The reversible integration of a plasmid into a phage genome via int-mediated recombination permits rapid and facile analysis of plasmid genes. Seventy-six independent mutants in the β-lactamase (bla) gene have been isolated and genetically mapped. The end points of deletions generated by partial digestion with restriction enzymes have been identified and this, together with the assignment of some nonsense mutations to specific amino acid residues, allows alignment of the genetic map with the DNA sequence.

The insertion of a high-copy-number plasmid into a lambdoid phage chromosome which lacks a functional repressor gene confers on the hybrid ‘phasmid’ the capacity to grow on an immune lysogen. This was found to be due to titration of repressor because of plasmid replication. We have exploited this property in order to isolate mutants that affect plasmid replication. These mutants have been mapped in a region that was previously characterized as necessary for plasmid replication and incompatibility properties. Some of the mutations could revert at frequencies characteristic of single-base-pair change mutations.

Two aspects of sperm phenotype were examined for tw32/+ males of the inbred strains C3H and C57BL/6 (B6). Sperm from fertile C3H-tw32/+ males very rarely achieved fertilization in vitro, while sperm from congenie C3H-+/+ males had no such difficulties. The presence of tw32 had no effect on ability of B6 sperm to undergo fertilization in vitro. In fertile hybrid males produced from crosses of B6 and C3H strains, tw32 significantly reduced, but did not inhibit completely, the sperms' ability to fertilize in vitro. The presence of tw32 decreased by a factor of two the frequency of abnormal sperm heads in males of the B6 strain, but doubled the frequency in the C3H strain. Hybrid males resembled the C3H strain in this respect. The presence of T or T2J had no effect on the level of sperm abnormalities in any strain. These results emphasize the importance of genetic background in expression of t haplotypes in sperm, and suggest that tw32 can influence a range of sperm characteristics, by interacting with products of other loci.

The pattern of response expected from fixation of mutant genes for quantitative traits in finite populations is investigated for a range of distributions of mutant gene effects. The eventual rate depends on the total variance of mutant effects per generation, but the initial rate and the variance of response is higher if the distribution of mutant effects has a large standard deviation or is leptokurtic. The difference between initial and eventual rates of response is greater with large population sizes.

For a range of assumptions, new mutants are unlikely to have much influence on response for 20 or so generations, but then may contribute substantially, such that no plateaux are obtained. However, information on the variance contributed by mutants is almost entirely on bristle number in Drosophila.

It is argued that the role of new mutants should be considered in designing breeding programmes, in particular in utilizing large populations.

The distribution of sex-determinants in field populations of Musca domestica domestica L. was studied in 62 samples of flies collected at 53 sites (animal farms) between 1975 and 1981 in an area stretching North–South from Denmark (+ Iceland) to Sicily.

Karyological observations and genetic analyses demonstrated the existence of three types of population along a latitudinal cline. Populations of Northern Europe were of the standard type (XX females and XY males) with the Y chromosome determining sex. Those of Central and Southern Italy from sites below 100 m.a.s.l. (metres above sea level) were autosomal (XX females and males), sex in them being determined by autosomal sex-determinants for both femaleness and maleness. In the large intermediate zone the populations were mixed and had several karyotypes in both sexes. In this zone an altitudinal gradient was also observed, with autosomal determinants less common at higher altitudes. Genetic tests showed, in the autosomal and in the mixed populations, the presence of two autosomal male factors: M III, the most common, on autosome III and M II, on autosome II.

The gradient in sex determinants found in flies of Western Europe appears to be a dynamic phenomenon of relatively recent origin. Both climatic influence and selective pressure with insecticides have probably contributed towards the micro-evolution of populations with different sex-determinants in the houseflies of the area studied.

During an extensive survey of drosophilid fauna in 1977, D. melanogaster was not collected in the Seychelles. However, a population was found in 1981 in Victoria city, suggesting a recent introduction of this species. With respect to allozyme frequencies or ethanol tolerance, this population is almost identical to European ones and very different from those living under a similar equatorial climate on the African continent. The frequencies of rare biochemical alleles perhaps suggested that this population was founded by a small number of flies, less than ten inseminated females. For various biometrical traits, the situation was not so clear: according to the trait considered, Seychellian flies are either intermediate between European and African populations or closer to the latter. These data suggest that a few flies, recently introduced from a temperate (European?) country, built up a big population which is now on the way to adapting itself to new tropical conditions. Such an involuntary experiment should afford a unique opportunity to distinguish the respective roles of drift and adaptation in the evolution of D. melanogaster geographic races.

The production of MSHA+ (type-1 fimbriate) recombinants was observed in transductional crosses between different pairs of naturally occurring strains of Salmonella paratyphi B. MSHA+ recombinants were readily produced in transductions from MSHA+ donor strains to MSHA− (type-2 fimbriate or non-fimbriate) recipient strains, and less frequently between some pairs of MSHA− strains. The genetic evidence suggests that there are at least three different clones among MSHA− strains of S. paratyphi B. Relationships between the different strains and their possible origins are discussed.

Two physiologically and biochemically distinct groups of α-amylase (E.C.3.2.1.1) isozymes are synthesized when isolated aleurone layers of barley are incubated with gibberellic acid (GA3). Isoelectric focusing of the α-amylases showed that the isoelectric points of the isozymes of one group were near pH 5, whereas those of the second group were close to pH 6. Using wheat–barley addition lines, the genes for these groups were located in barley chromosomes 1 and 6 respectively. Joint segregation in the F2 generation of appropriate crosses indicated that the isozymes within each group were inherited collectively, and were attributed to codominant alleles segregating at two presumably complex loci, α-Amy 2 and α-Amy 1.

The extent of genetic variation at these two loci was examined in 40 lines of Hordeum spontaneum (the wild progenitor of barley), and in a complex gene pool representative of H. vulgare (composite cross XXI). Variation at the α-Amy 1 locus was much more extensive than that at the α-Amy 2 locus. The genetic variation at both α-amylase loci exceeded that at the majority of other allozyme loci. However the α-amylase loci were less variable than the two loci coding for the seed storage protein, hordein. The wild species was found to contain much genetic diversity, which might be useful in modifying α-amylase activity by breeding. Parallels between the genetics and variation of α-amylase in barley and wheat were noted.

Selection against harmful mutations in large populations is studied assuming that the rate of fitness decrease grows with every new mutation added to a genome. Under this reasonable assumption (Mayr, 1970) the average fitness of a sexual population, without linkage between the loci, is higher, and the average equilibrium number of harmful mutations per individual lower, than in an asexual population. If a gamete contains on the average one or more new mutations, the resulting advantage of sexual reproduction and recombination seems to be sufficient to counterbalance the double advantage of parthenogenesis. Moreover, selection against harmful mutations is probably the most powerful factor preventing linkage disequilibrium even with epistatic interaction between the loci.