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261.
Inhibitors of nucleic acid synthesis were tested for the effect on the primary antibody response. Antibody formation by adult
spleen cells mixed with antigenin vitro and transferred to newborn rabbits was completely suppressed by 6-mercaptopurine administered in the initial phase of antibody
induction. The extent of inhibition decreased proportionately to the time interval between cell transfer and treatment with
6-MP. Different sensitivity of immunologically competent cells towards 6-MP at various periods of differentiation and interference
of 6-MP with synthesis of specific ribonucleic acid is suggested. Actinomycin D did not show any inhibitory effect on the
primary antibody responsein vivo which may be due either to the toxicity of the drug or to some other mechanism of synthesis of immunoglobulins than is known
for bacterial proteins. Mitomycin C was also ineffective in suppressing the inductive phase of antibody formation. From results
with 6-MP and mitomycin C it is concluded that DNA synthesis and mitotic division are not essential steps in the induction
of antibodies. 相似文献
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Ximena Tabares Heike Zimmermann Elisabeth Dietze Gregor Ratzmann Lukas Belz Andrea Vieth‐Hillebrand Lydie Dupont Heinz Wilkes Benjamin Mapani Ulrike Herzschuh 《Ecology and evolution》2020,10(2):962-979
- Shrub encroachment has far‐reaching ecological and economic consequences in many ecosystems worldwide. Yet, compositional changes associated with shrub encroachment are often overlooked despite having important effects on ecosystem functioning.
- We document the compositional change and potential drivers for a northern Namibian Combretum woodland transitioning into a Terminalia shrubland. We use a multiproxy record (pollen, sedimentary ancient DNA, biomarkers, compound‐specific carbon (δ13C) and deuterium (δD) isotopes, bulk carbon isotopes (δ13Corg), grain size, geochemical properties) from Lake Otjikoto at high taxonomical and temporal resolution.
- We provide evidence that state changes in semiarid environments may occur on a scale of one century and that transitions between stable states can span around 80 years and are characterized by a unique vegetation composition. We demonstrate that the current grass/woody ratio is exceptional for the last 170 years, as supported by n‐alkane distributions and the δ13C and δ13Corg records. Comparing vegetation records to environmental proxy data and census data, we infer a complex network of global and local drivers of vegetation change. While our δD record suggests physiological adaptations of woody species to higher atmospheric pCO2 concentration and drought, our vegetation records reflect the impact of broad‐scale logging for the mining industry, and the macrocharcoal record suggests a decrease in fire activity associated with the intensification of farming. Impact of selective grazing is reflected by changes in abundance and taxonomical composition of grasses and by an increase of nonpalatable and trampling‐resistant taxa. In addition, grain‐size and spore records suggest changes in the erodibility of soils because of reduced grass cover.
- Synthesis. We conclude that transitions to an encroached savanna state are supported by gradual environmental changes induced by management strategies, which affected the resilience of savanna ecosystems. In addition, feedback mechanisms that reflect the interplay between management legacies and climate change maintain the encroached state.
265.
Effects of space environment on embryonic growth up to hatching of salamander eggs fertilized and developed during orbital flights. 总被引:1,自引:0,他引:1
Lydie Gualandris-Parisot David Husson Alain Bautz Danielle Durand Paulette Kan Christian Aimar Herve Membre Anne-Marie Duprat Christian Dournon 《Biological Sciences in Space》2002,16(1):3-11
In vertebrates, only few experiments have been performed in microgravity to study the embryonic development from fertilization. To date, these concern only amphibian and fish. We report here a study on the embryonic development of Pleurodeles waltl (urodele amphibian) eggs oviposited in microgravity. The experiment was performed twice on board the Mir space station and the data obtained included video recording and morphological, histological and immunocytological analyses. The data confirm that the microgravity conditions have effects during the embryonic period, particularly during cleavage and neurulation, inducing irregular segmentation and abnormal closure of the neural tube. Moreover, we observed several abnormalities hither to undescribed corresponding to cortical cytoplasm movements, a decrease of cell adhesion and a loss of cells. These abnormalities were temporary and subsequently reversible. The young larvae that hatched during the flight displayed normal morphology and swimming behavior after landing. The results obtained in the urodele Pleurodeles waltl are in accordance with those observed earlier in the anuran Xenopus laevis and in the fish Oryzias latipes. 相似文献