A new CNR (caudatum non-reversal) mutant of Paramecium caudatum was isolated after gamma ray mutagenesis. This CNR lacks not only the transient inward Ca2+ current but also the sustained Ca2+ current. It was shown to complement the three known CNR mutants of P. caudatum (cnrA, cnrB and cnrC) by crossbreeding analyses. Thus, this new mutant belongs to a 4th CNR locus, designated cnrD. The defect of cnrD can be partially rescued by microinjection of cytoplasm from any of the three CNR mutants or the three Pawns (pwA, pwB and pwC) in P. tetraurelia. Since the three CNR genes have been shown to be different from the three Pawn genes by cytoplasmic complementation test (Haga et al. 1983), this result suggests that cnrD is the 7th non-excitable mutant in Paramecium. Thus, there are at least seven genes controlling Ca2+ channel function in Paramecium.

By designing 3′ ends of primers in PCR (polymerase chain reaction), a specific DNA fragment was selectively amplified in the presence of a 103-fold excess of highly homologous (sequence difference ca. 2 %) opponent DNA. This technique was applied in detecting paternal leakage of mitochondrial DNA (mtDNA) in intraspecific crosses of Drosophila simulans and interspecific crosses of Drosophila simulans and Drosophila mauritiana. The mtDNA types of their progeny were analysed by selective amplification of the paternal mtDNA fragment possessing a polymorphic restriction site and detecting its cleaved fragments. Paternal mtDNA was detected in the progeny of 14 out of 16 crosses. The present result indicates small but frequent inheritance of sperm mtDNA in Drosophila, which is supportive to our previous finding.

The method of heritability estimation proposed by Abplanalp (‘linear heritability’ estimation) is reviewed and the formulation simplified. It is equivalent to the regression of performance of sibs of an individual, itself excluded, on that of the individual. The method is found to be slightly biased in small samples but, in terms of sampling variance, can make efficient use of the data.

Wild house mice, Mus musculus, were bred (a) at 3 °C (‘Eskimo’) and (b) at 23 °C. Mice of the ninth generation bred at 23°C were transferred to the cold environment. Their young, and Eskimo of the same (tenth) generation, were mated to give the four possible types of pairs: controls; the two reciprocal hybrid pairings; and Eskimo. In the resulting (eleventh) generation there were therefore two hybrid classes, genetically identical but with different parentage. The growth and reproduction of the eleventh generation were studied. At all ages from birth, mice with Eskimo mothers were heavier than those with control mothers. They were also better breeders: (1) they matured earlier; (2) their litters were larger; (3) the mortality of their young in the nest was lower. In one feature there was heterosis: of the four classes, the hybrids with Eskimo mothers produced the largest litters. These and previous findings suggest rapid selection, in the cold, for changes in growth, reproductive physiology and other aspects of metabolism. The cold-adapted mice of a given generation differed from the controls partly as a result of favourable parental effects, which acted in conjunction with genetical differences. It is hypothesized that the ecological versatility of Mus musculus depends partly on the presence, in each population, of alternative genotypes which allow rapid adaptation to new conditions.

Inheritance of DDT resistance has been studied in crosses between the highly resistant ‘T’ strain of A. aegypti (constituted by inbreeding from the TRINIDAD DDT-resistant stock) and the ‘64’ susceptible strain.

Larval DDT resistance derives from a major gene RDDT1 on linkage group II, the order being RDDT1–s–y. Linkage group III may also contribute to larval resistance. Linkage group I makes no contribution.

Adult DDT resistance derives from a major gene RDDT2, 18·2 ± 2·1 units from the market blt on linkage group III. Linkage group II has no influence on adult resistance.

Selection with DDT to retain only RDDT1/+ segregants in larvae of backcrosses RDDT1/+×+/+ did not increase resistance in resulting adults, confirming the difference in genetic mechanism at the two stages.

The F1 progenies from reciprocal crosses between ‘T’ and ‘64’ differed slightly but significantly in larval resistance, modifying the influence of the major gene RDDT1 in the heterozygote.

