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1.
Dr. Karl Fritsch 《Plant Systematics and Evolution》1898,48(2):47-49
Ohne Zusammenfassung 相似文献
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James R. Anderson Catherine Fritsch Bernadette Favre 《Primates; journal of primatology》1990,31(4):611-615
An adult femaleLemur catta and an adult femaleEulemur fulvus were given edible rewards for scratching. Both subjects learned to scratch in order to obtain the rewards, showed diminished
rates of scratching during periods of extinction, and learned to scratch preferentially with one foot when required. TheLemur catta subject was more responsive to the changing experimental conditions than theEulemur fulvus. The conditionability of scratching in primates does not appear to be directly related to the widespread occurrence of scratching
in simian social contexts. 相似文献
5.
Peter Fritsch 《American journal of botany》1996,83(3):342-355
Various hypotheses have been put forward to explain the presence of sclerophyllous plant disjuncts between western North America and the Mediterranean region. The Madrean–Tethyan hypothesis postulates that the two regions were floristically connected in the Early to Middle Tertiary by way of a low-latitude migration route. Others deny the possibility of such a route, and instead postulate convergence to xerophytic conditions from more widespread mesophytic ancestors, or suggest long-distance dispersal scenarios. One example of a “Madrean–Tethyan link” between the two regions is composed of four species within the genus Styrax: S. officinalis subsp. officinalis from the Mediterranean region, S. officinalis subsp. redivivus and subsp. fulvescens from California, and three closely related species in Texas and northeastern Mexico (S. texanus, S. platanifolius, and S. youngiae). This group was examined with isozymes to assess whether patterns of genetic variation are consistent with those predicted by the Madrean–Tethyan hypothesis. Ten populations from California, six from the Mediterranean region, and three from Texas were sampled. Pairwise comparisons revealed mean genetic identity (I) estimates of 0.581 between Mediterranean and California populations, 0.470 between Mediterranean and Texas populations, and 0.640 between California and Texas populations. Two populations of a species thought by many to be the closest relative of S. officinalis on morphological grounds (S. jaliscanus) exhibited low I (0.299–0.321) relative to all other group comparisons. Intercontinentally disjunct populations of S. officinalis possessed an I value that warrants species status for the Californian and Mediterranean groups. Divergence time estimates between Madrean and Tethyan Styrax range from 5.0 to 13.8 Mya, too recent to be consistent with the Madrean–Tethyan hypothesis. However, alternative explanations for this disjunction are suboptimal in that they require the invocation of either long-distance dispersal, which appears unlikely in this group, or extinction. Nonetheless, the evidence presented here and in other recent studies casts substantial doubt on the Madrean–Tethyan hypothesis as a general explanation for the presence of Madrean and Tethyan taxa similar in overall appearance. More plants with Madrean–Tethyan distributions must be sampled before definitive conclusions regarding this aspect of Madrean and Tethyan vegetation can be reached. 相似文献
6.
We show that the voltage-gated K+ and Ca2+ currents of rat osteoblastic cells are strongly modulated by arachidonic acid (AA), and that these modulations are very sensitive to the AA concentration. At 2 or 3 μm, AA reduces the amplitude and accelerates the inactivation of the K+ current activated by depolarization; at higher concentrations (≥5 μm), AA still blocks this K+ current, but also induces a very large noninactivating K+ current. At 2 or 3 μm, AA enhances the T-type Ca2+ current, close to its threshold of activation, whereas at 10 μm, it blocks that current. AA (1–10 μm) also blocks the dihydropyridine-sensitive L-type Ca2+ current. Thus, the effect of AA on Ca2+ entry through voltage-gated Ca2+ channels can change qualitatively with the AA concentration: at 2 or 3 μm, AA will favor Ca2+ entry through T channels, both by lowering the voltage-gated K+ conductance and by increasing the T current, whereas at 10 μm, AA will prevent Ca2+ entry through voltage-gated Ca2+ channels, both by inducing a K+ conductance and by blocking Ca2+ channels. 相似文献
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Population genetics and phylogenetics of DNA sequence variation at multiple loci within the Drosophila melanogaster species complex 总被引:14,自引:1,他引:13
Two regions of the genome, a 1-kbp portion of the zeste locus and a 1.1-
kbp portion of the yolk protein 2 locus, were sequenced in six individuals
from each of four species: Drosophila melanogaster, D. simulans, D.
mauritiana, and D. sechellia. The species and strains were the same as
those of a previous study of a 1.9-kbp region of the period locus. No
evidence was found for recent balancing or directional selection or for the
accumulation of selected differences between species. Yolk protein 2 has a
high level of amino acid replacement variation and a low level of
synonymous variation, while zeste has the opposite pattern. This contrast
is consistent with information on gene function and patterns of codon bias.
Polymorphism levels are consistent with a ranking of effective population
sizes, from low to high, in the following order: D. sechellia, D.
melanogaster, D.mauritiana, and D. simulans. The apparent species
relationships are very similar to those suggested by the period locus
study. In particular, D. simulans appears to be a large population that is
still segregating variation that arose before the separation of D.
mauritiana and D. sechellia. It is estimated that the separation of
ancestral D. melanogaster from the other species occurred 2.5-3.4 Mya. The
separations of D. sechellia and D. mauritiana from ancestral D. simulans
appear to have occurred 0.58- 0.86 Mya, with D. mauritiana having diverged
from ancestral D. simulans 0.1 Myr more recently than D. sechellia.
相似文献
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The kinetics of formation and the structure of infectious DNA of spleen necrosis virus were determined. Nonintegrated infectious viral DNA first appeared 18 to 24 h after infection of dividing cells and persisted for more than 14 days. The nonintegrated infectious viral DNA was in the form of either a double-stranded linear DNA with a molecular weight of 6 X 10(6), detected in both the cytoplasm and nucleus, or a closed circular DNA of the same molecular weight, detected primarily in the nucleus. Integrated infectious viral DNA appeared soon after the nonintegrated infectious viral DNA and was the predominant form of infectious viral DNA late after infection. Integration of the spleen necrosis virus DNA into the chicken cell genome was demonstrated by three independent criteria. Nucleic acid hybridization indicated that the linear infectious viral DNA had a 5- to 10-fold higher specific infectivity than either the closed circular or integrated infectious viral DNA. Infectious viral DNA did not appear in infected stationary cells, indicating some cellular influence on the formation of infectious viral DNA. 相似文献