In Drosophila melanogaster, the P−M and I−R systems of hybrid dysgenesis are associated with high rates of transposition of P and I elements, respectively, in the germlines of dysgenic hybrids formed by crossing females of strains without active elements to males of strains containing them. Transposition rates are not markedly accelerated in the reciprocal, nondysgenic hybrids. Previous attempts to evaluate the extent to which hybrid dysgenesis-mediated P transposition contributes to mutational variance for quantitative characters by comparing the responses to selection of P−M dysgenic and nondysgenic hybrids have given variable results. This experimental design has been extended to include an additional quantitative trait and the I−R hybrid dysgenesis system. The selection responses of lines founded from both dysgenic and nondysgenic crosses showed features that would be expected from the increase in frequency of initially rare genes with major effects on the selected traits. These results differ from those of previous experiments which showed additional selection response only in lines started from dysgenic crosses, and can be explained by the occasional occurrence of large effect transposable element-induced polygenic mutations in both dysgenic and nondysgenic selection lines. High rates of transposition in populations founded from nondysgenic crosses may account for the apparently contradictory results of the earlier selection experiments, and an explanation is proposed for its occurrence.

Six experimental populations of D. pseudoobscura were maintained at three different temperatures for more than 8 years. All the populations have originally descended from the same founders. The populations were examined for evidences of genetic divergence. The pre-adult viabilities of the flies from these populations, and of their F1 and F2 hybrids, were studied at 16°C., 25°C., and 27°C. The results indicate that the populations have diverged genetically from each other. Some divergence is observed not only between populations kept at different temperatures, but also between the replicate populations kept at the same temperature.

The often remarkable similarity in structural gene products among related species has led to the hypothesis that species differences may reside largely in changes at regulatory gene loci. This hypothesis assumes that groups capable of speciating have allelic variation at regulatory loci in their natural populations. We have undertaken an analysis of the mode of regulation of the esterase 6 (Est 6) locus in Drosophila melanogaster to determine the nature and extent of regulatory gene variation in natural populations. Analyses of esterase 6 (EST 6) activity among strains carrying the same thermostability variants reveal that significant, specific-activity differences are present. Reciprocal crosses between lines having high and low EST 6 activity show that loci other than the Est 6 structural gene influence EST 6 activity. Analyses of male hybrids from crosses between D. melanogaster and simulans indicate that the X chromosome of these flies affects the expression of the Est 6 locus, resulting in unequal levels of enzyme activity from the two alleles. The effect is sex and tissue specific. Female hybrids carrying the X chromosomes of both species exhibit equal expression of the two Est 6 alleles. We have determined whether natural populations are polymorphic for X chromosomes which affect EST 6 activity by extracting single X chromosomes from wild-collected males and placing these chromosomes in identical genetic backgrounds. Stocks which are otherwise genetically identical but carry independently derived X chromosomes show significant differences in the activity of EST 6. These data suggest that regulatory loci may be commonly polymorphic in natural populations.

The paper describes a cattle serum antigen (McA2) located on a macroglobulin molecule which has its isoelectric point at pH 5 and is capable of interacting with the wheat germ lectin and concanavalin A.

The specificity is inherited in a simple Mendelian manner and the gene controlling its synthesis is allelic to the one controlling the synthesis of the McAl antigen.

The steel allele, contrasted (Slcon), arose in a neutron irradiation experiment. Slcon is fully penetrant and heterozygotes can be recognized at or soon after birth by darkly pigmented external genitalia in both sexes, while the adult coat tends to be a little lighter than normal. Homozygotes also have dark genitalia and a markedly diluted coat. Both eumelanin and phaeomelanin are affected, with reduced numbers of cortical and medullary pigment granules in the hairs. Contrasted also affects the haematopoietic system, causing slight macrocytic anaemia in the homozygote. Slcon homozygous males are fertile but testes weigh on average 20% less than in their heterozygous litter-mates. Homozygous females are usually sterile although if mated early (4½−6 weeks) they occasionally have a single litter. Ovarian sections showed a gradual degeneration of oocytes in Graafian follicles so that most had gone by 2 months. Similarly, vaginal smears indicated that after about three normal cycles homozygous females lapsed into a state of persistent dioestrus; injections with gonadotrophins did not prolong their period of fertility or cause a resumption of their oestrous cycles. The effects on fertility, pigmentation and haematology of contrasted when combined with other steel alleles are also described.

The intensity of negative interference in 28 crosses involving 14 allelic paba mutants in different combinations has been measured. Among selected cross-overs between two such mutants, when the distance between the mutants is large, additional exchanges on the distal side (relative to the centromere) greatly exceed those on the proximal side. With decreasing length of the interval of selection, the inequality disappears. It is suggested that this polarity may be the outcome of a undirectional process which imposes a time sequence on the recombination events within an effectively paired segment. Some alternative explanations of this result are discussed.

1. Lines with ten pairs of parents and selected at an intensity of 20% were exposed to 1000 r. of X-rays for 0, 2, 10 or 30 generations.

Lines which received some irradiation generally gave greater response than the unirradiated controls. The phenotypic variance in the irradiated lines was much higher than in the controls. There was little difference in behaviour between lines receiving ten generations of irradiation and those irradiated every generation. Lines receiving only two generations of irradiation had lower variances than the other irradiated lines, but in one of three replicates the response was greater than the corresponding continuously irradiated line.

3. Lethal frequencies were much higher in irradiated than unirradiated lines. Particular chromosome II and III lethals were at high frequencies in most of the irradiated lines but in only two out of five controls.

4. On relaxation, the mean of the irradiated lines generally declined considerably, but in the unirradiated lines there was only a very small regression.

5. It appears that most of the extra response and increased variance in the irradiated lines were caused by a few genes with large effect on bristle number.

An R factor for ampicillin and streptomycin resistance (AS) was identified in Salmonella enteritidis. The AS factor transferred freely to Escherichia coli K12, but only two of 260 K12(AS) clones from this cross would transfer AS to S. typhimurium, although all lines tested transferred it to S. enteritidis and K12. A study of one of the two exceptional lines“ revealed that it also transferred AS to S. paratyphi B, S. thompson and S. anatum. The R factor maintained its transferability when cycled between these serotypes and K12. Transfer to S. enteritidis, however, resulted in loss of the ability of AS to transfer to the heterologous serotypes, that is, it apparently became host specific for S. enteritidis. S. paratyphi B and S. anatum also imposed host specificity on AS, but S. typhimurium and S. thompson did not. The R factor would always enter S. enteritidis, whatever its previous salmonella host but, once it had done so, it became specific for S. enteritidis. AS could always transfer to K12, which did not seem to modify its host range. These phenomena are most easily explained by analogy with host-controlled modification of phage. Their possible significance in relation to apparent host specificity of R factors is discussed.

A modest theory of polygene selection is presented. Recently published data show that the curved model of gene action (presented in a previous paper) is insufficiently general. The curved model does, however, show that the scaling tests (used in the additive type of polygene analysis) do not guarantee additivity; they guarantee a state of indeterminacy, in which additivity is one of a range of reasonable possibilities. These different possibilities give entirely different predictions of selective advance. The failure (in practice) of polygene analysis does not reduce the value of polygene concepts.