The early developmental stages of the RDDT1/+ phenotype (up to the fourth larval stage) were more viable than the +/+ phenotype in backcross segregation. The difference in mortality probably exceeded 30%.

Starting from a completely homozygous population of Drosophila melanogaster, lines have been derived and subjected to 47 generations of divergent selection for abdominal bristle number (20 lines selected in each direction) or to 60–67 generations of inbreeding (100 B lines maintained by a single brother-sister mating, 100 C lines maintained by two double first cousin ma tings). In the selected lines, 25 were identified carrying at least 30 mutations affecting bristle number. A large fraction of these mutations (42 %) were lethals. Non-lethal mutations had smaller effects on the trait, were predominantly additive and had no detectable pleiotropic effects on fitness. In the inbred lines, 21 mutations affecting bristles were individually analysed. Deleterious mutations had the largest effects on the trait (irrespective of sign) and showed recessive gene action (complete or incomplete). The rest were predominantly additive and had smaller effects. Thus, both procedures identify a quasi-neutral class of additive mutations which should be close to that responsible for standing variation in natural populations. Moreover, the results indicate a leptokurtic distribution of mutant effects, consistent with a model of natural selection acting on bristles through pleiotropic effects of pertinent loci on fitness. Consequently, neutral additive alleles of considerable effect can be found segregating at intermediate frequencies in natural populations.

It is shown that the distribution of the sum of heterozygotes, due to mutant gene(s), that appear in a finite population is invariant under geographical structure, provided that the mutant gene has additive effect on fitness and migration does not change the genetic constitution as a whole population. The expected number of heterozygotes is 2N when Ns = 0 and gradually rises to 4N as Ns increases provided s remains small, where N = the total population size and s = selective advantage of a mutant gene. The distribution of the heterozygosity summed over those generations in which the gene frequency in the entire population is specified, is also shown to be invariant. In the case of a neutral mutant, the density is equal to 4(1 − Y) where Y is the frequency of the mutant in the whole population, and in the selectively advantageous case, it is approximately equal to a constant function 4, provided that the population size times selection coefficient is sufficiently large. These quantities conditional on the fixation of the mutant are shown to be invariant and some special cases are obtained explicitly.

A new allele of one of the metallothionein genes of D. melanogaster, Mtn•3, sheds light on the recent evolution of this gene. In comparison to the previously studied Mtnl allele found in Canton S, this new allele, Mtn•3, produces a transcript that is 49 bases longer and 65–70 % less abundant. We detected Mtn•3 in several laboratory strains as well as in isofemale lines derived from natural populations. Sequence comparison showed that Mtn•3 differs from Mtnl in that it has: (a) base-pair substitution and an extra 49 bp-segment in the 3' untranslated region, (b) a substitution in the coding region that replaces the terminal Glu40 in Mtnl with Lys40, and (c) two base-pair substitutions in the promoter region. The Mtn•3-type was detected in six species of the melanogaster group by restriction analysis, and this result was confirmed by sequencing the D. simulans Mtn gene. Thus Mtn•3, which produces a less abundant transcript, appears to be the oldest of the two alleles. We also found that the duplications previously isolated from natural populations all derived from Mtnl, the more recent allele. Thus, two evolutionary steps: Mtn•3 to Mtnl and Mtnl to Dp(Mtnl), are accompanied by an overall 5- to 6-fold increase of RNA accumulation. The two changes seem to have occurred in non-African populations since Mtn•3 but not Mtnl was detected in our sample from tropical Africa, while Mtnl and Dp (Mtnl) are prevalent in European and North American samples.

The sites of mutations affecting host-controlled modification (HCM) have been mapped in E. coli K and E. coli B by conjugation and transduction experiments between mutants. These mutations all map close to the serB locus on the opposite side to the marker thr. Non-parental HCM has been observed among colonies obtained from Pl transduction experiments between HCM mutants. Control experiments have shown that these non-parental recombinants can not be accounted for by reversion of either parent and must result from recombination between mutants. Several genetic models are suggested which could account for these recombinants and an attempt is made to distinguish between various models by testing for complementation between mutants in a zygotic induction complementation test.

