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Common shrews display two types of Robertsonian (Rb) heterozygosity: simple (where CIII configurations are formed at meiosis I) and complex (which have longer meiotic chains or rings). Based on an analysis of large sample sizes (over 100) of MII cells per specimen, we estimated the non-disjunction frequency in seven Rb homozygotes and 21 complex Rb heterozygotes (CIV and CV) of Sorex araneus Linnaeus, 1758. The analysis showed high betweenindividual variability. The mean level of non-disjunction in homozygotes (2.01%) was significantly lower than in CIV and CV heterozygotes (4.27% and 5.78%, respectively). The study demonstrated that non-disjunction frequency in male CIV and CV heterozygotes was similar to that in simple heterozygotes in the common shrew.
Density and distribution of the bank vole Clethrionomys gtareolus (Schreber, 1780) was studied in 1986-1989 on two forest plots (one of 5.4 and one of 5.8 ha) that differed with respect to their mosaic character. On both plots densities of bank voles were similar in different habitats. The same trap sites were used by voles in a similar way in successive study years. Spatial variations in habitat use were primarily related to the density of plant cover and its spatial distribution providing shelter from predators. Frequency and intensity of use of trap sites by voles was positively corre­lated with the percent cover of shrubs and tall herbaceous plants. It was negatively correlated with the percent area without herb cover and with the percent cover of photophilous herbs. Using the correlation method, it was possible to estimate the effect of a given environmental factor on the distribution of bank voles only when the range of spatial variation of this factor was large.
The research was carried out on 100 water bodies, from 1998 to 2000. There were 87 water reservoirs located in the area of Białystok, another 13 reservoirs were located in the Białystok surroundings, but outside the administrative boundaries (up to 2.5 km away). Several physical features of the breeding ponds in our study as well as their surroundings were described to determine the urbanization pressure in a given area. The frequency of occurrence of particular amphibian species and their number were determined. In the study area, amphibians belonging to 12 species (with one genetic hybrid) were found. Due to problems identifying "green frogs", they were put into one group for most of analyses. The most frequent amphibians within the administrative boundaries of Białystok were: the "green frogs" (observed in 66.6% of studied water bodies), the common frog (65.5%) and the moor frog (49.4%). These species occurred throughout the entire city. The rarest amphibian species were observed only on the outskirts of Białystok. They were: the crested newt (2.3%) and the natterjack (3.4%). The green toad is considered to be a species less sensitive to urbanization pressure. Paradoxically, it was rarely found in Białystok (9.2%) and only 22.2% of its breeding sites were located in the city centre.
In Central Poland, two similar chromosome races of the common shrewSorex araneus Linnaeus, 1758 were earlier described: Drnholec race (arm combinationsgm, hi, ko, nr) and Stobnica race (gm, hi, ko, np). Great similarity in size and G-banding patterns between thenr andnp metacentrics leave open to doubt the actual existence of both races in Poland. The present study, which is based on good quality karyotypes of common shrews from 18 sites, showed the presence of thenr arm combination. There is therefore strong evidence that thenp arm combination was wrongly described and thus the Stobnica race should not be considered valid.
We studied populations of Sorex araneus Linnaeus, 1758 in the upper Drwęca River valley and in the vicinity of Lake Szeląg Wielki. We found that the cline of frequency of metacentric np decays in the Drwęca River valley, which results in a smooth transition between the Stobnica race (karyotype hi, ko, gm, np) and the Laska race (hi, ko, gm). However, on the west shore of Lake Szeląg Wielki the cline of frequency of metacentric gm disappears. Reduction of frequencies of autosomes np and gm makes possible broadening of the ciines of metacentrics gr and mn, originating from the Łęgucki Mtyn race. This results in a reduction of the number of hybrids and maximization of frequency of recombinants (hi, ko, gr, mn), which reaches the value of 1.0 in the populations located on the west shore of the lake. We compared studied populations with those in the Pasłęka River valley, and those between the Pasłęka and Drwęca Rivers valleys. We also discussed the maximization of the frequency of recombinants as a mechanism enhancing fertility of hybrid populations.
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