Mean fitness and inbreeding depression values in multi-locus models of the control of fitness were studied, using both a model of mutation to deleterious alleles, and a model of heterozygote advantage. Synergistic fitness interactions between loci were assumed, to find out if this more biologically plausible model altered the conclusions we obtained previously using a model of multiplicative interactions. Systems of unlinked loci were assumed. We used deterministic computer calculations, and approximations based on normal or Poisson theory. These approximations gave good agreement with the exact results for some regions of the parameter space. In the mutational model, we found that the effect of synergism was to lower the number of mutant alleles per individual, and thus to increase the mean fitness, compared with the multiplicative case. Inbreeding depression, however, was increased. Similar effects on mean fitness and inbreeding depression were found for the case of heterozygote advantage. For that model, the results were qualitatively similar to those previously obtained assuming multiplicativity. With the mutational load model, however, the mean fitness sometimes decreased, and the inbreeding depression increased, at high selfing rates, after declining as the selfing rate increased from zero. We also studied the behaviour of modifier alleles that changed the selfing rate, introduced into equilibrium populations. In general, the results were similar to those with the multiplicative model, but in some cases an ESS selfing rate, with selfing slightly below one, existed. Finally, we derive an approximate expression for the inbreeding depression in completely selfing populations. This depends only on the mutation rate and the dominance coefficient and can therefore be used to obtain estimates of the mutation rate to mildly deleterious alleles for plant species.

Selection for high and low growth-rate was carried out during eight generations of asexual propagation by zoospores and seven generations of sexual reproduction by oospores. The fungus has previously been shown to be diploid during its vegetative phase. In the zoospore lines there was no significant variation and no response to selection, except for the occasional appearance of fast-growing sectors. A high line was established from such a sector; in its sexual progeny the inheritance of growth-rate was non-Mendelian. Propagation through self-fertilized oospores released very considerable genetic variation, and both high and low lines responded to selection. At first the variation within families, and the response to selection, increased with succeeding generations, despite the intense inbreeding. In later generations the high line became less variable, and the progeny oospore cultures resembled the fast-growing sectors. It is concluded that growth-rate is controlled by a polygenic system and by cytoplasmic determinants, a mutant form of which is responsible for the fast-sectoring phenotype.

The effect of nalidixic acid, a specific inhibitor of DNA synthesis, on Escherichia coli strain B (lon) and its u.v.-sensitive derivatives is examined. Strain B itself is sensitive to nalidixic acid, whereas its u.v.-resistant derivative B/r is resistant.

It is shown that in all exr A strains, in which u.v.-induced filamentation is suppressed, resistance to nalidixic acid is increased. Among exr A strains, Bs4 is exceptionally resistant to nalidixic acid. This is because nalidixic acid kills only growing cells and strain Bs4, a try auxotroph, may grow poorly under the conditions used to test nalidixic acid.

The uvr genes of the HCR strains Bs1, Bs8 and Bs12 do not suppress u.v.-induced filamentation nor do they affect the response to nalidixic acid. The uvr gene of strain Bs3 is unusual in increasing the tendency to filament and also sensitivity to nalidixic acid.

Strains Bs1, Bs3 and Bs8 are all doubly mutated from strain B, the second mutation (not uvr) being responsible for their increased resistance to nalidixic acid as well as partially or completely suppressing filamentation.

It is concluded that the cell division mechanism of (lon) strain B is sensitive to inhibition of DNA synthesis. Mutations which suppress the tendency of strain B to filament reduce its sensitivity to inhibition of DNA synthesis.

1. The maintenance of mu particles and metagons from Paramecium aurelia (stock 540, syngen 1) in another ciliate protozoan Didinium nasutum has been shown to occur.

2. It has been shown that mu particles could not be supported in Didinium without metagons. One particular strain of Didinium has, however, never been able to support mu even when metagons were present.

3. The continued production of metagons in Didinium was shown to take place even when the Didinium was fed on killer Paramecium of genotype m1m1m2m2.

4. After destruction of the metagons in Didinium by ribonuclease there was no subsequent reappearance of metagon activity.

5. Metagons and mu particles were eliminated from Didinium during encystment.

6. It is considered that the increase of metagons in Didinium is due to self-replication. In Paramecium the metagons replicate only slowly or not at all.

7. It is concluded that the metagon is a gene product in Paramecium which takes on at least one other property, replication, when introduced into Didinium.

We measured traits involved in physiological and behavioural thermoregulation in the 6 replicates of a selection experiment for large and small size (6-week weight) in mice, including control lines (18 lines in all). The observed genetic correlations between body size and thermoregulatory traits are consistent with a thermoregulatory advantage of large size, including decreased weight-specific food consumption and increased nest-building, with no change in body temperature. The differences in food consumption were closely paralleled by differences in amount of brown adipose tissue, strongly suggesting that much of the decreased efficiency of the small lines is due to heat production by brown fat. These results are consistent with available observations on natural populations, that selection for temperature adaptation probably has had some influence on body size in this species.

