In this paper I present the results of an experimental study of the effects of genotype and density on the mean and variance of offspring numbers in both sexes of the flour beetle, Tribolium castaneum. From the observed variance in offspring numbers the effective population size at several different densities is estimated using the methods of Crow & Morton (1955).

I found that both the mean and variance of offspring numbers varied with genotype and density. In general, males were more variable in offspring numbers than females and this variability increased with density. Individuals homozygous for the black body colour mutant, b/b, were less variable in offspring numbers than + / + individuals, but the latter produced more offspring at most densities. As density increased, + / + individuals became more variable in offspring numbers whereas b/b individuals were less sensitive in this regard. These findings are discussed in relation to the ecology of selection at the black and closely linked loci.

Alcohol dehydrogenase (Adh) allozyme frequencies and tolerance of adult flies to ethanol were measured in population samples of D. melanogaster from a winery in Southern Ontario. Samples were also tested from a number of non-winery sites. Adhfast frequencies decrease as one moves away from the winery, but this drop in allozyme frequency is statistically significant only for those areas that are more than 3 km distant from the winery. Tolerance of adult flies to ethanol also differed between winery and non-winery populations, but these differences were not statistically significant. The data presented here may help to resolve the seeming conflict between the two previous studies of Adh allozyme frequencies in natural populations from highand low-alcohol environments.

Geographic populations are not necessarily equivalent to genetic populations. Electrophoretic isozyme analysis shows that in the biennial, Verbascum virgatum, not only is spatial distribution a factor creating isolated populations, but time also is an isolating mechanism. Thus, in the biennial situation there is a possibility that selection can be rejected as the probable cause of variation, much as can random drift in cases involving spatial distributions. The magnitudeof difference infrequency of alleles between the odd and even year colonies studied was too large to be explained by random drift in populations of the size and short duration of those we observed. Similarly, it is unlikely that random fluctuations in selection intensity on one age class would produce a difference as large as that observed. It is possible, however, that variation was introduced (mutation or founder) when the population was much smaller and that the difference was trapped at a relatively high level when the population rapidly increased in size. Simulations and algebraic theory do not refute this idea. They also show that colony differentiation can occur with migration rates considerably greater than previously predicted.

The effect of migration and recombination on the equilibrium structure of populations subject to a common symmetric selection regime in all habitats is studied. Attention is restricted to a class of symmetric polymorphic equilibria which have been studied in two-deme systems by Bazykin (1972) and Karlin & McGregor (1972) for one locus and by Christiansen & Feldman (1975) for two loci. With increased migration and recombination the heterozygosity increases unless it is already at the maximum level. Although the populaton system as a whole is always at linkage equilibrium, the magnitude of linkage disequilibrium in the individual demes may either increase or decrease with more migration and recombination. In general, the less the migration and the less the recombination between interacting loci, the greater the possibilities of polymorphic equilibria.

In the course of an X-ray experiment, the normal allele of forked was transposed to the second chromosome, where it acts as a suppressor of forked. In this position, which is near the centromere, the duplication (Dp-f+) is subject to a variegated position effect. This was studied in its dependence on the hetero-euchromatin balance; the results agree with and extend those found for other position effects. In addition, we found regional preferences for variegation in the individual flies. The most interesting aspect of Dp-f+ is its tendency to transpose either to the homologous second chromosome or to Chromosome IV. In the latter position, Dp-f+ acts as a dominant near-lethal, so that the apparent selectivity of insertion sites is at least in part due to deleterious effects at insertion sites other than its original one. In a new, and presumably, centromere-far position of Dp-f+ on Chromosome II the variegated position effect disappeared and transposition was reduced in frequency or wholly abolished. The frequency of losses of Dp-f+ approximately equalled that of transpositions. Since there is good evidence that transpositions occurred pre-meiotically, the apparent losses of Dp-f+ may have been due to meiotic segregation separating the loss from the new insertion.

The experiments reported in this paper are primarily addressed to test the hypothesis of overcompensation; i.e. whether polymorphic populations exploit limiting environmental resources better than populations uniform for a single genotype. Overcomposition is an ecological consequence of some models of frequency-dependent selection. Secondarily, the experiments investigate whether overdominance exists at the Mdh-2 locus in Drosophila pseudoobscura.

Two types of experimental populations are established: ‘low-variability’ populations, in which all flies in a culture are offspring from only two laboratory strains; ‘high-variability’ populations, in which the flies in a culture are derived from 20 different strains. However, the overall degree of individual heterozygosity is the same in both types of populations. Three kinds of populations with respect to the Mdh-2 locus are established within each type; two are homozygous for either the 100 or the 112 allele, the third is heterozygous. A fourth kind of population exists among the high-variability populations; namely, populations in which all three Mdh-2 genotypes are present. The experiments are done at two densities; one quasi-optimal, the other highly competitive.

