The ecological significance of resting sites and the seasonal habitat change in polecats (Mustela putorius)

The daytime resting sites of 13 radio-tracked polecats ( Mustela putorius ) were recorded in a mountainous and a lowland study area in Switzerland. Information on breeding sites was collected from questionnaires distributed throughout the country. At all resting sites, the polecats were completely invisible from more than a metre distant. During summer, when polecats mainly live in forests, individuals used many different hiding-places (e.g. small self-dug burrows, woodpiles, heaps of branches and dry leaves, dense vegetation). Such places were used for short periods, and then abandoned. Above ground, the polecats sometimes built nests of dry grass or moss. In winter, the polecats slept mainly inside barns, stables and other buildings. These resting sites were changed less frequently. In rainy weather throughout the year, subterranean places were preferred. Breeding polecats were often found inside houses, and obviously did not avoid human presence.
The importance of different types of resting sites for polecats is discussed. In summer, the quality of a resting place is of less importance than its distance from the foraging area, but in winter, warm resting places are essential and buildings are therefore considered an important resource for polecats in Switzerland. The seasonal habitat change can more readily be explained as a consequence of thermoregulatory problems than of food availability, and the distribution of polecats in Switzerland may be affected by the availability of suitable winter resting places. The northern limit of polecat distribution and its historical changes can be explained by the changing availability of human buildings which provide winter resting sites.     

Photosensitivity of Chromatophores

Almost all major phyla of invertebrates and lower vertebratesdisplay a direct sensitivity of their chromatophores to lightby either dispersing or—in rare cases—aggregatingthe pigment granules within the cell. This "primary response"of color change is an accessory component of the "dermal lightsense" and characterizes the chromatophores as an independentreceptor for light and effector of the chromomotor response.Photosensitivity does not seem to be restricted to certain partsof the pigment cell but is rather supposed to be a ubiquitousproperty of the chromatophore. Experiments on partially illuminatedchromatophores show that the photopigment, whose chemical compositionis still unknown, is localised within the plasma membrane orthe cytoplasmic ground substance. Threshold responses for ajust visible reaction are much higher than for background responsesand have for some pigment cells been determined to be in a rangebetween 0.2 and 0.5 erg/sec/cm². The physiological significanceof the photosensory reaction is closely related with thermoregulationand the protection of underlying tissue against harmful radiation.The chain of events involved in photosensory transduction remainsto be further studied and can at present be interpreted onlyon the basis of related phenomena like retinal pigment migrationand the light-sensitivity of simple non-pigmented contractilesystems.     

Filamentous Proteins from Plant Sieve Tubes

PROTEIN filaments are characteristic structural components of the assimilatory conducting elements of angiosperm plants (“P protein” of Cronshaw and Esau¹). We have isolated filamentous structures from the phloem exudate of cut cucurbit stems². The presence of the filaments could be clearly demonstrated after negative staining with the electron microscope.     

Molecular Basis for Repressor Activity of Qβ Replicase

WITH the purification and characterization of viral replicases, a novel feature of nucleic acid polymerases—stringent template specificity—was recognized^1,2. Qβ replicase, the most extensively studied viral RNA polymerase^2–8, is now known to replicate Qβ RNA², the complementary Qβ minus strand⁹, RNA molecules described as “variants” of Qβ RNA^10,11 and a set of small RNAs of unknown origin which accumulate in Qβ-infected Escherichia coli, collectively designated as “6S RNA”¹². On the other hand, the RNA from phages related distantly, if at all, to Qβ^13,14, such as MS2 or R17 and of other viruses such as TMV² or AMV (Diggelmann and Weissmann, unpublished results) are completely inert as templates, as are ribosomal and tRNA from E. coli². Poly C and C-rich synthetic copolymers at high concentrations elicit synthesis which, however, remains restricted to the formation of a strand complementary to the template^15,16.     

