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911.
Does the mechanism of sex determination constrain the potential for sex manipulation? A test in geckos with contrasting sex-determining systems 总被引:1,自引:1,他引:0
The concentration of yolk steroids was suggested to influence offspring gender in oviparous animals subject to both temperature-dependent sex determination (TSD) and genotypic sex determination (GSD). However, the proposed mechanisms of steroid effects are thought to differ between TSD and GSD: a direct effect of oestrogens on gonad feminisation in TSD species vs a differential induction of male-producing or female-producing gametes in GSD species. Geckos offer an ideal opportunity for testing these suggested mechanisms. Closely related gecko species differ in their modes of sex determination. They lay clutches of two synchronously formed eggs; both eggs share equal steroid levels. If identical hormonal composition and environment during vitellogenesis, gravidity and incubation determine the sex of the progeny, siblings should share the same gender in both TSD and GSD geckos. We found strong support for this prediction in a TSD gecko species. Among clutches that were incubated at the temperature that produced both sexes, there were no clutches with siblings of the opposite sex. On the other hand, about half of the clutches yielded siblings of the opposite sex in four GSD species. These results suggest that sex-determining systems constrain the ability of the female to produce single-sex siblings and, hence, it seems that the GSD mechanism constrains the opportunities for sex ratio manipulation in geckos via yolk steroid manipulation. 相似文献
912.
913.
For the determination of safety characteristics of gases, vapors and dusts different types of ignition sources are used in international standards and guidelines. The paper presents test results of a comparative calorimetric and visual study between four different types of ignition sources. The ignition procedures were analyzed visually with a high-speed camera and electric recordings. In addition to that, the influence of the electrode-orientation, -distance as well as ignition energy on the reproducibility of the exploding wire igniter was tested.The exploding wire is already in use for standardized determination of safety characteristics of gases, first tests on the suitability of the exploding wire igniter for dust testing have been carried out but are not standardized yet. Using the exploding wire, the ignition energy can be varied from 2 J to 10 000 J (2 x 5000 J) and thus it could be used for gases, vapors, dusts and hybrid mixtures. Moreover it can be used at high initial pressures and it is the only ignition source with an easily measurable ignition energy release. Furthermore, it does not introduce another chemical reaction into the system.Finally, a proposal for a standard ignition source for explosion tests on hybrid mixtures is derived from the test results. 相似文献
914.
吹扫—捕集/气相色谱法测定水中挥发性卤代烃 总被引:6,自引:2,他引:6
建立了吹扫—捕集 /气相色谱 /电子捕获检测器联用测定水中 5种挥发性卤代烃分析方法 ,且按国家水质分析标准方法 (GB/T 1 71 3 0 -1 997)所确定的三氯甲烷、四氯化碳、三氯乙烯、四氯乙烯、三溴甲烷 5种挥发性卤代烃进行实验 ,采用 Supelcoport担体层析柱对去离子水处理 ,所获得的纯水无色谱干扰峰 ,本法按国家水质分析标准方法列出的 5种挥发性卤代烃进行测定在国内尚未见报导。当纯水的进样体积为 5 .0 ml时 ,其最低检出浓度为 1× 1 0 - 2~ 6× 1 0 - 4(μg/L)之间 ;相对误差在 1 .1 %~ 2 .4%之间 ;加标回收率在 82 .5 %~ 1 0 8%之间 ;相关系数在 0 .9980~ 0 .9999之间。可快速、高灵敏度、简便、准确、精密地测定水中 5种挥发性卤代烃 ,本法适用于饮用水和地表水中挥发性卤代烃的测定 相似文献
915.
916.
Kin conflict over caste determination in social Hymenoptera 总被引:2,自引:0,他引:2
We argue that caste determination, the process whereby females in the social Hymenoptera develop into either queens or workers,
is subject to kin-selected conflict. Potential conflict arises because developing females are more closely related to their
would-be offspring than to those of other females. Therefore, they may favour becoming queens contrary to the interests of
other developing females and of existing queens and workers. We suggest two contexts leading to potential caste conflict.
The first occurs when queens are reared in a reproductive phase following an ergonomic phase of worker production, while the
second occurs when queens and workers are reared simultaneously. The first context assumes that workers' per capita contribution
to colony survival and productivity falls with rising colony size. A critical feature influencing whether potential conflict
is realized is the extent to which developing females can determine their own caste (“self-determination”). Self-determination
is facilitated when female larvae control their own food intake and when queen-worker size dimorphism is low. We know of no
strong evidence for actual conflict over caste fate arising in the first context. However, stingless bees and polygynous ants
with excess queen-potential larvae that are either forced to develop as workers or are culled as adults demonstrate actual
caste conflict in the second context. Caste conflict does not preclude caste regulation for “the good of the colony”, but
such regulation is contingent on either the absence of potential conflict or on developing females losing control of their
caste fate.
Received: 22 March 1999 / Received in revised form: 15 June 1999 / Accepted: 19 June 1999 相似文献
917.
带有阶跃函数的GIM(1)预测模型及其应用 总被引:1,自引:0,他引:1
结合某水域地面水ROD5的预测实践,研究提出了带有阶跃函数的GIM(1)预测模型,并给出了估计这种模型参数的简便方法,从而扩大了GIM(1)模型在环境系统中的应用范围,实例表明,运用该方法得到的模型比常规GIM(1)具有更高的精度。 相似文献
918.
Using the Yellow River, China, the study explores the problem of the use of COD and BOD5 as water quality management parameters in the presence of very high levels of suspended sediment (TSS) that characterize
this river. Although the amount of natural organic matter per unit of suspended sediment of the Yellow River is not high,
the very high concentration of mineral sediment in the Yellow River results in a large concentration of organic matter, which
artificially inflates the laboratory values of COD and, as a consequent, leads to greatly exaggerated reports of pollution
of the Yellow River. BOD5 can more accurately reflect the pollution of the Yellow River than COD; however, measured values of BOD under-report the
actual values due to settling of the sediment in the incubation chamber resulting in values that are 21.6--38.3% less than
the actual values. Therefore corrections are required for laboratory COD and BOD values so that the values are not artifacts
of the sediment regime. Our work provides new insight into this phenomenon and demonstrates how correction factors may be
determined and used with pollution data. Our work also suggests that the actual pollution levels of the Yellow River are probably
not as high as reported by monitoring agencies. 相似文献
919.
920.