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排序方式: 共有71条查询结果,搜索用时 15 毫秒
1.
Summary Ramets of the understory herb, Aster acuminatus, were transplanted from two source populations into eight understory garden sites that varied in light and soil moisture levels. Ramet growth, clonal growth, flowering and survivorship were monitored for three growing seasons. Large differences among gardens in ramet growth, clonal growth and flowering developed in the first growing season and increased in the next two years. This variation was positively correlated with garden light level but not at all with soil moisture. Mortality rates were low in all gardens and showed that genets from any particular source could survive over a broad range of environmental conditions. There was no conclusive evidence for any source population differences in the capacity to survive or grow in different environments. The rapid, light-induced responses of transplanted ramets resulted in garden populations very similar in appearance to natural populations experiencing similar light regimes. These results combined with those from other studies of A. acuminatus provide strong evidence for the importance of light in explaining population patterns and dynamics in this species. 相似文献
2.
Ecosystem response to elevated CO(2) 总被引:1,自引:0,他引:1
Pitelka LF 《Trends in ecology & evolution》1994,9(6):204-207
3.
Fitness consequences of helping behavior in the western bluebird 总被引:5,自引:4,他引:1
We examined the fitness consequences of helping behavior inthe western bluebird (Sialia mexicana) at Hastings Reservationin Carmel Valley, California, USA, and tested hypotheses forhow helpers benefit from engaging in alloparental behavior.Both juvenile and adult western bluebirds occasionally helpat the nest During a 12 year period, all adult helpers and mostjuvenile helpers were male. Helpers usually fed at nests ofboth their parents and rarely helped when only one parent waspresent. The frequency of pairs with adult helpers was only7%, but nearly one-third of adult males helped among those withboth parents on the study area. At least 28% were breeders whosenests failed. The propensity to help appears to depend uponparental survival, male philopatry, and the breeding successof potential helpers. Feeding rates were not increased at nestswith juvenile helpers, apparendy because breeding males reducedtheir feeding rates. In contrast, adult helpers increased theoverall rates of food delivery to the nest in spite of a reductionin the number of feeding trips made by both male and femaleparents. Helpers did not derive any obvious direct fitness benefitsfrom helping, but they had greater indirect fitness than nonhelpersdue to increases in nestling growth rates and fledging successat their parents' nests. Helpers fledged fewer offspring intheir first nests than did nonhelpers, suggesting that theywere birds with reduced reproductive potential. Although wehave not yet measured the effect of extrapair fertilizationson the fitness benefits of helping, we calculated the differencein fitness between helpers and nonhelpers as a function of thepotential helper's paternity when breeding independently andhis father's paternity in the nest at which he might help. Inconjunction with constraints on breeding and indirect fitnessbenefits, we predict that relatedness of males to the youngin their own as well as their parents' nests will influencehelping behavior in western bluebirds. 相似文献
4.
Analyses of the rat mammary gland show that the increase in the milk-protein mRNAs during the development of lactation and the rapid disappearance of these sequences during involution are not accompanied by similar changes in the poly(A) content. During the development of lactation the casein mRNA is initially in great excess to the whey-protein mRNA and this differential expression of the genes for the two types of milk proteins is again observed during early involution. Since the amounts of poly(A) and of both milk-protein mRNAs are also similar to the amounts found in the gland during late pregnancy, these results indicate that during early involution the mammary gland has reverted to the pattern of mRNA metabolism that occurs during late pregnancy. 相似文献
5.
The translation of polyadenylated and of non-poly-adenylated RNA obtained from lactating rat mammary gland was almost totally inhibited by 0,5 mM 7-methylguanosine-5-phosphate in the wheat-germ cell-free system, This inhibition was maintained during the preparation of the 9S whey-protein mRNA and of the 12S and ISS casein mRNAs, Chemical decapping of these mRNAs caused a similar reduction of their activity . Although a large fraction of milk-protein mRNAs have been reported to lack 3-polyadenylation, these results show that the mRNAs in the mammary gland do contain a 5-terminal 7-methylguanosine cap. 相似文献
6.
Joanne T. Emerman Dorothy R. Pitelka 《In vitro cellular & developmental biology. Plant》1977,13(5):316-328
Summary Dissociated normal mammary epithelial cells from prelactating mice were plated on different substrates in various medium-serum-hormone
combinations to find conditions that would permit maintenance of morphological differentiation. Cells cultured on floating
collagen membranes in medium containing insulin, hydrocortisone and prolactin maintain differentiation through 1 month in
culture. The surface cells form a continuous epithelial pavement. Some epithelial cells below the surface layer rearrange
themselves to form alveolus-like structures. Cells at both sites display surface polarization; microvilli and tight junctions
are present at their medium-facing or luminal surface and a basal lamina separates the epithelial components from the gel
and stromal cells. Occasinal myoepithelial cells, characterized by myofilaments and plasmalemmal vesicles, are identified
at the basal surface of the secretory epithelium. In contrast, cells cultured on plastic, glass or collagen gels attached
to Petri dishes form a confluent epithelial sheet showing surface polarization, but lose secretory and myoepithelial specializations.