Populations with high overall levels of genetic variation consistently produce more flies than low-variability populations. The differences are significant at the low, but not at the high, density. Moreover, populations polymorphic for the Mdh-2 locus generally produce more flies than populations having only one Mdh-2 genotype. At high density, the Mdh-2 polymorphic populations have greater productivity than populations with anyone of the three genotypes, and the differences are statistically significant when the polymorphic populations are compared with either one of the two homozygotes or with the average of all three genotypes. In brief, overcompensatory effects – which may account for frequency-dependent selection – are observed in the experiment and may be a common phenomenon in nature.

Populations in which all individuals are heterozygous at the Mdh-2 locus produce in every case more flies than populations with only homozygotes for one or the other allele. The superiority of the heterozygotes is statistically significant for all comparisons at low density, but at high density it is statistically significant for the comparison with the homozygote rarer in nature and only in low variability populations.

The effect of nalidixic acid on conjugal recombination was studied in matings with recipient strains carrying recA200, a mutation which confers a thermosensitive Rec− phenotype. Addition of nalidixic acid to Hfr Nals × F−NalRrecA200 matings at low temperature (35 °C) caused a sharp 10- to 20-fold decline in the yield of recombinants if plating on selective agar was delayed. Two separate processes were identified as being responsible for this decline. Those merozygotes in which the transferred DNA was free of the donor cell lost the ability to form recombinants through inactivation of this DNA, an effect which could be prevented by using exonuclease deficient (recB sbcB) recipients or by prior growth of exonuclease proficient recipients in medium containing 0·25 M sodium chloride. No more than 50% of the observed loss of recombinants could be attributed to this effect. The remaining merozygotes lost their ability for recombinant formation provided mating pairs, and presumably the displaced donor DNA strand, remained intact. This process was thought to involve withdrawal of transferred DNA (DeHaan & Gross, 1962) and was studied in isolation in matings with recA+, or recA200 recB sbcB recipients. A mechanism involving re-annealing of the displaced Hfr DNA to the donor molecule as a result of nalidixic inhibition of gyrase activity in the donor causing relaxation of DNA supercoils is proposed to account for this withdrawal event.

In Paramecium caudatum, syngen 3, cells become mature (i.e. capable of undergoing conjugation) by about 50 fissions after conjugation. Matingtype-instability, another mating characteristic, begins to occur between 80 and 120 fissions after conjugation in heterozygotes for the mating type locus (Myohara & Hiwatashi, 1975). Mating-type instability also occurs in dominant homozygotes, earlier than in heterozygotes. In two different early maturing mutants the homozygotes become mature 20 fissions earlier than in wild-type clones, and mating-type instabiliiy also begins earlier than in wild-type clones. The number of fissions from maturity to the occurrence of the mating type change does not differ.

G6PD measurements (expressed per g haemoglobin) in both males and females of the parental lines C57BL/6J and C57L/J and four recombinant inbred (RI) lines between them showed segregation of G6PD levels as parental classes (one line showed the higher C57BL/6J activity, three lines were indistinguishable from C57L/J). Haemoglobin also varied significantly among the parental and RI lines but this variation did not explain the variation in G6PD. NADP levels (expressed per g wet wt of erythrocytes) correlated with G6PD levels but one RI line was intermediate in males while two RI lines were lower than either parental line in females. Thus, the major autosomal locus affecting G6PD levels which segregates between these two inbred lines probably does not act directly on NADP levels. The variation in the G6PD levels in the four RI lines co-segregated with the brown coat color locus but the association is not significant because of the small number of RI lines available.

Plasmids R68 and R68.45 were transferred from Pseudomonas aeruginosa to Escherichia coli by conjugation. R68.45 was able to mobilize the E. coli chromosome from different origins at a frequency of about lO−6/donor cell. With R68, no transfer of chromosomal genes could be detected. Plasmid R68.45 differs from its parent R68 only by an additional DNA segment, 2120 bp long, located close to the kanamycin resistance gene. By restriction enzyme analysis it was shown that the additional DNA segment of R68.45 is a duplication of a pre-existing DNA region of R68. The duplicated region is characterized by the following sequence of restriction sites: A–310 bp–SmaI–70 bp–PstI–795 bp–PstI–15 bp–KpnI–540 bp–HpaI–370 bp–B.

The endpoints A and B of the duplicated region were determined by a heteroduplex experiment between HindIII linearized molecules of R68 and R68.45. It is proposed that this duplication found in R68.45 is responsible for its chromosome mobilizing ability.

Several of the B–A translocations of maize produce a small-kernel phenotype which is associated with hypoploid endosperms from fertilizations by sperm which result from nondisjunctional events of the B centromere at the second microspore division. Lin (1975) demonstrated that the small-kernel phenotype was not a consequence of the deficient nature of said endosperms, but involved a differential effect of male and female chromosomal regions.

The TB-A effect has been examined for evidence of autonomy in mosaics for the most extreme such case known, i.e. the small-kernel effect produced by the compound B–A translocation, 1La-5S8041. Although reduced-sized kernels are formed when the paternal contribution of the 1L-5S element is totally lacking from the endosperm, sectorial loss has no detrimental effect on dry weight. This phenomenon is therefore considered to be nonautonomous.