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Condensation of mouse C-heterochromatin is inhibited in cells of established mouse lines if treated with 33258 Hoechst. The fact that after such treatment these regions appear in mitotic chromosomes as longthreaded segments enabled us to localize, with the silver staining technique, the NORs outside the C-heterochromatin. The number of NORs per cell and their identification was determined.

The wood lemming is unique in the following respects: (1) the sex ratio is unequal, the frequency of males is 0.20–0.30; (2) two types of females occur, type MF producing progeny of both sexes, type F producing daughters only; and (3) two sex chromosome types of females exist, XX and XY. The hypothesis presented accounts for these and other known facts concerning sex ratio, reproduction biology and chromosome constitution. It is suggested that an X-linked mutant gene affects the male-determining action of the Y, thus converting some XY animals into females. These females normally produce one kind of eggs only; by a mechanism of selective non-disjunction in the foetal ovary only X-carrying eggs are formed. Thus, XY females are of type F, XX of type MF.

In the wood lemming (Myopus schisticolor) three genetic types of sex chromosome constitution in females are postulated: XX, X*X and X*Y (X*=X with a mutation inactivating the male determining effect of the Y chromosome). Males are all XY. It is shown in the present paper that the two types of X chromosomes, X and X*, exhibit differences in the G-band patterns of their short arms. In addition, it was demonstrated in unbanded chromosomes that the short arm in X* is shorter than in X. The origin of these differences is still obscure; but they allow to identify and to distinguish the individual types of sex chromosome constitution, as of XX versus X*X females and of X*Y females versus XY males, on the basis of G-banded chromosome preparations from somatic cells.

Condensation of human chromosomes in phase G2 and early mitosis is inhibited by the fluorochrome 33258-Hoechst. This inhibitory effect is most apparent in primary diploid fibroblasts and lymphoblasts and least pronounced in peripheral blood lymphocytes. Condensation of the human Y chromosome, which contains a large heterochromatic region rich in A-T base pairs, is drastically inhibited by 33258-Hoechst treatment of fibroblasts and lymphoblasts. The difference in sensitivity of human chromosomes in different cell types to 33258-Hoechst probably reflects differences in the cell-membrane permeabilities to 33258-Hoechst.

Ein gleichmasig reproduzierbarer Wachstums- und Differenzierungs-abschnitt bei der Entwicklung der Dunenfeder des Huhnchenembryos in vitro wird dazu verwendet, die Wirkung des Adrenalind und des Colchicins auf das Wachstum eines organoiden Systems zu untersuchen und zu vergleichen.

A new fluorochrome that preferentially binds itself to the centromeric or the constitutive heterochromatin is described. This stain allows an easy assay, through fluorescence, of the “repetitive DNA” or bands, supposedly composed of constitutive heterochromatin, in insectivores, rodents and man, without following the in situ hybridization of Pardue & Gall (1970) or the DNA denaturation-renaturation processes of Arrighi & Hsu (1971). The staining patterns with this derivative of a Benzimidazole compound (“Hoechst 33258”) are induced in the chromosomes without incubation or pretreatment with SSC and are identical to those produced by other techniques. This stain may eventually contribute to elucidating the hitherto unknown molecular mechanisms involved in the relationship between the repetitious DNA sequences and the banding patterns, and to interpreting the mechanisms responsible for the chromosomal rearrangements and aberrations involving the peri-and non-pericentric regions of the chromosomes.

Autosomal trisomy in the mouse is invariably associated with fetal or early postnatal death. Hemopoietic stem cells from fetuses trisomic for chromosome 12 or 19 can be rescued by transplantation into lethally irradiated mice. These trisomic cells restore hemopoiesis, including lymphopoiesis, in the irradiated mice and establish a permanent and almost complete engraftment. There is no evidence that hemopoietic cells with trisomy 12 or 19 are cytogenetically unstable.

Mouse aggregation chimaeras consisting of trisomy 12 and normal euploid cells were produced. The analysis of one trisomy 12↔euploid chimaera, using biochemical and cytological markers, showed that the trisomic cells were able to participate in the formation of most tissues including the ovary. On the other hand, no trisomy 12 cells were found in lymphocyte populations, which is most likely due to early selection in this particular cell lineage. The viability of two adult trisomy 12 chimaeras demonstrates that trisomy 12 cells are able to develop beyond the fetal stage which is not observed in completely trisomic fetuses.

The fluorochrome Hoechst 33258 which binds preferentially to A-T base pairs, drastically inhibits the condensation of A-T-rich centromeric heterochromatin regions in mouse cell lines. The condensation of all other regions of these chromosomes is also inhibited to some extent. The human Y chromosome contains a large heterochromatic region, which is also rich in A-T base pairs. This chromosome is not affected by Hoechst 33258 in human leukocyte cell cultures. On the other hand, condensation of the multiple copies of human Y chromosome in the mouse-human cell hybrid RH-28Y-23 is inhibited and the chromosomes appear distorted in Hoechst 33258-treated cells.