If these dedifferentiated cells are subsequently maintained on floating collagen membranes, they redifferentiate. There is
little DNA synthesis in cells on collagen gels, in contrast to Petri-dish controls. Protein synthesis in cells on floating
collagen membranes increases over T0 values and remains constant through 7 days in culture whereas it decreases on attached gels; however, if the gels are freed
to float, protein synthesis increases sharply and parallels that seen on floating membranes.
The work was supported by USPHS Grants CA-05388 and CA-05045 from the National Cancer Institute, DHEW. 相似文献
7.
Substrate properties influencing ultrastructural differentiation of mammary epithelial cells in culture 总被引:20,自引:0,他引:20
Epithelial cells dissociated from mammary glands of midpregnant mice and cultured with lactogenic hormones on plastic or collagen gel substrates have been shown to vary in their extent of differentiation, as identified by the presence of secretory organelles and accumulation and secretion of casein. Morphological and biochemical differentiation was obtained on floating collagen gels. At least four unique factors provided by the floating collagen gel substrates are not found on plastic substrates: access of nutrients to basolateral cell surfaces, close proximity of cells to the medium surface and gas phase, interaction of epithelial cells with stromal elements, and substrate flexibility permitting cell shape change. In this study, we have attempted to assess the relative contributions of these factors in the ultrastructural differentiation of mammary cells in culture. None of these factors alone is responsible for the differentiation achieved when all are present. The novel aspect of this research is the identification of the cells' apparent requirements for basolateral access to nutrients and for freedom to assume a preferred shape in order to achieve differentiation. 相似文献
8.
Electrical coupling has been observed between cultured cells of the mouse mammary gland in five distinct physiological or pathological states. We have employed young primary cultures of cells dissociated from the following tissues: normal glands from young virgin or midpregnant females, hyperplastic alveolar nodules (believed to be precancerous) transplanted in gland-free mammary fat pads, and spontaneous mammary adenocarcinomas and their pulmonary metastases. All successfully impaled pairs of cells (a total of 97 pairs) were found to be ionically coupled. Furthermore, in normal and tumor cell cultures, electrical coupling was observed between dome-dome and dome-nondome cell pairs. This study correlates with electronmicroscopic studies of fresh normal, hyperplastic, and tumor samples, which show the presence of gap junctions in all three. 相似文献
9.
Hapten-sandwich labeling. II. Immunospecific attachment of cell surface markers suitable for scanning electron microscopy 总被引:5,自引:3,他引:2
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A hapten-sandwich procedure has been used for immunospecific labeling of cell surface antigens with markers visible by scanning electron microscopy. Antihapten antibody was used to link hapten-modified tobacco mosaic virus, bushy stunt virus, or hemocyanin to hapten-modified human erythrocytes. The antihapten antibody bridge was also used to link the hapten-virus marker to hapten-modified antibodies against mammary tumor virus on mouse mammary tumor cells, or against immunoglobulin receptors on mouse splenic lymphocytes. In all cases, labeling was highly specific. With this technique, it is possible to (a) compare morphological features of cells bearing differing cell surface antigens, and (b) examine the arrangement of specific antigenic sites on a cell surface or their distribution relative to membrane structures such as microvilli. 相似文献
10.
Mark S. Castro Keith N. Eshleman Louis F. Pitelka Geoff Frech Molly Ramsey William S. Currie Karen Kuers Jeffrey A. Simmons Bob R. Pohlad Carolyn L. Thomas David M. Johnson 《Biogeochemistry》2007,84(3):333-348
The objective of this study was to evaluate the nitrogen (N) biogeochemistry of an 18–22 year old forested watershed in western
Maryland. We hypothesized that this watershed should not exhibit symptoms of N saturation. This watershed was a strong source
of nitrate (NO3
−) to the stream in all years, with a mean annual export of 9.5 kg N ha−1 year−1 and a range of 4.4–18.4 kg N ha−1 year−1. During the 2001 and 2002 water years, wet deposition of inorganic N was 9.0 kg N ha−1 year−1 and 6.3 kg N ha−1 year−1, respectively. Watershed N export rates in 2001 and 2002 water years were 4.2 kg N ha−1 year−1 and 5.3 kg N ha−1 year−1, respectively. During the wetter water years of 2003 and 2004, the watershed exported 15.0 kg N ha−1 year−1 and 18.4 kg N ha−1 year−1, rates that exceeded annual wet deposition of N by a factor of two (7.5 kg N ha−1 year−1 in 2003) and three (5.5 kg N ha−1 year−1 in 2004). Consistent with the high rates of N export, were high concentrations (2.1–3.3%) of N in foliage, wood (0.3%) and
fine roots, low C:N ratios in the forest floor (17–24) and mineral soil (14), high percentages (83–96%) of the amount of mineralized
N that was nitrified and elevated N concentrations (up to 3 mg N l−1) in soil solution. Although this watershed contained a young aggrading forest, it exhibited several symptoms of N saturation
commonly observed in more mature forests. 相似文献