The known limits of the distal imprinting region of mouse Chromosome (Chr) 2 are defined by the breakpoints of the translocations T(2;8)2Wa, (T2Wa), and T(2;16)28H, (T28H), in distal H3, and proximal H4 respectively. We have shown that T2Wa and T(2;4)1Go, (T1Go), which has a breakpoint in central H3 map close to a, non-agouti. Ada, adenosine deaminase, lies very near the proximal boundary and Ra, ragged, maps very close to the distal boundary, and is less than 0-2 cM from wasted, wst. From the current data Ada can be taken as the proximal, and Ra as the distal gene marker of the imprinting region on the linkage map. From consensus maps twenty three other markers, including fourteen genes, lie between Ada and Ra, some of which may be useful in investigations of imprinting. Of the markers included in the study reported here, four, Ada, ls, lethal spotting, Ra and wst lie or probably lie within the region but none display any evidence of imprinting. We suggest that recombination frequency is elevated in distal Chr 2, because in none of the crosses could the most closely linked marker be ordered in relation to the translocation breakpoint due to the high frequency of double crossovers.

Two series of mouse chimaeras were produced by aggregating pairs of eight-cell embryos that differed at the Gpi-1s locus, encoding glucose phosphate isomerase (GPI-1); the paired embryos were respectively homozygous Gpi-1sa/Gpi-1sa and Gpi-1sb/Gpi-1sb. Chimaeric blastocysts were transferred to pseudopregnant females, that were homozygous Gpi-1se/Gpi-1se and produced only GPI-1 C enzyme. Quantitative electrophoresis of GPI-1 was used to estimate the contribution of each embryo (GPI-1 A and GPI-1B enzyme activity) to the foetus, placenta and other extraembryonic tissues of 12½ day chimaericconceptuses. For both series of chimaeras, the distributions of %GPI-1A in different tissues were classified as (1) balanced and typical, (2) balanced but atypical or (3) unbalanced. One series of chimaeras was clearly unbalanced, so that the cells derived from the (C57BL × CBA/Ca)F2 embryo (Gpi-1sb/Gpi-1sb) predominated over those derived from the BALB/c inbred strain (Gpi-1sa/Gpi-1sa) in most foetuses. Two significant observations were made concerning this unbalanced series. Firstly, the mean composition of the placenta and other extraembryonic tissues was similar to that in the foetus i.e. also unbalanced with (C57BL × CBA/Ca)F2 (abbreviated to BF2) cells predominating. Secondly, despite this generalizeddeficiency of BALB/c cells, there were differences in the frequency of non-chimaeric tissues between different developmental lineages. In 20/34 chimaeric conceptuses in the unbalanced series only BF2 cells were detected in the foetus, whereas both BF2 and BALB/c cells were present in at least one of the extraembryonic tissues. This group of chimaeras, therefore, shows some similarities to human confined mosaicism. Although chimaerism occurred more often in the primitive endoderm (hypoblast) lineage (yolk sac endoderm and parietal endoderm) than in the placenta, this may also be the case in human mosaics. The mosaic status of the human yolk sac endoderm is usually unknown so it is possible that mosaicism often occurs in the yolk sac endoderm as well as the trophectoderm in human ‘confined placental mosaicism’. The uniformly unbalanced phenotype seen in the mouse chimaeras may be a result of generalized cell selection against BALB/c cells in all tissues. As an alternative explanation, we propose that most of the BALB/c cells in the blastocyst are allocated to the mural trophectoderm, which has a limited mitotic potential and so contributes little to the mid-gestation conceptus. Further work is required to test these possibilities.

DNA fingerprint is a pattern of a variable number of bands (DNA fragments) with different sizes on a Southern gel for each individual, generated by one or many VNTR loci. Genetic divergence between individuals within and between populations can be studied in terms of number of shared bands between individuals. Using a population genetic model we show that the expectations of measures of genetic distance between populations based on band sharing data from DNA fingerprint patterns are functions of composite parameters M = 4Nv, and time of divergence (t) between populations, where N is the effective size of the populations, and ν, the mutation rate. The expected genetic distance remains linear with time of divergence at least up to N generations as long as the average heterozygosity at the DNA fingerprint loci remains at or below 90%. Neither incomplete knowledge of the allele frequencies at each locus, nor the unknown number of loci underlying DNA fingerprint pattern, compromise these evolutionary dynamics of DNA fingerprint patterns. Applications of this theory to data on three human populations, and review of literature indicate that co-migration of alleles, and the presence of syntenic loci underlying the fingerprint pattern have little impact of the reliability of evolutionary conclusions from DNA fingerprint studies.

