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An expression is derived for determining the probability of survival of a new favourable mutation in a large random-mating population with overlapping generations. For a gene of small effect, in a near-stationary population, an approximate formula similar to the usual one for discrete generations is obtained. The implications of these results for the evolution of life histories are discussed, using the partial derivatives of the chance of survival of a gene, with respect to changes in age-specific fecundities and survival probabilities. The properties of these derivatives are very similar to those of the derivatives of the intrinsic rate of increase, analysed by Hamilton (1966), thus providing a genetical basis for his conclusions concerning the evolution of life histories.
A mutant of Tetrahymena with heat-sensitive phagocytosis was obtained using a tantalum-particle enrichment procedure. The mutant phenotype is most likely determined by a somatic (macronuclear) mutation(s). The inability of the mutant to sustain cell division and to phagocytize at 37 °C are most likely determined by the same mutation. The phenotype of the mutant is stably inherited under vegetative propagation at 30 °C. At 37 °C, the mutation affects the development of the oral apparatus, the phagocytotic organelle. This mutant has proven useful for the study of cellular functions related to phagocytosis.
Manganese and cobalt are capable of inducing ρ− mutations* in non-growing cells of Saccharomyces cerevisiae, but their mutagenic action is much stronger in growing cells. At a given concentration cobalt and manganese can be either strongly mutagenic or non-mutagenic, depending on the cell density.
Most of the ρ− mutants induced with manganese and a considerable proportion of those induced with cobalt are suppressive and/or transmit drug resistance markers, so they must still carry mitochondrial DNA. Cobalt can decrease suppressiveness with low efficiency and eliminate drug resistance markers from established ρ− clones.
A mathematical method for evaluating the probability that a locus is monomorphic for the same allele in related species is developed under the neutral mutation hypothesis. A formula for the proportion of identically monomorphic loci in related species is also worked out. The results of the application of this method to Drosophila data do not support Prakash & Lewontin's (1968) contention that the strong association between gene arrangements (inversion chromosomes) and alleles at protein loci is evidence of coadaptation of genes in the inverted segment of chromosomes. Similarly, unlike Haigh & Maynard Smith's (1972) contention, the monomorphism of the haemoglobin α chain locus in man can be accommodated with the neutral mutation hypothesis without invoking the bottleneck effect.
There is subnormal dispersion in the distributions of the combinations of the sexes in some samples of litters of pigs, rabbits and mice. For instance, consider litters of exactly size 8. As contrasted with binomial expectation, there are too many with exactly 4 males and 4 females, and too few unisexual litters. It is argued that this supports the hypothesis (independently proposed by the author and by Guerrero) that P, the probability that a zygote will be male, varies with the time at which it is formed within the cycle. It is noted that data of Kaufman seem to support the hypothesis.
Strains of Aspergillus nidulans with a duplicate segment are mitotically unstable; they produce phenotypically improved variants following deletions in either duplicate segment, and morphologically deteriorated types. The number of variants produced is characteristic of each duplication strain under the same conditions. After ultraviolet treatment two variants, one more stable and the other less stable than the original strain, were selected. Genetic analysis showed that the increased instability in the less stable variant was due to a translocation involving linkage groups V and VIII. The increased stability of the more stable variant was due to a recessive factor (stf–1) located in linkage group VIII. In the homozygous condition this factor also reduces the number of sectors in a diploid strain. The possible genetic mechanisms explaining the instability alterations are discussed.
Six hundred and ten gynandromorphs were produced in which an X chromosome loss uncovered the vermilion mutation. The mosaic patterns observed indicate that wild type ocelli are incapable of kynurenine production and that, in addition to the eyes, postembryonic kynurenine producing cells originate from two separate regions of the blastoderm. The positions of these regions on the genetic fate map of Drosophila melanogaster correspond to the embryonic precursors which give rise to the kynurenine producing cells of the larval fat body and Malpighian tubes.
The genetic control of resistance to the fungus, Cercosporella herpotrichoides, causing the eyespot disease of wheat was studied using the Chinese Spring (Cappelle-Desprez) chromosome substitution lines and F2 monosomic families of hybrids between Cappelle-Desprez and Mara. Chromosome 7A of Cappelle-Desprez was found to increase resistance to eyespot in both types of test. Chromosomes 2B and 5D of Cappelle-Desprez gave increased resistance compared to their homologues in Mara on an F2 background. When substituted into Chinese Spring neither of these chromosomes appeared to increase resistance. Chromosome 1A of Cappelle-Desprez in a Chinese Spring background increased the level of infection. Dominance was towards resistance and the presence of between-chromosome interaction could be deduced.
Alcohol dehydrogenase activity in Drosophila melanogaster may be considered as a quantitative character, since it shows many features typically associated with such traits. Although strains with the electrophoretically fast phenotype generally have activities greater than those with the slow phenotype, presumably reflecting differences in the nucleotide sequences of the structural alleles, within each electrophoretic class there is considerable variation in activity. The expression of the structural gene, in terms of ADH activity, is to some extent regulated by its genetic background. Strains homozygous for particular structural alleles respond to divergent directional selection for ADH activity. Modifiers have been located to the X, second and third chromosomes.
Genetic analysis of seven dominant short tailed mutations independently induced by radiation of male mice showed that six were allelic to T (Brachyury) but not identical to it. Homozygotes for each mutant die at least 2 days earlier than T/T homozygotes; two that were studied histologically are indistinguishable from one another. The development of these abnormal embryos is arrested by seven days of gestation, when cells of embryonic ectoderm cease proliferation and become pycnotic. Endoderm and extra-embryonic ectoderm do not seem to be primarily affected, and survive and grow for at least 2 days more. Serological studies of one of these mutations suggest that it is a deletion. A review is presented of these and other T-like mutations that have been described; from this it appears that five different categories of T-like mutants are discernible.
Chlorpromazine and several other phenothiazines at sub-bacteriocidal concentrations were found to cure an Escherichia coli F′ lac+ strain of its plasmid efficiently. Curing was most efficient at high pH and in complex medium when 70% or more of the bacteria were plasmid-free after 24 h growth of cultures in the presence of the drug.