A 12-year-old boy with primary reticulum cell sarcoma of the testis who is alive and free of tumor 32 months after orchiectomy, radiation therapy to inguinal, iliac and retroperitoneal lymph nodes and chemotherapy is reported. Seven previously documented cases of childhood primary testicular lymphoma are reviewed. The patient with reticulum cell and one with lymphosarcoma were alive and tumor-free 4 and 18 months respectively. The other 5 with primary testicular lymphosarcoma died. Four survived for periods up to 1 year, 3 developing generalized lymphosarcoma and 1 leukemia. The fifth was tumor-free for 4 years and developed leukemia 2 weeks before death. The small number of cases precludes evaluation of survival in relation to various modes of therapy. Primary lymphoma of testis, no evidence of lymphoma elsewhere at time of orchiectomy, does exist. Its relatively benign biologic behavior in some remains an enigma.

SUMMARYGenetic divergences between the wild mouse populations with various Robertsonian translocations from the Poschiavo Valley, Yugoslavia, Milan and the Apenninies, were estimated based on the mitochondrial (mt) DNAs. The mtDNAs isolated from the liver were analysed by agarose slab-gel electrophoresis after digestion with eight kinds of restriction endonucleases: BamHI, EcoRI, HindII, HindIII, PstI, HpaI, HpaII and BgII. These preparations were further used to make restriction maps, from which sequence divergence between each Rb variation was calculated to be 0·2–2·2%. These rather larger values appear to be in conflict with the present concept that the Rb variations occurred during the last several thousand years. Both, however, might be reconciled by assuming genetic introgression of the founder with a small number of Rb translocations into other subspecies populations genetically remote and the subsequent rapid accumulation of Rb translocations unique to each population due to an unknown mechanism occurring specifically in the intersubspecies hybrids betweenM. m. domesticusand the otherM. m. subspecies. This was the case also in a new Rb (9.15) translocation obtained from Ogasawara Islands in Japan which was the intersubspecies hybrid betweenM. m. molossinusandM. m. domesticus.

THE wood lemming, Myopus schisticolor Liljeborg, is distinguished by an aberrant sex ratio, with a considerable excess of females, and by the fact that some females produce only daughters1,2. Fredga and his associates3 have provided a basis for understanding both these characteristics. They observed that 82 out of 181 female wood lemmings studied (45%) had an XY sex chromosome constitution, with chromosomal G-banding and C-banding patterns indistinguishable from those of XY males. On the other hand, the XY females were anatomically normal and indistinguishable from XX females. Furthermore, meiotic studies3 showed that the germ line in the somatically XY females was XX. The second X chromosome in the germ cells must have arisen by non-disjunction from the single X present in the XY cells. Thus all progeny from such females must have received copies of the same X chromosome. How does one reconcile these observations with the apparent function of the Y chromosome in mammalian sex determination4, or with more recent evidence that a particular Y-linked gene which controls presence of H–Y antigen is critical for differentiation of the male gonad? We have approached these questions serologically by typing male and female wood lemmings for expression of H–Y antigen, and we have found that all female wood lemmings are H–Y−.

Hypoplasia and retardation of the fetus are prominent developmental features in trisomies of the mouse. They are caused by disturbances and depression of growth in vivo. In a study of growth parameters of two trisomic mice. Ts12 and Ts19, cell cultures were initiated from Ts12, Ts19 and normal embryos at late developmental stages. Kinetics of growth of these cells under different conditions show that growth is not significantly affected in either of the trisomies. These results indicate that the trisomy-dependent growth impairment in vivo is not expressed under the experimental growth conditions used in vitro. It remains possible, though, that other autosomal trisomies of the mouse may show a different behaviour in this respect. If so, this may be considered specific to the tested trisomic chromosome and not as a general phenomenon of any trisomy.

THE wood lemming (Myopus schisticolor Lilljeborg), a small rodent inhabiting mossy forests of northern coniferous areas of Eurasia, is unique with respect to sex ratio, showing a definite prevalence of females, and with respect to the occurrence of two types of female, one producing daughters only, the other producing progeny of both sexes1,2. We thought that these phenomena might be associated with chromosome constitution, and as the chromosomes of this species were poorly known, we have made a detailed study of its karyotype. Some additional observations, pertinent both to the occurrence of chromosome variation and to reproduction, will be reported elsewhere. We report here two remarkable findings: (1) the existence of two types of fertile female with different somatic sex chromosome constitutions—the orthodox female complement, XX, and the male complement, XY, and (2) evidence for selective non-disjunction in XY females, leading to the formation of X-type egg cells only.