Conserved patterns of bending in satellite and nucleosome positioning DNA.

Intrinsic DNA curvature has previously been implicated in the condensation of satellite DNA in chromatin. In this article, electrophoretic methods and computer programs for predicting DNA structure from nucleotide sequence were used to determine if curvature is a conserved feature of satellite DNA. The results revealed that satellite sequences display wide variation in magnitude of intrinsic bending. Less than half of the satellites examined were strongly bent when compared with control DNA. However, a conserved pattern of bending was seen in all 57 satellite sequences that were studied. The pattern consisted of repeating units of two 50-60-base pair bending elements which were separated by a 20-30-base pair region of low curvature. This pattern resembles qualitatively the bending of DNA in the nucleosome where the helix is folded approximately two turns around the histone octomer with the turns interrupted by a less bent segment in the center of the particle. The distance between successive pairs of bending elements was also similar to the average spacing of nucleosomes in chromatin. Thus, the conserved structures could play some role in the positioning of nucleosomes along satellite chromatin. In order to strengthen the correlation between DNA structure and nucleosome positioning, sequences were examined which have been shown to position nucleosomes at single major sites. This analysis revealed a conserved pattern of DNA structure resembling the two peak units seen in satellites. In addition, the nucleotide sequence patterns responsible for the conserved patterns of bending were similar in both satellite and nucleosome positioning DNA. Likewise, nucleotide sequence patterns that are thought to direct the rotational orientation of DNA in the nucleosome were similar in both sequence sets. These results are considered in terms of a general model for the role of DNA bending and nucleotide sequence in the control of nucleosome positioning and chromatin condensation in eukaryotes.

Distinctive features of the gametes and reproductive tracts of the Asian musk shrew, Suncus murinus.

Insectivora are of special interest as the most primitive of the eutherian mammals, but essentially nothing is known of their gamete function. In this respect, the Asian musk shrew (Suncus murinus), investigated in the present study, displays many idiosyncrasies. In the epididymis, the giant acrosome undergoes further stabilization, its unusual resilience being especially evident in a "rim" created by a persistent close alignment of the outer acrosomal and overlying plasma membranes. However, until spermatozoa reached a gland on the vas deferens, no post-testicular change was demonstrable in the sperm head surface, the unusual nature of which was indicated by a dorso-ventral differentiation, by an inability to auto-agglutinate or to bind to the homologous zona pellucida, and by an insensitivity to anti-sperm immunoglobulin IgG in fresh serum. At mating, only about 1 x 10(6) spermatozoa are inseminated as far as the anterior vagina with plug formation. Within the small (6 mm) fallopian tube, the isthmus and ampulla are sharply delineated by their contractile activity and epithelial character; there is evidence of some sperm entry into isthmic crypts and a tendency for ipsilateral ovarian control of sperm transport to the tubal ampulla. The cumulus oophorus does not undergo preovulatory mucification and expansion, is characterized by persistent intercellular gap junctions, and is insensitive to hyaluronidase and trypsin. It is unclear how the compact cumulus is penetrated at fertilization. The giant acrosome contains acrosin and an unusually temperature-dependent cumulytic activity; it is intact in motile ampullary spermatozoa but appears to be discharged before reaching the zona pellucida. Since eggs were not penetrated in the presence of ampullary spermatozoa until 4-10 h after ovulation, Suncus spermatozoa spend a long period in the female before they can fertilize. The determinants of sperm function, including capacitation and the acrosome reaction (AR), may depend on a different set of controls in Suncus, perhaps as a legacy of the resilient giant acrosome. This possibility could be examined in other Crociduran and Soricine shrews selected according to acrosome size.

Retinal cysts in the eye of the Asian musk shrew, Suncus murinus, (Mammalia, Insectivora).

The development of retinal cysts in the eye primordia of Suncus murinus embryos was studied. Retinal infoldings were first identified in the 17.5-day-old embryo (crown rump length--CRL = 6.3 mm). At this time, the apex of the developing retinal fold could be identified attached to the lens. The pigmented epithelium did not take part in the formation of the cyst. The retinal fold closed, to form a cyst, by day 23 of embryonic development (CRL = 14.0 mm). The newly formed retinal cyst remained continuous with the retina and extended toward the lens where it attached to the lens epithelium. Retinal cysts had an oval, bilaterally symmetrical shape and a narrow lumen, which, lined by the internal limiting membrane, occasionally contained cellular debris. The dimensions of the cysts were usually 100 to 150 microns (diameter) by 200 to 250 microns (length). Retinal cysts appeared in 45 per cent of the embryos examined.

Biology of the scrotum. IV. Testis location and temperature sensitivity.

Whereas abdominal temperatures have no effect on spermatogenesis in testicondid mammals but inhibit the process completely in scrotal mammals, the inguinal testes of the naturally cryptorchid musk shrew and degu have been found to display an intermediate response. Twelve to 18 weeks after transposition to the abdomen (and so about 1.5 degrees C above its normal inguinal temperature) the weight of the testis was reduced by 27% (musk shrew) and 52% (degu). Nonetheless, spermatogenesis continued in both, though at a lower rate and with a greater proportion of abnormal spermatozoa formed. The ultrastructural anomalies evident in some spermatids of the degu testis transposed to the abdomen were reminiscent of those seen commonly in the germinal epithelium of "normal" men. Natural cryptorchids may thus be useful models in which to discover whether spermatogenesis at a temperature somewhat above the norm for the species compromises the ability of fertilizing spermatozoa to support normal development. The fact that deep-body temperature induces only a partial suppression of function in the inguinal testis of natural cryptorchids shows that there is no, absolute distinction between the scrotal and ascrotal states among mammals with respect to the temperature sensitivity of the testis. That visible sensitivity seems more likely to reflect the adaptation of the metabolically dynamic germinal epithelium to function optimally at the temperature of the location to which the testis migrates, rather than any fundamental incompatability between spermatogenesis and the temperature imposed by homeothermy. External migration of the testis itself may originally have helped the sperm storage region of the associated epididymis to project from and so attain a cooler environment than that beneath the body surface.

Sensitivity of European microtines to injected CdCl2.

Basic histologic and metabolic indices of damage to the testes by CdCl2 were observed in six species of rodents: Lagurus lagurus, Pitymys subterraneus, Microtus oeconomus, M. agrestis, Clethrionomys glareolus (and Mus musculus for comparison). All species showed discoloration of the subcapsular tissues and damage to the testicular parenchyma. Testes of all species receiving CdCl2 utilized from 43% to 76% less oxygen than did those from untreated individuals. Cadmium treatment preceded few significant temperature shifts. The voles observed experienced differential sensitivity to CdCl2 and should be quite suitable for further studies of the mechanism of action of CdCl2 as a testicular poison.

Ovarian hormone: lack of effect on reproductive structures of female Asian musk shrews.

Uterine and vaginal weights and histologies are not altered by ovariectomy or estrogen treatment in the Asian musk shrew (Suncus murinus). In addition, ovariectomized shrews mate. Thus, the role of ovarian hormones in the control of the reproductive status of this species does not conform to the accepted mammalian pattern.