A mathematical theory of population genetics accounting for the genes transmitted through mitochondria or chloroplasts has been studied. In the model it is assumed that a population consists of Nm males and Nf females, the genetic contribution from a male is β and that from a female 1 – β, and each cell line contains n effective copies of a gene in its cytoplasm. Assuming selective neutrality and an infinite alleles model, it is shown that the sum (H) of squares of allelic frequencies within an individual and the corresponding sum (Q) for the entire population are, at equilibrium, given by

and

where ρ = 2β(1−β), Ne = {β2/Nm+(1−β)2 / Nf}−1, λ is the number of somatic cell divisions in one generation, and v is the mutation rate per cell division. If the genes are transmitted entirely through the female the formulae reduce to Ĥ ≃ 1/(1 + 2nv) and Q^ ≃ 1/{1 + (2Neλ+ 2n)υ}. Non-equilibrium behaviours of H and Q^ are also studied in the case of a panmictic population. These results are extended to geographically structured models, and applied to existing experimental data.

This study extends previous observations that chromosome loss in somaticcells of juveniles of the pacific oyster Crassostrea gigas is associated with reduced growth rate. All four studies designed to examine this association (two usingrandom population samples and two using full sibs) produced the same result. This consistent effect appears to be unrelated with the commonly, but not consistently, observed correlation between degree of allozyme heterozygosity and growth. We propose thatthe inverse relationship between aneuploidy and growth is due to the unmasking of deleterious recessive genes caused by ‘progressive haploidization’ of somatic cells. Because unmasking of deleterious recessive genes by random chromosome lossisunlikely in polyploid cells, this hypothesis may also provide an explanation for theobservation that artificially produced polyploid shellfish usually grow at faster rates than normal diploid ones.

Seven imprinted genes are currently known in the mouse but none have been identified yet in the distal imprinting region of mouse Chromosome (Chr) 2, a region which shows striking linkage conservation with human chromosome 20q13. Both maternal duplication/paternal deficiency and its reciprocal for distal Chr 2 lead to mice with abnormal body shapes and behavioural abnormalities. We have tested a number of candidate genes, that are either likely or known to lie within the distal imprinting region, for monoallelic expression. These included 3 genes (Cebpb, E2f1 and Tcf4) that express transcription factors, 2 genes (Cyp24 and Pck1) that are involved in growth, 5 genes (Acra4, Edn3, Kcnb1, Mc3r and Ntsr) where a defect could lead to neurological and probably behavioural problems, and 3 genes (Cd40, Plcg1 and Rcad) that are less obvious candidates but sequence information was available for designing primers to test their expression. On/off expression of each gene was tested by reverse transcription–polymerase chain reaction (RT–PCR) analysis of RNA extracted from tissues of mice with maternal duplication/paternal deficiency and its reciprocal for the distal region of Chr 2. None of the 13 genes is monoallelically expressed in the appropriate tissues before and shortly after birth which suggests that these genes are not imprinted later in development. This study has narrowed down the search for imprinted genes, and valuable information on which genes have been tested for on/off expression is provided. Since there is considerable evidence of conservation of imprinting between mouse and human, we would predict that the 13 genes are not imprinted in human. Five of the genes: E2f1, Tcf4, Kcnb1, Cd40 and Rcad, have not yet been mapped in human. However, because of the striking linkage conservation observed between mouse Chr 2 and human chromosome 20, we would expect these genes to map on human chromosome 20q13.

The kinetics of spermatogenesis in the mouse, and the possible genetic controls, have been investigated in different genotypes by means of the method of velocity sedimentation at unit gravity to separate testis cells and nuclei labelled with tritiated thymidine. The progression of the radioactivity through different sedimentation classes of cells provides a measure of their kinetics. The results demonstrate kinetic differences at the stage of spermatocyte differentiation between the C57BL/6 and AKR inbred strains of Mus musculus musculus and in randombred mice of the subspecies Mus m. molossinus. These kinetic differences are controlled by genetic factors which are not linked to the Y chromosome. The relevant autosomal alleles of C57BL/6 are dominant over those of AKR in F1 hybrids.