Dissections were made of 16-day-pregnant female mice with the object of discovering the developmental stage at which litter size is reduced by inbreeding. Counts were made of the numbers of corpora lutea, implantation sites, and live embryos, and comparisons were made between females with inbreeding coefficients of 50–60% and non-inbred females. Except in one group the embryos were all non-inbred, so that the comparisons showed the effect of inbreeding in the mother of the litter. No influence of inbreeding in the male parent was found.

The only difference found between inbred and non-inbred females was in the number of eggs or embryos lost before implantation. The greater pre-implantation loss in inbred females was enough to account for the smaller number of young born alive in their litters.

There was no difference between the inbred and non-inbred females in the ovulation rate, measured by the number of corpora lutea, or in the post-implantation mortality of the embryos.

There was a positive correlation between ovulation rate and weight at 6 weeks. For reasons explained in the Discussion, the inbred females did not differ in weight from the non-inbred females. If, under other conditions, the weight declined on inbreeding, the ovulation rate would be expected to decline also.

This paper presents evidence that the alcohol dehydrogenases (ADHs) in barley are specified by three loci. Six distinct ADH isozymes are observed following native slab polyacrylamide gel electrophoresis of crude extracts from flooded wild-type roots. Three of these isozymes are missing in flooded roots of plants homozygous for the Adhl-M9 mutation. The results also indicate that a simple binomial model (incorporating random dimerization and no inhibitive interaction of the two subunit species within heterodimers) is unable to account for the distribution of the total ADH activity between the ADH isozymes observed. Finally, the level and distribution of ADH activity in heterozygous (Adhl+ / Adhl-M9) flooded roots is not what would be expected if these contain only one-half of the available ADH1 protomers and the same frequency of available ADH2 and ADH3 protomers as is contained in the flooded roots of wild-type homozygotes (Adhl + / Adhl + ).

A ‘two locus two allele’ model is developed with special reference to the introgression of extranuclear genomes between two species of finite size. The model assumes that one locus, coded by a nuclear genome, causes the reproductive barrier while the other locus, coded by an extranuclear genome, causes nucleo-cytoplasmic incompatibility in particular genotypes. To fully study this model, simulations are conducted, and a diffusion equation is derived when introgression or extranuclear gene flow occurs in one direction. It is shown that although selection against the nuclear genome can reduce the levels of extranuclear gene flow and retard the introgression process, the dynamics are very similar to those without such selection. In contrast, the nucleo-cytoplasmic incompatibility directly affects the dynamics of introgressing extranuclear genomes: in large populations the ability of incompatibility to overcome extranuclear gene flow is conspicuous, but in small populations it is overshadowed by random sampling drift. Paternal leakage of extranuclear genomes, if present, is of evolutionary importance only when the male's migration rate is much larger than the female's. When the sizes of two populations are unequal, the introgression is most likely to occur from the larger population to the smaller one in the absence of mating preferences of backcross progeny. Recent observations on interspecific mitochondrial transfer in various species do not support the ubiquitousness of nucleo-cytoplasmic incompatibility as an efficient reproductive barrier.

Mutants of Pseudomonas aeruginosa isolated as being resistant to p-fluorophenylalanine (FPA-r) are pleiotropic, showing patterns of phenotypic alteration of host-controlled modification, growth rate at 37 and 43 °C, ability to grow on minimal medium, response to antibiotics and ribosomal characteristics. Alterations in host-controlled modification are also found in FPA-r mutants of Escherichia coli B.

The pleiotropy arising in these FPA-r mutants of P. aeruginosa appears to result from alterations in the specificity of protein synthesis. Phenotypic variations in host-controlled modification of the type found in the FPA-r mutants may provide a method for the detection of suppressor mutations which act by miscoding.