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Despite marked improvements in the interpretation of systematic relationships within Eutheria, particular nodes, including Paenungulata (Hyracoidea, Sirenia and Proboscidea), remain ambiguous. The combination of a rapid radiation, a deep divergence and an extensive morphological diversification has resulted in a limited phylogenetic signal confounding resolution within this clade both at the morphological and nucleotide levels. Cross-species chromosome painting was used to delineate regions of homology between Loxodonta africana (2 n =56), Procavia capensis (2 n =54), Trichechus manatus latirostris (2 n =48) and an outgroup taxon, the aardvark ( Orycteropus afer , 2 n =20). Changes specific to each lineage were identified and although the presence of a minimum of 11 synapomorphies confirmed the monophyly of Paenungulata, no change characterizing intrapaenungulate relationships was evident. The reconstruction of an ancestral paenungulate karyotype and the estimation of rates of chromosomal evolution indicate a reduced rate of genomic repatterning following the paenungulate radiation. In comparison to data available for other mammalian taxa, the paenungulate rate of chromosomal evolution is slow to moderate. As a consequence, the absence of a chromosomal character uniting two paenungulates (at the level of resolution characterized in this study) may be due to a reduced rate of chromosomal change relative to the length of time separating successive divergence events.

Comparative cytogenetics and fluorescence in situ hybridization (FISH) were used to track structural rearrangements in the X chromosomes of two African antelope species, the eland (Taurotragus oryx; tribe Tragelaphini) and the suni (Neotragus moschatus; tribe Neotragini). Using two microdissected cattle chromosome painting probes (one specific for Xp-containing sequences corresponding to Xp24→p12 of the cattle X, and one specific for Xq-containing sequences corresponding to Xq12→qter), we show that intrachromosomal rearrangements distinguish the X chromosomes of these species. Furthermore, there is clear evidence that, superimposed on this background of intrachromosomal evolutionary change, the sequences contained in the cattle Xp painting probe appear to have moved as a complete unit during the repatteming of the bovid X. Although we are unable to infer the order of the rearranged chromosomal segments, given the large regions of homology detected by the chromosome paints, we nonetheless believe that this approach (combining conventional and molecular cytogenetic studies on the X chromosome) will prove useful for inferring phylogenetic relationships in the family Bovidae, particularly as more probes become available.

Epididymal spermatozoa from 12 species of mammals were stained using silver nitrate and examined with the light microscope. Silver nitrate differentiates many of the gross morphological features of spermatozoa, including the acrosome, subacrosomal region, perforatorium, postacrosomal sheath, neck, dense outer fibers of the core of the midpiece, annulus, principal piece, and end piece. Silver-staining patterns of spermatozoa reveal both species-specific and strain-specific differences, particularly of the sperm head. The biochemical basis of silver staining may be due in part to the presence of sulfhydryl- and disulfide-rich proteins; however, it cannot be explained entirely by the presence of these moieties. The detail obtained using silver nitrate staining, coupled with the ease and rapidity of the procedure, should be useful to workers in many areas of biological and medical research.

We have used aqueous silver nitrate to stain preparations of formalin-fixed meiotic cells. In doing so we have “rediscovered” a number of spermatid organelles, such as nuclear rings, acrosomal cap rings, centriolar adjuncts, and developing annuli, apparently not seen in numerous other recent studies of silver-stained meiotic cells. The availability of these structures for light microscopy coupled with the ease of preparation of the samples should facilitate studies on spermatid development, cytochemistry, and differentiation.

Silver-stained pachytene cells of male cotton rats, Sigmodon julviventer, heterozygous for a naturally occurring Robertsonian fusion, were examined by light microscopy. Silver staining clearly revealed synaptonemal complexes of all autosomal bivalents and of the translocation trivalent, as well as X-Y conformations within the sex vesicle. The synaptonemal complex of the trivalent conforms closely in morphology to that observed by electron microscopy in hybrids of other species. In all cases, the proximal telomeric knobs of the two acrocentric elements were paired and projected in the same direction from the metacentric element. The geometry of trivalent pairing in pachytene may predispose to normal disjunction and, therefore, account for the high fertility observed in 5. julviventer heterozygotes.

Splenic lymphocytes from Sprague Dawley and Fischer 344 rats were exposed to two chemicals known to induce common fragile site expression in man: fluorodeoxyuridine (in conjunction with the enhancing effects of caffeine) and aphidicolin. Of 39 sites that were significantly damaged in excess, 12 meet the criteria for fragility proposed in this investigation. Rat fragile sites appear to differ from those in man in that no common hierarchical frequency of expression is evident from the two methods of induction. In addition, a comparison of published cancer-specific chromosome breakpoints from a variety of rat tumors reveals little or no apparent concordance with the identified fragile sites. The rat is an animal model in which multiple common fragile sites can be induced and, as such, will be valuable for testing hypotheses concerning the biological basis of chromosomal fragility.

Nineteen fragile sites induced by aphidicolin in lymphocyte cultures from the laboratory mouse are documented. These sites are compared with previously described fragile sites induced in mouse fibroblast systems, and then with those reported on chromosomes which have been evolutionarily conserved between the mouse and the laboratory rat. Of a total of 38 fragile sites thus far identified in mouse fibroblasts and lymphocytes, only 4 sites are common to the two cell types; 34 sites show no correspondence of loci. The reason for this discrepancy is unclear, but it is possible that these data may indicate some degree of tissue specificity of fragile site expression in the mouse. Eight autosomes in the mouse and rat retain straightforward and nearly complete banding homology. To test the hypothesis that fragile sites are conserved between the two species, we compared these eight autosomes with regard to number and distribution of fragile site loci. A total of 30 fragile sites is distributed over the conserved chromosomes. Only 4 (possibly 5) are common to both species; 18 are found in the rat but not the mouse, and 4 are found in the mouse but not the rat. Of the 4 shared sites, notable differences in frequency of expression exist. Our comparisons show that: (1) a small number of fragile sites is conserved; (2) a large number of fragile sites is not conserved, and (3) some sites which are conserved are quite different in the frequency at which they are expressed in the two species, indicating that the sites themselves may have undergone evolutionary change.(ABSTRACT TRUNCATED AT 250 WORDS)