Arginine-requiring mutants of Neurospora crassa were isolated using a strain partially impaired in an enzyme of the arginine pathway (bradytroph). Among these, five strains were found which carry mutations at a new locus, cpc-1+. The recessive cpc-1 alleles interfere with the cross-pathway control of amino acid biosynthetic enzymes. The enzymes studied, three of arginine and one each of histidine and lysine biosynthesis, fail to derepress under conditions which normally result in elevation of enzyme concentration, namely arginine, histidine or tryptophan limitation. Enzymes not involved in amino acid biosynthesis are still able to derepress in the presence of cpc-1. In wild-type backgound, i.e. with the bradytroph replaced, cpc-1 strains lose the original arginine-requirement. cpc-1 mutations confer sensitivity of growth to 3-amino-1,2,4-triazole.

A least-squares diallel analysis of the offspring of crosses of seven inbred lines of house mice was performed for the mean widths of the second (M2) and third (M3) mandibular molars. Significant differences among lines, as well as between inbreds and hybrids (positive heterosis) were found for these characters. Estimates of the heritabilities (from general combining ability variances) were high for both teeth as were estimates of maternal effects. Specific combining ability and reciprocal effects were significant only for the M3. Additionally, X-irradiation reduced the mean number of individuals per litter, and thereby indirectly influenced the widths of both molars.

An analysis of populations of Drosophila melanogaster which had ceased to respond to selection for increased scutellar bristle number has disclosed the existence of an overall negative genetic correlation between replicate lines in the frequency of the two major component bristle types, viz. anteriors and posteriors. Negative phenotypic correlations among component bristle sites have also been detected within populations. A model involving competition among sites for the available resources of a particular limiting substrate is therefore proposed. Genetic changes have been effected in exceptional populations which lead either to an increase in the rate of production of the limiting substrate, or to a lowering of the concentration of the substrate necessary for bristle initiation. The allelic substitutions concerned are recessive, and have been described as decanalizing alleles in view of their effects on both bristle number variability and a measure of developmental regulation at individual anterior sites. Genetic variation has also been demonstrated for the mean allocation of resources to each of the four component bristle types, viz. anteriore interstitials, posteriors and apicals. A brief discussion is given of the implications of the model for breeding practice in domestic species.

In crosses in which the ratio of Hfr to F- cells was varied it was found that with one group of Hfr strains the yield of recombinants increased linearly with donor concentration until the number of donor and recipient cells were approximately equal. With other Hfr strains marked deviations from linearity were observed at Hfr: F− ratios greater than 1:5 and in crosses with one particular F− strain the yield of recombinants actually decreased progressively as the Hfr concentration was raised beyond this value. The deviations from linearity observed with this group of strains have been shown to be due to damage to F− cells which mate simultaneously with several Hfr cells.

The relationship between parental age and recombination frequency in the offspring of mice has been studied using backcrosses to heterozygous males or females. The mice were allowed to breed until they became infertile. Data for several chromosomes were analysed, mostly from a stock carrying five markers covering approximately three-quarters of the length of chromosome 2.

Analyses of two-point recombination ratios or of multi-point interference ratios revealed no consistent age-related heterogeneity in offspring of male or female heterozygotes. A significant age-related heterogeneity was detected in the offspring of female heterozygotes but not of males, when the number of offspring with no detectable recombination was compared to those with one or more. This difference between the sexes could be related to earlier cytological observations on chiasma frequency in oocytes and spermatocytes. The significance of these findings for the analysis of follicular growth in mammals and in the origin of human trisomic conditions is discussed.

