A computer simulation of the genetic analysis of the asexual life cycle of Phycomyces' heterokaryons has been carried out. We have studied experimentally the relationship between the nuclear proportion in heterokaryotic mycelia containing prototrophic and auxotrophic nuclei and their growth rates. We discuss possible evolutionary implications of heterokaryosis in coenocytic fungi and the genetic applications of the quantitative complementation technique.

Gene frequency changes were followed in 17 experimental cage populations of Drosophila melanogaster polymorphic for the two loci G6PD and 6PGD. From these cages it was observed that selection was favouring the F alleles of both loci when the cages were started with low frequencies of these alleles (0·20F and 0·50F). A viability experiment and an extinction experiment also provided evidence that the F alleles of both loci were favoured. It is argued that the observed selection is not due to selective differences in genetic background of the different alleles, but is acting on the isozyme loci themselves.

The effectiveness of selection for increased pupa weight in Tribolium was compared for three different selection systems. In all three systems the same number of breeding individuals was used each generation. Population L was a large random mating population with 24 males and 48 females selected each generation. The C4 and C8 populations were each divided into 6 subpopulations (lines) consisting of 4 males and 8 fem ales. Each of the three populations was replicated. In C4, selection for pupa weight was within lines for three generations, followed by a generation of among-line selection when the best two out of six lines were selected. These lines were then crossed to produce 6 new subpopulations, and the cycle was repeated. The C8 population was handled in exactly the same manner except that seven generations of selection within lines were practised before each generation of among-line selection. Selection response for the 42-generation period was significantly greater in the L population than in either subdivided population. No consistent differences among the selection systems were apparent when evaluating short-term response for the first 12 generations of the experiment. The results were interpreted as indicating that the influence of multiple-peak epistasis was not of major importance for this trait in determining ultimate response to selection when starting from a base population of previously unselected lines and utilizing a within- and among-line selection regime.

Five biometrical traits (thorax length, wing length and width, sternopleural and abdominal chaetae numbers) were measured on 13 equatorial African strains and 30 French strains. In all cases highly significant differences were observed between the two geographic groups. These results are added to previously known variations concerning adult weight and ovariole number. In each place, the genetic particularities of the wild populations seem to be maintained by the selective pressure from environmental conditions, resulting in a homeostatic focusing of the best fitted average genotype. Analysis within each group showed that variations between strains were in most cases poorly or not correlated, so that partial or total genetic independence between the various traits measured seems likely.

The frequency of monogenic resistance to anticoagulant rodenticides in Rattus norvegicus in an area straddling the England–Wales border was monitored from 1967 until 1975. Rats were trapped on farms and tested in the laboratory by administering a dose of warfarin lethal to susceptibles. The mean incidence of resistance was 44% and did not change significantly, despite the extensive use of anticoagulants by farmers during the 9-year period. In 1975 more refined techniques showed that the frequencies of susceptible (SS) and resistant (RR) homozygotes were significantly below the Hardy–Weinberg expectations and simple estimates of the relative fitness ratios for the RR, RS and SS phenotypes were 0·37, 1·0 and 0·68 respectively. In two relatively isolated valleys, where selection with anticoagulants was minimal, the frequency of resistance decreased significantly from 57% to 39% during 1973–5. The results are consistent with the hypothesis that a balanced polymorphism is being maintained. Selection against susceptible homozygotes by the use of anticoagulant rodenticides, and against the resistant homozygote due to its high susceptibility to a primary deficiency of vitamin K gives the heterozygotes a selective advantage. A number of ecological factors that influence the incidence of the resistance are discussed briefly.

5-Bromouracil was shown to enhance the mutation frequency in an ultraviolet sensitive strain of Neurospora crassa (uvs-2) that was previously irradiated with ultraviolet light. The design of the experiment eliminated light or dark repair as possible hypotheses to account for the observed enhancement. Since a similar effect was noted at the rib-1 locus in N. crassa, which is not a UV-sensitive strain, it appears that the error prone repair mechanism in N. crassa may be related to post-replication repair. This enhancement may be attributed to the interference of 5-bromouracil with post-replication repair. It is also postulated that the maximum increase in the mutation frequency occurs when the greatest number of unrepaired pre-mutational UV lesions are repaired in the presence of 5-bromouracil.

