Genetic mapping by means of mitotic haploidization (induced by parafluoropkenylalanine) and mitotic crossing-over was carried out with the fission yeast Schizosaccharomyces pombe. Thirty-two different genetic markers were involved in this investigation; some meiotic linkage relationships had been previously reported (Leupold, Megnet) for 16 of these loci. Mitotic haploidization experiments resulted in the genetic identification of six chromosomes in the haploid complement.

Furthermore, in an attempt to study the mechanism of action of parafluorophenylalanine (pFPA) on mitotic haploidization, pedigree analyses were performed by micromanipulation of diploid cells growing in the presence of pFPA. Haploid cells were detected after 40 hours of contact with the analogue and many lethal pedigree branches were observed. These observations seem to agree with Käfer's (1961) and Lhoa's (1968) suggestion that mitotic haploidization in Fungi is achieved by progressive loss of chromosomes throughout cell divisions.

If a polymorphic locus is maintained in finite populations by frequency-dependent selection with selective neutrality at equilibrium, it is generally accompanied by two genetic loads, i.e. the dysmetric and the drift loads. The former arises because the fitness of the population may not be at a maximum at the equilibrium gene frequency and the latter because genetic drift in small populations displaces the gene frequency from its equilibrium value.

In some simple models of frequency-dependent selection considered, the drift load is independent of selection coefficients and is approximately equal to (n−1)/(2Ne), where n is the number of alleles and Ne is the effective population size.

Mutants of Ustilago maydis have been isolated both, deficient and derepressed for nitrate reduction. Those deficient in enzyme fall into six groups, one of which is the structural gene. Enzyme which has proved to be more labile than that of wild-type has been isolated from a temperature-sensitive mutant at this locus. All the mutants in the structural gene have xanthine dehydrogenase activity and the situation closely parallels that of Aspergillus nidulans.

The derepressed mutants fall into four complementation groups and all are partially derepressed in that they are further inducible by nitrate. Full derepression can be conferred by induction of a second mutation. In one analysed case the second reinforcing mutation proved to be pheno-typically similar to the first one when separated from it.

Associative overdominance arises at an intrinsically neutral locus through its non-random association with overdominant loci. In finite populations, even if fitness is additive between loci, non-random association will be created by random genetic drift.

The magnitude of such associative overdominance is roughly proportional to the sum of between the neutral and the surrounding over-dominant loci, where is the squared standard linkage deviation, defined between any two loci by the relation

in which p and 1 – p are frequencies of alleles A1 and A2 in the first locus, q and 1 – q are frequencies of alleles B1 and B2 in the second locus, and D is the coefficient of linkage disequilibrium. A theory was developed based on diffusion models which enables us to obtain formulae for under various conditions, and Monte Carlo experiments were performed to check the validity of those formulae.

It was shown that if A1 and A2 are strongly overdominant while B1 and B2 are selectively neutral, we have approximately

provided that 4Nec ≫ 1, where Ne is the effective population size and c is the recombination fraction between the two loci. This approximation formula is also valid between two strongly overdominant as well as weakly overdominant loci, if 4Nec ≫ 1.

The significance of associative overdominance for the maintenance of genetic variability in natural populations was discussed, and it was shown that Nes′, that is, the product between effective population size and the coefficient of associative overdominance, remanis constant with varying Ne, if the total segregational (overdominant) load is kept constant.

The amount of linkage disequilibrium expected due to random drift in experimental populations was also discussed, and it was shown that in the first generation, if it is produced by extracting n chromosomes from a large parental population in which D = 0.

Resistance to mercury has been shown to be associated with the presence of the sex factor (FP) in Pseudomonas aeniginosa. The resistance is expressed in all infected cells even though they do not necessarily function as donors.

For selection of freshly infected recipients when streptomycin or nalidixic acid were used for contra-selection of the donors, the level of mercury had to be reduced to between m/20000 and m/40000 when 500 units of streptomycin were present per ml and to m/40000 when 1·0 mg of nalidixic acid was included in each ml of medium. Normally donor strains would grow in the presence of Hg ions at a concentration of m/2000, which is 10–20 times greater than the concentration in the two selective media.

The kinetics of transfer of the sex factor were investigated and it was shown that transfer began almost as soon as the cells were mixed and was completed in 20 min when 5 min were allowed for pairing.

