Twenty reversed, two‐ship expanding spread profiles (ESPs) with maximum source‐receiver offsets of ∼100 km were collected in three transects across the rifted northern margin of the South China Sea. Source‐receiver offset versus two‐way travel time (X‐T) data were mapped into the intercept time versus ray parameter (τ‐p) domain, and velocity‐depth solutions were obtained by a combination of τ‐sum inversion in the τ‐p domain and ray tracing in both the τ‐p and X‐T domains. Arrivals from the Moho were detected on 17 of the ESPs. The depths to Moho determined for individual ESP interpretations have reproducibilities of ±0.1 km to ±3 km; in most cases the Moho depth has been determined to within ±1.5 km. Moho depths determined in this investigation represent a significant improvement over previous estimates of Moho along the margin from gravity data. Variations in present‐day crustal thickness (measured from top of prerift basement to Moho) are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent South China Sea Basin. The ESP interpretations reveal that across the eastern portion of the south China margin, the crust appears to thin more or less continuously toward the continent‐ocean boundary. In the west, ESP interpretations also show a general trend of seaward crustal thinning but, in addition, indicate at least two instances of focused, localized crustal thinning. Crustal velocities and the relative proportion of upper crust (VP< 6.4 km/s) and lower crust (VP> 6.4 km/s) are used to identify areas of the south China margin with similar and contrasting crustal structures. Variations in these properties are believed to result primarily from contrasting, prerift crustal structure across the margin. However, magmatic underplating during rifting, depth dependent extension, and Pleistocene igneous intrusions may also have contributed to the variations in present crustal structure. Reliable information about variations in crustal thickness and velocity structure across and along the south China margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin.
263 SummaryPollen-and seed-mediated transgene flow is a concern in plant biotechnology. We report here a highly efficient 'genetically modified (GM)-gene-deletor' system to remove all functional transgenes from pollen, seed or both. With the three pollen-and / or seed-specific gene promoters tested, the phage CRE/ loxP or yeast FLP/ FRT system alone was inefficient in excising transgenes from tobacco pollen and/or seed, with no transgenic event having 100% efficiency. When loxP-FRT fusion sequences were used as recognition sites, simultaneous expression of both FLP and CRE reduced the average excision efficiency, but the expression of FLP or CRE alone increased the average excision efficiency, with many transgenic events being 100% efficient based on more than 25 000 T 1 progeny examined per event. The 'GM-gene-deletor' reported here may be used to produce 'non-transgenic' pollen and/or seed from transgenic plants and to provide a bioconfinement tool for transgenic crops and perennials, with special applicability towards vegetatively propagated plants and trees.
Multichannel seismic data and gravity data have been used to construct crustal thickness profiles for three transects (eastern, central, western) across the rifted northern margin of the South China Sea. The present‐day crustal configuration of the margin is then interpreted by modeling the effects of two end‐member classes of extension processes, pure shear and simple shear. The applicability of each of these processes to the extension of the south China margin has been evaluated by comparing model predictions of subsidence and heat flow with observations across the margin. Neither of these end‐member models satisfactorily fits the observed data on the eastern and central transects across the south China margin when typical values for standard input parameters are used; the resulting heat flow is significantly underestimated by both models. In the case of a pure shear model, heat flow observations may be matched either by assuming an uncommonly thin initial steady state lithospheric thickness (∼60 km) or by assuming an unusually large crustal radiogenic heat production within the original, unextended continental crust. A perhaps more reasonable alternative scenario presumes the existence of an initially slightly thinner than “normal” steady state lithosphere (thicknesses of ∼90–100 km) in conjunction with a significant amount of upper crustal radiogenic heat production. Such heat production could be accommodated by the presence of Cretaceous granitic bodies (hypothesized) within the basement beneath the south China margin. In the case of a simple shear model, however, the observed high heat flow on the rifted south China margin may only be matched if the steady state lithospheric thickness is assumed to be uncommonly thin (∼60 km). Because the observed geophysical data characterizing lithospheric extension may be matched using more realistic input parameters in the pure shear case, pure shear extension is preferred over simple shear extension as the dominant mechanism for explaining the large‐scale rifting of the south China margin. For extension within the crust, however, combinations of both processes are not only possible, but probable, given published seismic evidence for through‐going crustal faults on the south China margin.
Here we report the effect of the 35S promoter sequence on activities of the tissue- and organ-specific gene promoters in tobacco plants. In the absence of the 35S promoter sequence the AAP2 promoter is active only in vascular tissues as indicated by expression of the AAP2:GUS gene. With the 35S promoter sequence in the same T-plasmid, transgenic plants exhibit twofold to fivefold increase in AAP2 promoter activity and the promoter becomes active in all tissue types. Transgenic plants hosting the ovary-specific AGL5:iaaM gene (iaaM coding an auxin biosynthetic gene) showed a wild-type phenotype except production of seedless fruits, whereas plants hosting the AGL5:iaaM gene along with the 35S promoter sequence showed drastic morphological alterations. RT-PCR analysis confirms that the phenotype was caused by activation of the AGL5:iaaM gene in non-ovary organs including roots, stems and flowers. When the pollen-, ovule- and early embryo-specific PAB5:barnase gene (barnase coding a RNase gene) was transformed, the presence of 35S promoter sequence drastically reduced transformation efficiencies. However, the transformation efficiencies were restored in the absence of 35S promoter, indicating that the 35S promoter might activate the expression of PAB5:barnase in non-reproductive organs such as calli and shoot primordia. Furthermore, if the 35S promoter sequence was replaced with the NOS promoter sequence, no alteration in AAP2, AGL5 or PAB5 promoter activities was observed. Our results demonstrate that the 35S promoter sequence can convert an adjacent tissue- and organ-specific gene promoter into a globally active promoter.
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