A new method for the isolation of temperature-sensitive mutants of Physarum polycephalum is described. It involves enrichment and prescreening of mutagenized amoebae followed by screening at both the plasmodial and amoebal stage. A total of 74 temperature-sensitive strains were recovered of which 26 were temperature-sensitive only as plasmodia, 35 only as amoebae and 13 in both stages. After a shift to the nonpermissive temperature, DNA and protein synthesis were followed in temperature-sensitive plasmodia to discover if the lesion affected functions of the nuclear cycle.

Genetic variation contained in a multigene family was theoretically investigated from the standpoint of population genetics. Unequal crossover is assumed to be responsible for the coincidental evolution of mutant genes in a chromosome. When the allowed latitude of the duplicated or deleted number of gene units at unequal crossover is 10 ˜ 15% of the total gene number in a chromosome, the arrangement of gene lineage in a chromosome is shown to be roughly random. The equilibrium properties of genetic variation or the probability of identity of two genes within a family (clonality) were studied under mutation, unequal crossover, interchromosomal crossover and sampling of gametes. The clonality of a multigene family within a chromosome is shown to be approximately

in which α = 2k/n2 with k = effective number of cycles of unequal crossover and with n = number of gene units in a family, v is the mutation rate per gene unit, β is the rate of interchromosomal crossover per family and Ne is the effective size of the population, all measured by the rate per generation. The clonality of a gene family between two different chromosomes becomes approximately C1 = C0/(l + 4Neν). Some models of natural selection which lowers the clonality or increases genetic variation in a multigene family were investigated. It was shown that natural selection may be quite effective in increasing genetic variation in a gene family.

The (I; II) duplication strain of Aspergillus nidulans contains a terminal segment of chromosome I twice, one copy being in the normal position, and the other one translocated to a tip of chromosome II. We show that in selfed cleistothecia (homozygous for the duplication) crossing-over between the two segments of the duplication followed by the appropriate disjunction and distribution of the meiotic chromosomes, results in segregation of recessive markers in the duplication. Alternative segregation mechanisms like the deletion mechanism responsible for mitotic non-conformity, cannot explain the observations. The average segregation frequency of the marker yA2 is 11%. Using the mathematical model outlined in the Appendix to this paper, we estimated the recombination frequency between the translocation breakpoint and the yA locus from the segregation frequency. The essential feature of the model is the computation of the probabilities of combinations of different tetrad types in quadrivalents. Application of the model leads to the qualitative conclusion that the meiotic recombination frequency within the duplication is increased over the normal level. Elsewhere in the genome the frequencies are either unchanged or decreased. Also in crosses heterozygous for the duplication, an increase by at least a factor of two is found. As judged from the appearance of certain recombinant classes, the quadrivalent frequency in homozygous and heterozygous crosses is 2/3 (random pairing) or slightly higher. The increased recombination is attributed to the same lesions that are responsible for the occurrence of deletions in duplication strains of A. nidulans.

Mice homozygous for the asebia mutation (ab/ab), which have defective sebaceous glands, display abnormalities in several other aspects of the integument. Histological sections showed that hyperplasia of the cellular layers and the stratum corneum of the epidermis is apparent at birth and increases markedly with age. Enlarged intercellular spaces are also noted in the epidermis. The thicker dermal layer of the asebic mice is characterized by increased vascularity, increased cellularity and the abnormal morphology of a large proportion of the ‘fibroblast’ population. Electron microscopy demonstrated the many abnormalities in the dermal fibroblasts as well as large amounts of cellular debris in the surrounding matrix. Collagen and elastin show alterations at the light microscopic and ultrastructural levels. Many features of the asebic dermis resemble those found with mild inflammation and with the lysosomal storage diseases. Changes in the dermis of asebic foetuses were noted prior to epidermal alterations and may mediate the latter.

The combined effect of multi-niche selection and cyclical selection is studied in a deterministic model. Sufficient conditions for protected polymorphism are derived for diploid populations with and without dominance and for haploid populations. These conditions appear to be broader than in the cases of only multi-niche selection or only cyclical selection.

A series of derepressed mutants of the tetracycline resistance (T) determinant in R-factor R57 have been found to be repressor-negative and recessive to the T determinant in R6. It is shown that these (Tdr) mutants are dominant to the inducible T determinant in RPl, indicating that the T determinants in R57 and RP1 code for different repressors of the resistance gene. The same Tdr determinants are unstable in cells carrying both the R57 mutant and RP1, probably due to selection against the dominant Tdr gene because it depresses the growth rate of the host cell compared with its T+ homologue. It is suggested that the T determinants giving high-level resistance in R57, R6 and R100 form one homologous group, probably disseminated by the transposon TnlO, while T determinants giving a much lower level of resistance, such as that in RP1, form a separate group, which may include those in R46, and R199. It is proposed that the gene responsible for tetracycline resistance should be designated tetA and the repressor gene tetI. The R57 Tdr mutants then have the genotype tetI−tetA+.

Two alleles of a gene (rac) unlinked to the mating-type locus (mt) have been identified in strains of Physarum polycephalum from different laboratories. Heterothallic strains differing in mt alleles cross more rapidly if they differ also in their rac alleles. The recovery of hybrid plasmodia from crosses between apogamic (mth) and heterothallic strains is more likely to be achieved if strains of different rac genotype are used.

The previously isolated plasmid pND3, arising from recombination between the TOL catabolic plasmid and the R plasmid R91, was transduced by pf16 in Pseudomonas putida. Apparent transductional shortening was evident in 25% of the transduced pND3 plasmids. Transductants were isolated which had segregated the antibiotic resistance marker, transfer ability and some of the catabolic functions of the parent plasmid.

D. melanogaster imagoes were collected weekly throughout the summer and early fall of 1976. Their Adh genotypes were determined by electrophoresis. The frequency of the Adh4 isoallele fluctuated throughout the period of study. Correlation coefficients assuming no delay, one week delay, two weeks' delay, three weeks' delay, and a four-week delay of the effect of temperature on the frequency of the Adh4 isoallele were insignificant. It was concluded that temperature alone may not be a selective factor but may be selective in combination with other environmental factors.