from each of 111 hybrid cleistothecia from 21 different crosses between 7 heterokaryon incompatible isolates were tested for biparental inheritance. All mitochondrial inheritance was strictly uniparental. Not one instance of paternal inheritance of mitochondria was observed. The implications of our results for the theory that uniparental inheritance evolved to avoid cytoplasmic conflictare discussed. Possible explanations for the maintenance of strict uniparental inheritance of mitochondria in an inbreeding homothallic organism are suggested. The chloramphenicol resistance marker was inherited preferentially to the oligomycin resistance marker probably due to the inhibited energy production of mitochondria with the oligomycin resistance mutation. The maternal parent was determined for 93 hybrid cleistothecia from 17 crosses between 7 different strains. Contraryto previous reports A. nidulans strains functioned as both maternal and paternal parent in most crosses.

Haploid amoebae of Physarum polycephalum may form plasmodia by crossing, a sexual process that involves cellular and nuclear fusions, or by selfing, an asexual process in which the development of a single amoeba into a plasmodium may involve neither cellular nor nuclear fusion. Mutant strains (npf) in which selfing is suppressed were previously assigned to several functional groups on the basis of their ability to cross with one another in certain combinations. In the present study hybrid, diploid amoebae were isolated from both crossing-compatible and incompatible mixtures of npf mutants. The diploid amoebae from mixtures of compatible strains readily formed plasmodia by selfing, but selfing was suppressed in the diploids from incompatible mixtures. Thus the crossing tests between npf mutants may be viewed as complementation tests: their results reflect the differing selfing abilities of the hybrid, diploid amoebae that formed in each mixture. Genetical and environmental factors affecting the efficiency of formation of diploid amoebae were studied, and the diploids were shown to be stable during repeated subcultures. Although diploid amoebae carrying complementing npf mutations readily formed plasmodia by selfing at 26 δC, they could be cultured without plasmodium formation at 30 δC, a temperature that also inhibited selfing of the haploid npf+ strains. Ways are discussed of exploiting this combination of properties in a general procedure for isolating and testing diploids for dominance and complementation of amoebal mutations in P. polycephalum.

The shapes of T1 and T2 vertebrae from unselected Q strain mice and from strains selected for large and small body size were studied by Fourier analysis in order to ascertain whether shape change was produced by size selection. The vertebrae of large, small and control strains were easily distinguishable, but between replicate groups shape differences were less marked. The main component of shape change was size related, but mice unselected for size also showed a non-size-related shape change.

The necessary and sufficient conditions for the stability of the equilibrium point with no linkage disequilibrium are obtained for the three locus model with multiplicative fitnesses. It is shown that there are six inequalities that must be satisfied in order for this equilibrium to be stable. Three of the inequalities require that there be heterozygotic superiority at all loci. The other three are exactly those inequalities which are required for each pair of loci to be stable with linkage equilibrium if they are considered to be an isolated two locus system. Thus, all the information needed to determine the stability of this equilibrium with three loci is contained in one and two locus theory.

Highly variable DNA polymorphisms called microsatellites are rapidly becoming the marker of choice in population genetic studies. Until now, microsatellites have not been utilized for Drosophila studies. We have identified eight polymorphic microsatellite loci in Drosophila melanogaster and used them to characterize the genetic variation in a wild population from the Tyrrell's winery in Australia. Microsatellites were isolated from a partial genomic DNA library. All microsatellites consist of (AC)n repeats ranging from n = 2 to n = 24. Six loci were assigned to chromosomal location by genetic mapping, with three loci on chromosome II, one locus on chromosome III and two loci on the X chromosome. Up to four microsatellite loci were multiplexed in the same reaction. Microsatellite variation is substantially greater than allozyme variation in the Tyrrell's Drosophila population. 80% of the microsatellite loci examined are polymorphic, compared with 28% of allozymes. The mean number of alleles per polymorphic locus is 5·2 in microsatellites compared with 30 in allozymes. The average observed heterozygosity of polymorphic microsatellites is 47% compared with 26% for allozymes. Microsatellite variation in Drosophila melanogaster is similar to that reported for other insects. Higher variability commends microsatellites over allozymes for genetic studies in Drosophila melanogaster.

Expressions for the probability and average time of detection of a recessive visible gene in populations where there is partial selfing or partial full-sib mating are presented. A small increase in the proportion of inbred matings greatly reduces the average time until detection and increases the proportion detected. Unless the proportion of inbred matings or the population size is very small, the time and proportion detected are approximately independent of the population size.

We propose a general hypothesis involving properties of circular DNA which can explain such phenomena as the petite mutation, suppressiveness, and the polarity observed in mitochondrial recombination in the yeast Saccharomyces cerevisiae. This hypothesis involves excision and insertion events between circular DNA molecules as well as structural rearrangements in the DNA generated by these events. The special properties of circular DNA have been considered in analysing recombination, and a number of results are obtained which are not intuitively apparent.

This hypothesis can be applied to any situation involving circular DNA such as bacterial plasmids and cytoplasmic circular DNAs, where the opportunity exists for recombination and rearrangement events.

The locus of the gene striated, Str, on the mouse X-chromosome, was previously reported to be on the side of tabby (Ta) away from bent-tail (Bn). Results given in the present paper show this report to have been incorrect, and that the order of loci is Bn-Str-Ta-(Mo-Blo). In addition, the position of the translocation break in Searle's translocation (T1GH) has been confirmed, the order of loci with respect to T16H being Bn-T16H break-Ta-Blo.