A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes

The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ～34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2-4.5?km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1?km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.     

构建可扩展的网络服务：理论和实践

随着计算机网络变得越来越复杂，在网络上部署大规模的服务也愈加困难。为此本书在介绍了可扩展的网络基础设施和在网络系统设计领域内的大量实验内容的前提下，提出了一个分层可扩展的服务模型，并且评价了在该结构模型下运行的各种服务的性能。除此之外，本书还囊括了一系列的理论结果，并且可以应用在真实的网络环境下，如构建全网范围内的测量、监测服务以及构建更好的p2p网络的策略。     

Requirement of Bmpr1a for Müllerian duct regression during male sexual development

Elimination of the developing female reproductive tract in male fetuses is an essential step in mammalian sexual differentiation. In males, the fetal testis produces the transforming growth factor beta (TGF-beta) family member anti-Müllerian hormone (Amh, also known as Müllerian-inhibiting substance (Mis)), which causes regression of the Müllerian ducts, the primordia of the oviducts, uterus and upper vagina. Amh induces regression by binding to a specific type II receptor (Amhr2) expressed in the mesenchyme surrounding the ductal epithelium. Mutations in AMH or AMHR2 in humans and mice disrupt signaling, producing male pseudohermaphrodites that possess oviducts and uteri. The type I receptor and Smad proteins that are required in vivo for Müllerian duct regression have not yet been identified. Here we show that targeted disruption of the widely expressed type I bone morphogenetic protein (BMP) receptor Bmpr1a (also known as Alk3) in the mesenchymal cells of the Müllerian ducts leads to retention of oviducts and uteri in males. These results identify Bmpr1a as a type I receptor for Amh-induced regression of Müllerian ducts. Because Bmpr1a is evolutionarily conserved, these findings indicate that a component of the BMP signaling pathway has been co-opted during evolution for male sexual development in amniotes.