Using silver staining and the whole mount electron microscopy technique of squashed chromosomes, we studied the substructural organization and behaviour of chromatid cores in mitotic chromosomes of spermatogonia of the grasshopper Oedaleus infernalis during mitosis. It was found that the formation of mitotic chromatid cores takes place during the transition from prophase to prometaphase. Each chromosome contains two compact chromatid cores which are surrounded by a halo of dispersed argyrophilic material emanating radially from the cores. In early metaphase the chromatid core usually appears as an extended, slender network running longitudinally through the entire length of the chromatid, while in late metaphase the core frequently has a spiral appearance. In addition, our results revealed the existence of interconnections between sister chromatid cores along their entire length, as a result of which sister chromatid cores appear as a single interconnected core network in mitotic metaphase chromosomes. At this stage the core occupies a lateral position in each chromatid. However, during the transition from metaphase to anaphase, the interconnections are gradually released to allow the individualization of sister chromatid cores and the segregation of chromosomes. The core comes to occupy a central position in each segregated chromatid. These findings demonstrate the presence of an intrinsic interconnected core network within metaphase chromosomes which could be involved in the maintenance and segregation of chromosomes during mitosis.

The insertion site numbers of the transposable elements (TEs) copia, mdgl, 412 and gypsy were determined in various natural populations of Drosophila melanogaster and D. simulans by in situ hybridization. We showed that, while all elements except gypsy had many insertion sites scattered over the chromosomes in D. melanogaster, only the 412 element in D. simulans presented a high number of insertions, and this number was lower than in D. melanogaster. This low 412 site number per genome in D. simulans was associated with a lower proportion of insertions on the X chromosome in comparison with D. melanogaster, as determined in diploid genomes (0·090 for D. simulans against 0·137 for D. melanogaster) and in haploid genomes (0·102 against 0·146), each value being, moreover, lower than the value of 0·20 expected on the hypothesis of no selection against insertional mutations. These results suggest that selection is a major mechanism explaining 412 copy number regulation in Drosophila, and is stronger in D. simulans than in D. melanogaster.

Z-(ll)-pentacosene, Drosophila virilis sex pheromone, is predominant among the female cuticular hydrocarbons and can elicit male courtship behaviours. To evaluate the genetic basis of its production, interspecific crosses between D. novamexicana and genetically marked D. virilis were made and hydrocarbon profiles of their backcross progeny were analysed. The production of Z-(ll)-pentacosene was autosomally controlled and was recessive. Of the six D. virilis chromosomes only the second and the third chromosomes showed significant contributions to sex pheromone production, and acted additively. Analysis of recombinant females indicated that the locus on the second chromosome mapped to the proximity of position 2–218.

We investigated the genetic variability for phally polymorphism within and between natural populations of the hermaphrodite self-fertile freshwater snail Bulinus truncatus. Phally polymorphism is characterized by the co-occurrence in natural populations of regular hermaphrodite individuals (euphallic) and individuals deprived of the male copulatory organ (aphallic). The two morphs can both self-fertilize and outcross. However, aphallic individuals cannot outcross as males. We examined the variation of the aphally ratio in 22 natural populations from Niger over two successive years. During the second years, populations were sampled three times at 3 week intervals. The aphally ratio was highly variable among populations at a given sampling data and remained relatively stable over time. For 10 of these populations, one population from Corsica and two from Sardinia, we also estimated the between- and within- population variability, analysing the aphally ratio of 346 families under laboratory conditions. The aphally ratio varied significantly among populations and was highly correlated with the aphally ratio of the natural populations. Some within-population variability, associated with a high value of the broad sense heritability, was observed in four populations out of 13. In these populations, aphallic individuals produced significantly more aphallic offspring than euphallic individuals. Our results indicate a strong genetic basis for aphally, with large genetic differences among populations and some genetic variability for aphally within populations. We discuss the adaptive and stochastic factors that may shape the distribution of the genetic variability for aphally.