Mutations, sulA and sulB, that suppress the UV sensitivity conferred by the lon mutation have been isolated and precisely positioned on the linkage map of Escherichia coli. The E. coli B strains Bs-3 and Bs-8 have been shown to possess sulA mutations. Also the E. coli K12 strain J6271 that possesses a suppressor of the lon mutation, previously designated as suf, has been shown to be a sulA mutation. A series of methylmethane sulphonate resistant derivatives of an E. coli K12 lon strain has been isolated and genetically characterized. In addition to sulA mutations, a second suppressor sulB was identified and located between leu and azi genes on the chromosome. Neither sulA or sulB mutations result in increased sensitivity to the antibiotics ampicillin, rifampicin, or actinomyein D, nor do they have any significant effect upon the overproduction of mucopolysaccharide caused by the lon mutation. Under some growth conditions the sulB mutation causes cells to be temperature sensitive for the cell division process at 42 °C.

Breeding stocks were re-established from embryos of various mutant and inbred strains of mice after prolonged storage in liquid nitrogen at the 8-cell stage even with strains where only a proportion of the progeny were of the desired type, i.e. in the ModP and XO strains. Gonadotrophin treatment failed to produce superovulation consistently in any of the strains tested. Although the initial survival of embryos after thawing and culture to the morula and blastocyst stage was highest for embryos from XO mothers (61%), these embryos suffered the heaviest early postimplantation loss after transfer (61%). The proportion of embryos, originally frozen, developing into foetuses and offspring was variable (13% HT, 14% PT, 20% ModP, 14% XO, 21% CBA and CBA-T6) and lower than previously reported for hybrid 3H1 embryos (20–30%). The sex ratio of the liveborn young was within the normal expected limits except for the Modp strain, where it differed significantly from the exected 2♂:1♀ ratio. The proportion of young of the desired type from the frozen embryos of Modp and XO females was less than expected (17 and 3% respectively). In all cases a normal breeding stock was reestablished whose performance was within normal limits for each strain. Even without further improvements in embryo collection and the freezing technique per se, the storage of embryos in liquid nitrogen is an extremely economic way of preserving mouse genetic stocks.

A spontaneous mutation conferring white plasmodial colour on the Myxomycete Physarum polycephalum has been analysed. The mutant whi-1 allele is recessive to whi+ in both heterokaryotic and heterozygous plasmodia. The whi locus is unlinked to mt, npfA, fusA and leu.

The detection, in unordered octads, of 6+:2m and 2+:6m ratios with postmeiotic segregation, and of aberrant 4:4s and their use in corresponding-site interference studies

BY AGLAIA GHIKAS AND B. C. LAMB

Department of Botany, Imperial College, London SWl 2BB

Vol. 29, No. 3 p. 273.

The first line should read ‘originally PD or NPD 4:4(4)s,’.

Sterile mutation in Drosophila melanogaster

BY TAKAO K. WATANABE AND WON HO LEE

National Institute of Genetics, Mishima 411, Japan

Vol. 30, No. 2.

p. 110, second line: for ‘Avery Period’ read ‘P. J. Avery’.

p. 111, 4 lines from foot of page: formula should read

p. 112, line 4: start of line should read

The contribution of plasmid and host genes to plasmid-mediated interference with phage growth

BY VICTORIA HUGHES AND G. G. MEYNELL

Biological Laboratory, University of Kent, Canterbury, Kent CT2 4tNJ, England

Vol. 30, No. 2.

p. 180, line 12: ‘2000μg streptomycin’ should read ‘200μg streptomycin’.

The basis for an apparent auxotrophy for reduced sulphur metabolites in sF−mutants of Aspergillus nidulans

BY HERBERT N. ARTS, JR.

Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3EH, England

Vol. 30, No. 2.

p. 209, note added in proof line 9: ‘does not protect sF-211 strains’ should read ‘does protect sF-211 strains’.