To investigate the cellular action of the Miωh allele in the mouse with regard to its effects upon coat color patterns, we generated a series of aggregation chimeras, using embryos that differ in their mi locus genotype. We have obtained 11 chimeras Miωh/ + C/C↔ + / + c/c and 8 chimeras + / + C/C↔ + / + c/c. Chimerism was determined by coat and retinal pigment epithelium mosaicism and by the electrophoretic analysis of GPI-1 isoenzymes. In Miωh/+ C/C↔, +/+ c/c mice white coat color prevailed due to the higher percentage of unpigmented areas and the higher percentage of unpigmented hairs in pigmented areas. Our data indicate that a single Miωh gene dose decreases the melanoblast proliferative activity, causing the lightening of coat pigmentation. In Miωh/ + C/C↔+/+ c/c mice a few pigmented hairs were often detected on the belly where Miωh / + mice always had a white spot. This suggests that in the chimeras the presence of some non-Miωh cells in the skin of the belly allows pigment cells to develop. Using embryos of two substrains of Miωh/Miωh mice that differ in their Gpi-1 locus genotype we have produced 8 Miωh/ + ↔ Miωh/Miωh chimeras. In all these chimeras coat color patterns resembled those of Miωh/ + heterozygotes despite the higher percentage of the Miwh/Miωh component in three chimeras. Mosaic hairs were absent in the chimeras. This shows that the chimeras have only one Miωh/ + melanoblast population which actively proliferates and colonizes almost all hair follicles. Thus the Miωh/Miωh dermis and epidermis do not suppress proliferation and differentiation of the Miωh/ + melanoblasts except the certain area on the belly.

The interpulse interval (IPI) of courtship song in the Drosophila auraria complex is the only parameter that is consistently species-specific among the several courtship elements examined within the complex. The genetic basis of the species-specific courtship song was examined by analysing the song of interspecific hybrids and of backcross progeny. IPI of all interspecific hybrids except two showed intermediate values, suggesting autosomal control of species-specific IPI. However, significant deviation for shorter IPI from midparent was found in thirteen out of 20 crosses. The chromosomal analysis between D. auraria and D. biauraria revealed that the two major autosomes had significantly large effects on IPI, but the sex chromosome and cytoplasm had no effect. Since no interaction was detected, it is concluded that each autosome acts additively in the determination of species-specific IPI. The common ancestors of the D. auraria complex may also have had autosomal control of IPI, which has been conserved during speciation in the complex.

According to the classical genetic analysis in Paramecium caudatum by Tsukii & Hiwatashi (1983), the E mating type of each syngen is expressed when the cell bears alleles specific for syngen at the Mt locus. The O mating type is expressed when cells are homozygous for the null allele, mt, at the Mt locus. In such mt/mt cells the O syngen specificity is determined by alleles at two other loci called MA and MB. Inthe study reported here, macronucleoplasmic transplantation technique was used to test the above hypothesis. When macronucleoplasm of type E3 (mating type E of syngen 3) was injected into a macronucleus of type O12 (mating type O of syngen 12), some recipients changed to type E of the donor syngen but some others changed to type E of the recipient syngen. Thus, syngen specificity of donor macronucleoplasm controlling type E was converted into that of the recipients, even though the latter has no gene that controls type E. When this transformant expressing type E of the recipiexnt syngen was re-injected back into E of the other syngen, the expression of the converted mating type in some way continued in the recipient. This suggests that syngen specificity of gene Mt of the donor was changed to that of the recipients by intersyngenic transplantation of macronucleoplasm. We also obtained results suggesting that the gene dosage ratio of Mt to mt or Mt to MA and MB may be important for syngen specific expression of type E.

The singedvery weak mutation was created by the sequential addition of two P transposable elements to the singed gene. The mutation can be somatically unstable through the action of a dominant maternal effect mutation on the second chromosome. It is also unstable in the germ line in these conditions. Sequencing of the region of the P insertions in the mutation reveals that the two inserted elements have single internal deletions, and the larger of the two is a copy of the KP element. The mutation will generate, at high frequencies, strongly singed and pseudo-wild type products by reversions occurred in the germline. These are the result of the precise excision of the smaller and the larger elements respectively. By PCR amplification of dissected thoraces we show that the somatic instability of the mutation, from a weak to a strong singed phenotype, is also caused by the excision of the smaller of the two elements.

The dominant gene Fused (Fu) produces skeletal abnormalities during embryonic development. It was previously shown that C57BL/6 mice contain a suppressor of Fu, which acts after fertilization. Chimaeras were used to study whether this gene would suppress the Fu phenotype after the 8-cell stage of embryo development. We found no effect of the suppressor gene on Fu phenotype (its degree and frequency of expression) in chimaeric mice. We conclude that either the suppressor gene from C57BL/6 mice can only influence Fu expression at the intracellular level or Fu expression is determined before the 8-cell embryonic stage.