White-spored mutants of Ascobolus immersus were used to study postmeiotic segregation within a gene. It was found that in two-point crosses between mutants showing postmeiotic segregation, polarization in recombinant asci resulting from this type of segregation resembled that in normal (even-type) conversion. However, the ratio of recombinant asci with even segregation (6:2 asci) to those with odd segregation (7:1 asci) was much higher than might have been expected on the basis of the frequencies of various recombinant asci found in crosses between the same mutants and the wild-type strain. It seems that none of the current models of recombination gives a satisfactory explanation of the results obtained.

We have searched for evidence of historical transpositions of Ac-like sequences in four standard maize lines using the recombinant-inbred mapping technique. Thirty restriction fragments were mapped using EcoR I, EcoR V, and Hind III. The four inbreds contained 24 fragments which mapped to independent sites within each line; the other 6 fragments probably represented multiple mappings of the same element. Possible allelism between lines reduced this number to a minimum of 15 different sites containing Ac-like elements. The distribution of these sequences does not fit the expected Poisson distribution; instead, an unusually large number of these elements were found on chromosome 4. The other sequences were scattered randomly throughout the genome. With few exceptions, each line had sequences in different locations; however, the overall distribution of Ac-like sequences was similar for all lines. The non-random distribution of Ac-like sequences suggests that they have undergone a limited number of transpositions in maize; the distribution is incompatible with either complete immobility or frequent transposition.

A large number of mutants that affect the morphology of the ascus of Neurospora crassa has been isolated by means of a screening technique that utilizes the filtration principle to separate colonial type mutants from wild type. In relationships with each other and with wild-type alleles, the mutant genes exhibit zygote dominance and recessiveness. A zygote complementation test for placing the recessive mutants into functional groupings is described. Results obtained by use of the test place the mutants into seven functional groups. Dominance relations in the developing ascus of Neurospora are discussed.

Evidence is presented which indicates that the phenylalanine analog 2-amino-3-phenylbutanoic acid (APBA) induces mutation to APBA, streptomycin, and neamine resistance in the green alga Chlamydomonas eugametos. Apparent chromosome abnormalities (reciprocal translocations and inversions) were also found among some clones recovered after APBA treatment. The two, mutation and chromosome aberration, are separable.

The three theories of dosage compensation in Drosophila are examined. Data are presented supporting a developmental interpretation. The reason why such a mechanism is applicable in insects but not in mammals is discussed.

Recessive lethal mutations and mutations at the gol-1 locus were induced in the zebrafish by exposure of mature sperm to the alkylating agent ethyl nitrosourea (ENU). Embryonic lethal phenotypes were recognized among the parthenogenetic progeny of mutagenized animals or among the progeny of daughters of mutagenized animals. Novel specific locus mutations were identified by the failure of mutagenized chromosomes to complement pre-existing mutant alleles at the gol-1 locus. Each mutagenized individual harboured approximately 10 embryonic lethal mutations in its germ line and about 1 in 500 mutagenized animals harboured a new mutation at the gol-1 locus. Three lines of evidence indicate that the majority of mutations that were recovered following treatment of mature sperm with ENU were probably point mutations. First, the soma and germ lines of mutagenized animals were mosaic, as expected following simple alkylation of sperm DNA. Second, mutations induced by ENU at the gol-1 locus affected pigmentation but not viability, unlike the majority of mutations induced at this locus with y-irradiation. Third, the ratio of specific locus: recessive lethal mutations induced by ENU was approximately 50-fold lower than the ratio observed following mutagenesis with y-rays. Comparison of the incidence with which embryonic recessive lethal mutations were induced with the incidence with which specific locus mutations arose indicates that there are greater than 5000 genes essential to the development and viability of the zebrafish embryo.

The substructural organization of chromosome cores or nonhistone proteins was studied within intact metaphase chromosomes at the second meiotic division in the grasshopper Trilophidia annulata by silver staining as well as light microscopy and whole mount electron microscopy of squash chromosomes. Our results revealed that the metaphase II chromosome contains a longitudinal, helical coiling core structure. Probably the two last organizational levels of the core packaging are achieved by helical coiling. The core structure retains the morphological characteristics of the original metaphase chromosome, surrounded by a halo of dispersed materials, which may be composed mainly of nonhistone proteins. The kinetochore is found to be connected with the chromosome core. The present findings combined with our previous observations on the helical structure of metaphase II chromosomes suggest that the folding path of the internal core structure in metaphase chromosomes is consistent with the final helical arrangement of the chromosome itself. These observations also imply that in condensed metaphase chromosomes nonhistone protein may form a compact network structure with helical appearance, which extends throughout the entire chromosome.