Seven ascospore colour mutants of the 164 locus of Ascobolus immersus were mapped, and their basic conversion frequencies established. Polarization of the sum of the basic conversion frequencies was observed. The recombination frequencies from the two-point crosses (in repulsion) are nonadditive. The influence of one mutant on the frequency of conversion of another in crosses in repulsion and coupling was pronounced. The frequency of crossing over between a pair of mutants was higher in the cross in repulsion than in coupling. A total of 535 asci containing a pair of wild-type spores were analysed from five crosses in repulsion. 193 6w:2d asci originated from the conversion of the proximal allele, 43 6w:2d asci from conversion of the distal allele and 277 asci exhibited reciprocal recombination. 54 6w:2d asci from one cross in coupling were analysed. These asci were classified and assigned to four classes: proximal allele conversion, distal allele conversion, reciprocal recombination and simultaneous conversion of wild type-alleles to mutants. In all classes of recombinant asci analysed the frequencies of additional exchanges in the adjacent intervals were higher than in random samples of asci. The wild-type chromatid was involved in the additional exchanges in the majority of crosses with a frequency exceeding 60%.

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 selection of fi+ resistance factors and transfer factors, when introduced into K 12F+, showed a range of inhibitory activity of lysis by the male-specific phage μ 2. This range can be used to subdivide the fi+ factors into fi+1, fi+2, fi+3 and fi+4 classes, according to the degree of inhibition of μ 2 lysis. To this subdivision can be added restriction of the ‘female-specific’ phage φ 2.

Introduction of all the fi+ factors tested into K 12 HfrH totally inhibited lysis by μ 2 in spot tests, but with two fi+1 and one fi+2 factors visible lysis was obtained in agar-layer tests. These three factors caused least inhibition of transfer of pro by HfrH. It can be assumed that both tests reflect lower inhibition of sex fimbrial formation by these fi+ factors than by the remainder.

The fi− factors, when introduced into K 12, can be subdivided on the basis of restrictive effects on phage φ 2.

These effects can be added to phage restriction in the salmonellae for the purposes of further classification of the transfer factors and R-factors.

Following an experiment in which mice had been selected for the length of the mitochondrial section (midpiece) of their sperm tails, an attempt has been made to determine experimentally the adaptive significance of this character; this was by the artificial insemination of mixtures of selected and unselected (control) spermatozoa, and the subsequent disclosure of their competitive fertilizing ability in the paternity of the offspring. After the birth of nearly 500 offspring, there was no indication that the control cells were—by this criterion—functionally superior. From the discrepancy between the pairs of males sampled, however, it is possible that other biological or technical factors have been important.

The b locus of Ustilago maydis is both an incompatibility locus and a pathogenicity locus. There are several different b factors, and they are randomly distributed and equally frequent in the natural population. A sample of 62 lines contained 18 b groups and permitted an estimate of no more than 25 b groups in the natural population. All lines within the same b group are indistinguishable in their mating behaviour. No linkage was detected between b and any of the four biochemical markers tested. No intrafactor recombinants at b were found among more than 6700 progeny. Differences in degree of pathogenicity between solopathogenic diploids and comparable haploid by haploid crosses were detected.

By analogy with the situation in coccids it is suggested that in mammalian XO embryos the single X turns heterochromatic in some cells, but that such a change does not result in cell death because the X then reverts back to an euchromatic and active state. This testable alternative to the Gartler–Sparkes hypothesis would imply that the anomalies of the XO Turner syndrome are largely due to imbalance of sex-linked genes rather than developmental damage resulting from cell death and that mammalian X inactivation might become reversed in response to special developmental needs.

Klebsiella aerogenes strain V9A carries determinants AK and TK, giving resistance to ampicillin and tetracycline, and a plasmid FKlac, but no active sex factor. F− and I-type sex factors were able to transfer TK from V9A to Escherichia coli K12 and between strains of K12, and TK behaved as a separate plasmid with its own replicon. AK could not be transferred, except possibly by a sex factor carrying its own A determinant, but the evidence for such transfer was inconclusive. It is suggested that AK is either a chromosomal gene or is in a plasmid with a cell attachment in Klebsiella not represented in E. coli K12. AK produces a β-lactamase.