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.

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 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.

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.

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.

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.

Genet. Res., Camb. (1994), 63, pp. 87–99

Genotypically unbalanced diploid ↔ diploidfoetal mouse chimaeras:

possible relevance to human confined mosaicism

John D. West, and Jean H. Flockhart

The sentence starting in the middle of line 16 of the Summary should read

In 20/38 chimaeric conceptuses in the unbalanced