The R factors, R56 and R64, interfere with the growth of several otherwise unrelated phages, including λ, P22 and T7. In addition, λvir is inhibited more strongly in λ lysogens carrying either R factor when the prophage has an intact O – P region. Interference does not occur in certain str A R+ hosts but may reappear if they also carry other mutations affecting ribosomal proteins. Host mutants not permitting interference were isolated but were not strA. Non-interfering plasmid mutants were also isolated, of which some also failed to cause interference after transfer to a different host genus.

The gene conversion model reported by Birky & Skavaril (1976) has been analytically studied by using the theory of diffusion models of Kimura (1964) in population genetics. It has been shown that the fate of new mutations in systems with multiple genomes may be satisfactorily treated by the diffusion model.

Five loci have been identified in Saccharomyces cerevisiae whose function reduces suppressor activity in strains carrying ochre super-suppressor mutations. Recessive mutations which allow an increased level of suppression occur at these loci. In such mutants, termed allosuppressors, the serine-inserting suppressor SUPQ5 suppresses ochre mutations in a [psi−] background and Class I tyrosine-inserting suppressors are lethal or have a reduced viability. Mutations at two allosuppressor loci, sal3 and sal4, have a lethal interaction with one another and with the extrachromosomal determinant [psi+]. This interaction is expressed in the absence of any suppressor mutation. All the mutant alleles of one allosuppressor locus sal3 are cold sensitive. One allosuppressor mutation, sal4.2, is temperature-sensitive for growth, as well as for other aspects of its phenotypic expression; namely the expression of SUPQ5 and the lethal interactions with Class I super-suppressors, with [psi+] and with sal3. At low temperature (24 °C), sal3-sal4.2 double mutants weakly suppress the ochre mutation ade2.1, but do not suppress his5.2 or lys1.1. It is argued that the site of function of the products of these loci is ribosomal and that they are involved in chain termination at UAA codons. It is inferred that the [psi+] factor or its product affects protein synthesis by interaction with the ribosome.

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.

sF− mutants of Aspergillus nidulans are subject to toxicity by a metabolite derived from sulphate but are not blocked in cysteine biosynthesis. Bearing in mind the unavoidable presence of sulphate in agar-solidified media, their apparent auxotrophy for thiosulphate, L-cysteine or L-methionine stems from the ability of these reduced sulphur-containing metabolites to repress, directly or indirectly, the syntheses of sulphate permease and the enzymes of sulphate assimilation, thereby preventing synthesis of the toxic metabolite.

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’.