The evolution of sweet chestnut (Castanea sativa Mill.) appears to be a complex mixture of long-range gene flow, natural and artificial selection, and local effects of isolation by distance. In this paper we present the most complete analysis to date on the genetic structure and variability of 52 populations of chestnut spanning the entire European area of distribution. The study is based on the use of isozyme data. Our samples came from four major zones, possibly representing relevant steps in the evolution and spread of sweet chestnut in Europe: (i) eastern Turkey, the supposed center of origin of the species; (ii) western Turkey, the area in which human domestication started; (iii) Italy, where domesticated chestnut was first introduced to the rest of Europe by the Romans; and (iv) France, representing the latest phases of the expansion, close to the northern limit of the taxon.

As previous studies based only on Italian and some Turkish populations suggested, the electrophoretic data are consistent with a series of episodes of west- and north-ward migration. The early expansion from the center of origin was probably slow, resulting from natural diffusion of the species. Most of the original genetic variation has been conserved during this phase. Successive episodes of colonization of western Turkey and then of the rest of Europe were probably the more rapid result of human activity. These later stages were associated with genetic drift that reduced the overall heterozygosity of the extant populations. No evidence for selection could be found at the large geographical scale of this study, although previous regional works have shown spatial patterns of allelic frequencies at a few loci and phenotypic differentiation consistent with the action of past selective pressures.

Patterns of P element establishment and evolution were compared in populations of D. melanogaster and D. simulans. For each species, mixed populations were initiated with M strain flies lacking P elements together with P strain flies having similar P element copy numbers and phenotypes. The mixed populations were subsequently maintained under similar environmental conditions. On the basis of gonadal sterility assays, P elements tended to be significantly more active in D. melanogaster than in D. simulans populations. This activity difference between the two species was positively associated with P element copy number, determined by restriction enzyme analysis, and transposition frequency, as determined by a transposition assay. Host factors are the most likely explanation for the observed species variation. Difficulty of establishment may be a factor determining the absence of P elements in natural populations of D. simulans.

We looked for polymorphism of the prothoracicotropic hormone gene locus (Ptth) among inbred strains of the silkmoth, Bombyx mori, by in vitro DNA amplification (polymerase chain reaction), and found three alleles, PtthA, PtthB and PtthC. The PtthA allele contained a third intron consisting of 680 bp and a fourth intron of 350 bp. PtthB contained the same size third intron but a longer fourth intron of 490 bp, while Ptthc had a longer third intron of 1080 bp and a shorter fourth intron of 350 bp. In 29 strains which we examined, 9 strains had PtthA, 8 strains had PtthB, and 2 strains had Ptthc. The other 10 strains had heterogeneous genotypes with the same 3 alleles.

In many reptiles, sex determination is temperature-sensitive. This phenomenon has been shown to take place in the laboratory as well as in nature, but its effect on natural populations remains questionable. In the turtle Emys orbicularis, the effects of temperature override a weak mechanism of genetic sex determination which is revealed in incubation at pivotal temperature. At this temperature, the sexual phenotype is concordant with the expression of the serologically defined H-Y antigen (H-Ys) in non-gonadal tissues; males are H-Ys negative (H-Y−) whereas females are H-Ys positive (H-Y+). To estimate the importance of sexual inversion (sexual phenotype and H-Ys expression discordant) in populations of Brenne (France), the frequencies of male and female sexual phenotypes among H-Ys phenotypes were determined. The frequencies of sex reversed individuals are low, only 6 % of phenotypic females being H-Y− and 11 % of phenotypic males being H-Y+. According to these data, two theoretical models have been constructed to estimate the contribution to sex determination of individuals in relation to their genotype. The first model excludes any influence of incubation temperature and sexual phenotype on the fitness of individuals. The second one considers that these parameters influence fitness because this model has been previously shown to favour environmental sex determination. In both models, it appears that sex determination can be viewed as genotypic and monogenic with some individuals sexually inverted by theaction of temperature. One category of homozygous animals differentiates mainly into one sex, and the heterozygous animals differentiate mainly into the other sex. The second category of homozygotes has a low frequency in the populations and can differentiate as male or female without high constraint. Then it is estimated that in Brenne approximately 83% of the eggs are incubated in conditions allowing the genetic component to influence sex determination.

