Houseflies collected from eight pig-breeding farms were used to investigate the nature of sex determinants in fly populations of South-East England. Earlier observations had shown that their sex determination mechanism was not of the standard (XX females, XY males) type.

Most flies of both sexes were XX; the male determining Y chromosome of standard populations was rare. Test-crosses to females of standard multimarked strains and crosses using aneuploid (OX) flies identified two dominant male determinants, one on autosome 3 (M III) and another on the X chromosome (Xm), and provided the first demonstration in this species of an active involvement of the X chromosome in sex determination. A small secondary constriction on X appeared to indicate reliably the presence of Xm. Most individuals in field populations were Xm homozygotes, implying the presence of an unlocated female determinant F,† epistatic to Xm and M III.

M III was less common and differed in frequency between samples. Its increased frequency in a strain selected in the laboratory with the pyrethroid insecticide permethrin might be due either to genetic drift, or to linkage between M III and a gene on autosome 3 that confers resistance to pyrethroids in houseflies.

The relative frequency of retention of two mitochondrial loci, determining resistance to oligomycin (oli1) and erythromycin (ery1), has been analysed in petite (rho(−)) mutants derived from a number of unrelated strains of Saccharomyces cerevisiae. The frequency of retention of one marker relative to the other in spontaneous petites showed marked variation dependent on the strain of origin. The differences between strains in this characteristic were shown to be mitochondrially determined. Further, for individual strains, the relative retention of the markers in petites derived after UV-irradiation varied considerably in several cases from that observed with spontaneous petites. The observations on relative marker retention and the varied effects of UV-irradiation are discussed in terms of possible structural differences in the mitochondrial genomes of the various strains.

The expression of electrophoretic variant forms of the X-linked enzyme phosphoglycerate kinase (PGK-1) was examined during the ontogeny of the female germ-line following X-chromosome reactivation. Non-growing oocytes from foetal and neonatal ovaries of heterozygous females show a higher PGK-1A isozyme activity, a reflection of prior non-random X-inactivation favouring activity of the X-chromosome carrying the Pgk-1a allele and Xcec locus. On oocyte growth, the total PGK-1 enzyme activity increases 40–50 fold and the pattern of PGK-1 isozyme expression changes giving an electrophoretic pattern now skewed in favour of the PGK-1B isozymic form. Activity of the PGK-1B isozyme exceeds that of PGK-1A in all growing oocytes with a total activity greater than 0·075 nmol h−1 oocyte−1. Mice homozygous for either Pgk-1 allele show similar PGK-1 specific activities in somatic tissues where one X-chromosome is active, but oocytes of Pgk-lb homozygotes show a higher specific activity compared to those of Pgk-1a homozygotes. Thus increased activity of the PGK-1B isozyme relative to the PGK-1A isozyme is specific to growing oocytes.

The hypothesis is advanced that the evolutionary stability of the unusual sex-determining mechanisms of the Wood Lemming (Myopus schisticolor) and of the Varying Lemming (Dicrostonyx torquatus) is a direct consequence of certain characteristic features of their population dynamics, and that these include phases of unrestrained population growth and of mass dispersal. Computer simulations confirm the feasibility of such an explanation. Predictions of this hypothesis are found to differ in a potentially testable manner from those of the ‘inbreeding’ hypothesis of Stenseth (1978). The demonstration of such a direct link between population ecology and evolutionary genetics would, if substantiated, be exceptional in mammals.

A sex-linked recessive anaemia (symbol sla) in the mouse is responsible for a rather uncharacteristic blood picture which persists virtually unchanged throughout life. The red cell count is reduced to about three-quarters of the normal value, the cells produced differ from normal ones by a slight reduction in the mean corpuscular Hb concentration and a slight reduction in the mean cell volume. As the mean cell diameter is more strongly reduced, it is concluded that the cells must be thicker than normal. There is a reduction of haemopoietic tissue both in the liver and in the bone marrow.

Salmonella typhimurium strains of biotype 25x have been shown in transductional cross experiments to be clonal in the Nad character. The ancestral bacterium, probably of biotype 25a, mutated to a requirement for nicotinamide and subsequently diversified in phage type and secondary biotype characters. Such a sequence of events indicates interconversion among phage types 6, 16, 46, 49, 73, 76 and 135. Strains in biotypes 1x, 9ix, 17x, 17dx, 19dx and 25hix yielded Nad+ recombinants in interbiotype crosses, suggesting that each originated as an independent mutant line.

Individual and within-full-sib family selection for low sternopleural bristle number was carried out for 17 generations, with six replicate lines for each selection method. Our results can be summarized as follows: (1) the response to selection was exhausted very quickly, (2) the additive variance of the selected lines declined rapidly, (3) the variation in response to selection decreased as selection progressed, (4) genetic differences among replicates at the selection limit were small, (5) individual selection resulted in a higher initial response than within-family selection, but similar limits were achieved with both procedures. These observations are consistent with the hypothesis that the pattern of response to selection is due to the segregation in the base population of only a few loci with large effects, at intermediate frequencies.

Difference in bristle-making abilities in extreme scute and in wild-type Drosophila were investigated using flies mosaic for the two kinds of tissues. It was found that both wild-type and scute tissues were autonomous with respect to bristle differentiation. From the experimental evidence it was postulated that the differences in bristle-making abilities between the two genotypes might be due to differences in tissue competence rather than differences in pre-pattern or in level of bristle-making substances.

Ditelocentric accessions of the Chinese Spring cultivar of Triticum aestivum were analysed electrophoretically for peptidase (PEPT) and amino peptidase (AMP = LAP) activity. Isozymic activity was missing in the accessions CSDT6AS and CSDT6BS when stained for PEPT. This was taken as evidence that the structural genes encoding these isozymes are located on the long arms of chromosomes 6A and 6B. These genes have been named Pept-A1 and Pept-B1, respectively. Isozymic activity was missing in the CSDT6BL accession when stained for AMP. This result reconfirms the previously published location of the gene Amp-B1 on the short arm of chromosome 6B and demonstrates that the two sets of loci are clearly different.

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.

Salmonella typhimurium phage P22 can recombine with a serologically and morphologically unrelated Salmonella phage Fels 2 to yield the hybrid phage F22 at a frequency of about 10−12. F22 has inherited the entire late genes (protein coat and tail structural genes) of Fels 2 but carries some P22 early genes. P22 genes in the F22 phage were indentified by marker rescue of various P22 mutants. The F22 genome carries the x-erf-c-18-12 segment of the P22 genome. A number of P22 amber mutants were also tested to support these data. The F22 region homologous to P22 was mapped by scoring the ratio of P22 backcross recombinant types in lysates of F22 lysogens superinfected with P22c2ts12. The ratio of the distances between these markers and the ends of the homologous region was determined. Furthermore a new F22 hybrid designated F22dis, containing both P22 immunity regions (c and Im), was isolated at a frequency of about 10−4 by superinfecting F22 lysogens with P22. F22dis phage has lost some Fels 2 gene(s) which have been replaced by the P22 segment containing the Im region, resulting in formation of a defective hybrid phage.