The probability of fixation of a mutation with selective advantage s will be reduced by substitutions at other loci. The effect of a single substitution, with selective advantage , can be approximated as a sudden reduction in the frequency of the favourable allele, by a fraction w = 1 −(s/S)r/s (where r is the recombination rate). An expression for the effect of a given sequence of such catastrophes is derived. This also applies to the ecological prxoblem of finding the probability that a small population will survive, despite occasional disasters. It is shown that if substitutions occur at a rate Δ, and are scattered randomly over a genetic map of length R, then an allele is unlikely to be fixed if its advantage is less than a critical value, Scrit = (π2/6)(2ΔS/(Rlog(S/s))). This threshold depends primarily on the variance in fitness per unit map length dueto substitutions, var(W)/R = 2ΔS/R. With no recombination, the fixation probability can be calculated for a finite population. If Δ > s, it is of the same order as for a neutral allele ( ≈ Δ/(2N(Δ−s))), whilst if , fixation probability is much higher than for a neutral allele, but much lower than in the absence of hitch-hiking . These results suggest that hitch-hiking may substantially impede the accumulation of weakly favoured adaptations.

An important question in evolutionary biology concerns the manner and tempo in which organismal and/or genetic changes that promote evolutionary divergence occur. One recent hypothesis, termed rectangular evolution, holds that most significant evolutionary change occurs during occasional or periodic speciation episodes, with long periods of evolutionary stability in the interim. An alternative view, termed phyletic gradualism, holds that evolutionary divergences proceed by the slow and even accumulation of genetic differences within populations of established species. Two brief tests of rectangular evolution are presented using chromosomal data from North American cyprinid fishes (minnows), a group known to have experienced heterogeneous rates of splitting. Within the rapidly speciated genus Notropis, rates of chromosomal evolution appear slower relative to other, less rapidly speciated confamilial genera. Species of Notropis also are less divergent chromosomally, on the average, than are species from other cyprinid genera. These results are in compatible with a rectangular mode of chromosomal divergence these fishes. The results also reveal inconsistencies with a gradual mode chromosomal divergence, but at present this hypothesis cannot be falsified. Consideration of these and other data suggests that different levels of the cyprinid genome may follow independent evolutionary paths.

Multiple recombinational events within a gene were studied by tetrad analysis of multipoint intragenic crosses. It was found that a considerarle proportion of double-site conversions can arise as two separate but correlated events. The same was true for conversion and crossing-over associated with it. The data point to a possibility of multiple recombinational events involving both conversions and cross-overs, occurring as successive rounds of recombination within a single recombinational process. Some of the results suggest that more than two chromatids may be involved in such a process. The results are discussed in terms of different recombination models.

Selection against harmful mutations in large populations is studied assuming that the rate of fitness decrease grows with every new mutation added to a genome. Under this reasonable assumption (Mayr, 1970) the average fitness of a sexual population, without linkage between the loci, is higher, and the average equilibrium number of harmful mutations per individual lower, than in an asexual population. If a gamete contains on the average one or more new mutations, the resulting advantage of sexual reproduction and recombination seems to be sufficient to counterbalance the double advantage of parthenogenesis. Moreover, selection against harmful mutations is probably the most powerful factor preventing linkage disequilibrium even with epistatic interaction between the loci.

A mutant strain of Drosophila melanogaster with five markers on the X-chromosome was found to be more sensitive than the wild type when infected with an insect-pathogenic strain of Serratia marcescens. Two of the five mutations in this fly strain, cut and miniature, were found to be responsible for this sensitivity. A double-mutant, with both cut and miniature, was as sensitive to Serratia infection as was the original sensitive Drosophila strain with all five mutations. Recombinant flies with other alleles of cut and miniature were also sensitive. A revertant of cut was found to be less sensitive than the parental flies. Our insect pathogenic strain of Serratia produces several proteases and a chitinase. A bacterial mutant, lacking proteases and chitinase, was found to be less virulent than wild-type bacteria. When pupal shells from resistant and cut-miniature flies were incubated with a mixture of protease and chitinase there was a release of N-acetyl glucosamine, and 50% more material was liberated from pupal shells of sensitive flies. Sensitive flies reared on sucrose infected with Serratia showed bacteria in their hemolymph earlier than wild-type flies. We conclude that Drosophila genes for cut and miniature are associated with the sensitivity to Serratia infection, presumably because the gut peritrophic membrane is more susceptible to bacterial proteases and chitinase.