The insertion site polymorphism of the copia, mdg1, mdg3, gypsy, and P transposable elements was analysed by in situ hybridization to the polytene chromosomes in genomes of males from a natural population of Drosophila melanogaster. Parameters of various theoretical models of the population biology of transposable elements were estimated from our data, and different hypotheses explaining TE copy number containment were tested. The copia, mdg1 and gypsy elements show evidence for a deficiency of insertions on the X chromosomes, a result consistent with selection against the mutational effects of insertions. On the contrary, mdg3 and P copy numbers fit a neutral model with a balance between regulated transposition and excisions. There is no strong evidence of a systematic accumulation of elements in the distal and proximal regions of the chromosomes where crossing over and ectopic exchanges are reduced. For all chromosome arms but 3L, however, the TE site density increases from the proximal to the distal parts of the chromosomes (the centromeric regions were excluded in this analysis) with sometimes a sharp decrease in density at the extreme tip, following in part the exchange coefficient. The way the copy number of TEs is contained in genomes depends thus on the element considered, and on various forces acting simultaneously, indicating that models of TE dynamics should include details of each element.

Analysis of the transposition behaviour of the P and hobo elements borne by the 31·1/CyL4 MRF (P), 23·5Δ/CyL4 MRF (hobo) and 23·5*/Cy MRF (hobo) strains in the progeny of dysgenic crosses with two ME laboratory strains (Berlin-k and dp b en bw) at 25 °C revealed that: (a) the two ME laboratory strains affect differently the transposition rates of P and hobo elements. More precisely, P element transposition is higher in heterozygotes with dp b en bw than in those with Berlin-k. In contrast, the transposition rate of hobo elements is higher in Berlin-k than in dp b cn bw heterozygotes. (b) Like P, hobo has the potential to transpose at high frequencies and to nonhomologous chromosomes, (c) The dysgenically inactive hobo elements of the 3·11 MRF strain transpose more frequently than the dysgenically active hobo elements of the 23·5 MRF strains in certain crosses, (d) There are insertion hot spots for P and hobo elements. For the P elements there are enough data to suggest that the insertion hot spots are different in the two EM strains. The data are discussed on the basis of the involvement of putative host factors in transposition regulation of the P and hobo elements.

A computer model is developed that simulates Marker Assisted Selection (MAS) in a population produced by a cross between two inbred lines. Selection is based on an index that incorporates both phenotypic and molecular information. Molecular markers contributing to the index and their relative weights are determined by multiple regression of individual phenotype on the markers. The model is applied to investigate the efficiency of MAS as affected by several factors including total number of markers in the genome, number of markers contributing to the index, population size and heritability of the character. It is demonstrated that selection based on genetic markers can effectively utilize the linkage disequilibrium between genetic markers and QTLs created by crossing inbred lines. Selection is more efficient if markers contributing to the index are re-evaluated each generation than if they are evaluated only once. Increasing the total number of markers in the genome as well as the number of markers contributing to the index does not necessarily result in a higher efficiency of selection. Moreover, too many markers may result in a weaker response to selection. Population size is shown to be the most important factor affecting the efficiency of MAS.

Maximum likelihood estimation methods with an individual animal model were used to analyse a bi-directional selection experiment, with control, for cannon bone length in Scottish Blackface sheep. A method is described for partitioning the likelihood to allow within- and between-line estimates of genetic variance. It is concluded that both sources of information made substantial contributions to the precision of the base population heritability estimate. The implications for different experimental designs and varying heritability are discussed.

This paper analyses the effects of selection against deleterious alleles maintained by mutation (‘ background selection’) on rates of evolution and levels of genetic diversity at weakly selected, completely linked, loci. General formulae are derived for the expected rates of gene substitution and genetic diversity, relative to the neutral case, as a function of selection and dominance coefficients at the loci in question, and of the frequency of gametes that are free of deleterious mutations with respect to the loci responsible for background selection. As in the neutral case, most effects of background selection can be predicted by considering the effective size of the population to be multiplied by the frequency of mutation-free gametes. Levels of genetic diversity can be sharply reduced by background selection, with the result that values for sites under selection approach those for neutral variants subject to the same regime of background selection. Rates of fixation of slightly deleterious mutations are increased by background selection, and rates of fixation of advantageous mutations are reduced. The properties of sex-linked and autosomal asexual and self-fertilizing populations are considered. The implications of these results for the interpretation of studies of molecular evolution and variation are discussed.

Rates of transposition and excision of the Drosophila melanogaster retrotransposon elements mdg3, 297, Doc, roo and copia were estimated directly, by in situ hybridization analysis of their cytological insertion sites in 31 replicates of a highly inbred line that had accumulated spontaneous mutations for approximately 160generations. Estimated transposition rates of Doc, roo and copia were, respectively, 4·2 × 10−5, 3·1 × 10−3 and 1·3 − 10−3; no transpositions of 297 nor mdg3 were observed. Rates of transposition of copia varied significantly among sublines. Excisions were only observed for roo elements, at a rate of 9·0 × 10−6 per element per generation. Copy number averaged over these element families increased 5·9 %; therefore, in these lines the magnitude of the forces opposing transposable element multiplication were weaker than transposition rates.