Ribosomal RNA Turnover in Contact Inhibited Cells

CONTACT inhibition of animal cell growth is accompanied by a decreased rate of incorporation of nucleosides into RNA^1–3. Contact inhibited cells, however, transport exogenously-supplied nucleosides more slowly than do rapidly growing cells^4,5, suggesting that the rate of incorporation of isotopically labelled precursors into total cellular RNA may be a poor measure of the absolute rate of RNA synthesis by these cells. Recently, Emerson⁶ determined the actual rates of synthesis of ribosomal RNA (rRNA) and of the rapidly labelled heterogeneous species (HnRNA) by labelling with ³H-adenosine and measuring both the specific activity of the ATP pool and the rate of incorporation of isotope into the various RNA species. He concluded that contact inhibited cells synthesize ribosomal precursor RNA two to four times more slowly than do rapidly growing cells, but that there is little if any reduction in the instantaneous rate of synthesis of HnRNA by the non-growing cells. We have independently reached the same conclusion from simultaneous measurements on the specific radioactivity of the UTP pool and the rate of ³H-uridine incorporation into RNAs (unpublished work of Edlin and myself). However, although synthesis of the 45S precursor to ribosomal RNA is reduced two to four times in contact inhibited cells, the rate of cell multiplication and the rate of rRNA accumulation are reduced ten times. This suggests either “wastage”⁷ of newly synthesized 45S rRNA precursor, or turnover of ribosomes in contact inhibited cells Two lines of evidence suggest that “wastage” of 45S RNA does not play a significant role in this system. (1) The rate of synthesis of 45S RNA in both growing and contact inhibited cells agrees well with that expected from the observed rates of synthesis of 28S and 18S RNAs (unpublished work of Edlin and myself). Emerson has made similar calculations⁶. (2) 45S RNA labelled with a 20 min pulse of ³H-uridine is converted in the presence of actinomycin D to 28S and 18S RNAs with the same efficiency (approximately 50%) in both growing and contact inhibited cells. These results indicate that, in order to maintain a balanced complement of ribosomal RNAs, contact inhibited cells must turn over their ribosomes. We present evidence here that rRNA is stable in rapidly growing chick cells, but begins to turn over with a half-life of approximately 35–45 h as cells approach confluence and become contact inhibited.     

Chromosomes tell half of the story: the correlation between karyotype rearrangements and genetic diversity in sedges,a group with holocentric chromosomes

Chromosome rearrangements may affect the rate and patterns of gene flow within species, through reduced fitness of structural heterozygotes or by reducing recombination rates in rearranged areas of the genome. While the effects of chromosome rearrangements on gene flow have been studied in a wide range of organisms with monocentric chromosomes, the effects of rearrangements in holocentric chromosomes—chromosomes in which centromeric activity is distributed along the length of the chromosome—have not. We collected chromosome number and molecular genetic data in Carex scoparia, an eastern North American plant species with holocentric chromosomes and highly variable karyotype (2n = 56–70). There are no deep genetic breaks within C. scoparia that would suggest cryptic species differentiation. However, genetic distance between individuals is positively correlated with chromosome number difference and geographic distance. A positive correlation is also found between chromosome number and genetic distance in the western North American C. pachystachya (2n = 74–81). These findings suggest that geographic distance and the number of karyotype rearrangements separating populations affect the rate of gene flow between those populations. This is the first study to quantify the effects of holocentric chromosome rearrangements on the partitioning of intraspecific genetic variance.     

The Complete Amino Acid Sequence of Actins from Bovine Aorta, Bovine Heart, Bovine Fast Skeletal Muscle, and Rabbit, Slow Skeletal Muscle

Complete amino acid sequences for four mammalian muscle actins are reported: bovine skeletal muscle actin, bovine cardiac actin, the major component of bovine aorta actin, and rabbit slow skeletal muscle actin. The number of different actins in a higher mammal for which full amino acid sequences are now available is therefore increased from two to five. Screening of different smooth muscle tissues revealed in addition to the aorta type actin a second smooth muscle actin, which appears very similar if not identical to chicken gizzard actin. Since the sequence of chicken gizzard actin is known, six different actins are presently characterized in a higher mammal.
The two smooth muscle actins—bovine aorta actin and chicken gizzard actin—differ by only three amino acid substitutions, all located in the amino-terminal end. In the rest of their sequences both smooth muscle actins share the same four amino acid substitutions, which distinguish them from skeletal muscle actin. Cardiac muscle actin differs from skeletal muscle actin by only four amino acid exchanges. No amino acid substitutions were found when actins from rabbit fast and slow skeletal muscle were compared.
In addition we summarize the amino acid substitution patterns of the six different mammalian actins and discuss their tissue specificity. The results show a very close relationship between the four muscle actins in comparison to the nonmuscle actins. The amino substitution patterns indicate that skeletal muscle actin is the highest differentiated actin form, whereas smooth muscle actins show a noticeably closer relation to nonmuscle actins. By these criteria cardiac muscle actin lies between skeletal muscle actin and smooth muscle actins.