Previous studies have shown that mice with paternal disomy for chromosome 11 are consistently larger at birth than their normal sibs, whereas mice with the maternal disomy are consistently smaller. An imprinting effect with monoallelic expression of some gene/s affecting growth was indicated. Here we show that the size differences become established prior to birth and are only maintained subsequently, indicating that the gene repression is limited to prenatal development. Fetal analysis was limited to 12·5–17·5 days post coitum. However by extrapolating the data backwards it could be calculated that both the maternal and paternal size effects might commence as early as 7 days post coitum, although possibly slightly later. It may be deduced that initiation of expression of the gene/s responsible may occur at about this time in development. The two disomy growth rates were mirror-images of each other, suggesting that expressed gene dosage is the underlying cause. Differential growth of the placentas of the two disomies was also found, and extrapolation of these data backwards suggested that the placental size differences were initiated later in development than those for the fetuses. The differential placental growth of the maternal and paternal disomies may therefore have developed independently or emerged as a consequence of the differential fetal growth. In either event it would seem that the expression of the responsible gene occurs in the fetus itself to cause the anomalies of growth. The data therefore provide information on the temporal and tissue specificity of the gene/s responsible for the chromosome 11 imprinting effects. Possible candidate genes are discussed.

Material sampled along 11 rivers of the western part of Salix viminalis L. natural range (in Poland, Germany and Austria), as well as in stands in Sweden and Belgium, was assayed for 15 isozyme loci and cuttings were installed in two field experiments located in a nursery south of Uppsala, where growth traits were measured. These data were used to test hypotheses on the origin of Swedish populations, on the part played by rivers in the genetic differentiation and on the relative differentiation at isozyme and quantitative trait loci. Although significant, the overall population differentiation was low, the FST value being around 4%. Much higher FST values were observed between subpopulations from southern (Skåne) and central Sweden. This strong population differentiation, accompanied by significant linkage disequilibria, suggests the recent and diverse origin of Swedish populations. Degrees of differentiation between and within Polish river systems were of the same magnitude, indicating the presence of gene flow between river systems. Flow-regulated waterways, associated with higher human disturbance, may well explain why populations along rivers of the western part of the study area exhibited significant differentiation patterns while no differentiation could be detected along the less disturbed riparian habitats of eastern Poland. Finally, higher FST values were obtained for quantitative trait loci than for isozyme loci but, with two notable exceptions, their 95 % confidence intervals overlapped.

Mapping quantitative trait loci (QTLs) for binary traits in backcross and F2 populations was investigated using stochastic stimulation. Data were analysed using either linear regression or a generalized linear model. Parameters which were varied in the simulations were the population size (200 and 500), heritability in the backcross or F2 population (0·01, 0·05, 0·10), marker spacing (10 and 20 cM) and the incidence of the trait (0·50, 0·25, 0·10). The methods gave very similar results in terms of estimates of the QTL location and QTL effects and power of QTL detection, and it was concluded that in practice treating the zero-one data as continuous and using standard linear regression was efficient.

A model of pleiotropy with N diallelic loci contributing additively to N quantitative traits and stabilizing selection acting on each of the traits is considered. Every locus has a major contribution to one trait and a minor contribution to the rest of them, while every trait is controlled by one major locus and N−1 minor loci. It is demonstrated that a stable equilibrium with the allelic frequency equal to 0·5 in all N loci can be maintained in such a model for a wide range of parameters. Such a ‘totally polymorphic’ equilibrium is maintained for practically any strength of selection and any recombination, if the relative contribution by a minor locus to a trait is less than 20 % of the contribution by a major locus. The dynamic behaviour of the model is shown to be quite complex with a possibility under sufficiently strong selection of multiple stable equilibria and positive linkage disequilibria between loci. It is also suggested that pleiotropy among loci controlling traits experiencing direct selection can be responsible for apparent selection on neutral traits also controlled by these loci.