The conserved Hippo kinase pathway plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. Whereas the function of the core kinase cascade, consisting of the serine/threonine kinases Hippo and Warts, in phosphorylating and thereby inactivating the transcriptional coactivator Yorkie is well established, much less is known about the upstream events that regulate Hippo signaling activity. The FERM domain proteins Expanded and Merlin appear to represent two different signaling branches that feed into the Hippo pathway. Signaling by the atypical cadherin Fat may act via Expanded, but how Merlin is regulated has remained elusive. Here, we show that the WW domain protein Kibra is a Hippo signaling component upstream of Hippo and Merlin. Kibra acts synergistically with Expanded, and it physically interacts with Merlin. Thus, Kibra predominantly acts in the Merlin branch upstream of the core kinase cascade to regulate Hippo signaling.
Sex-determining cascades are supposed to have evolved in a retrograde manner from bottom to top. Wilkins' 1995 hypothesis finds support from our comparative studies in Drosophila melanogaster and Musca domestica, two dipteran species that separated some 120 million years ago. The sex-determining cascades in these flies differ at the level of the primary sex-determining signal and their targets, Sxl in Drosophila and F in Musca. Here we present evidence that they converge at the level of the terminal regulator, doublesex ( dsx), which conveys the selected sexual fate to the differentiation genes. The dsx homologue in Musca, Md-dsx, encodes male-specific (MdDSX(M)) and female-specific (MdDSX(F)) protein variants which correspond in structure to those in Drosophila. Sex-specific regulation of Md-dsx is controlled by the switch gene F via a splicing mechanism that is similar but in some relevant aspects different from that in Drosophila. MdDSX(F) expression can activate the vitellogenin genes in Drosophila and Musca males, and MdDSX(M) expression in Drosophila females can cause male-like pigmentation of posterior tergites, suggesting that these Musca dsx variants are conserved not only in structure but also in function. Furthermore, downregulation of Md-dsx activity in Musca by injecting dsRNA into embryos leads to intersexual differentiation of the gonads. These results strongly support a role of Md-dsx as the final regulatory gene in the sex-determining hierarchy of the housefly.
Research on drought impact on tree functioning is focused primarily on water and carbon (C) dynamics. Changes in nutrient uptake might also affect tree performance under drought and there is a need to explore underlying mechanisms. We investigated effects of drought on a) in-situ nitrogen (N)-uptake accounting for both, N availability to fine-roots in soil and actual N-uptake, b) physiological N-uptake capacity of roots, and c) the availability of new assimilates to fine roots influencing the N-uptake capacity using 15N and 13C labelling. We assessed saplings of six different tree species (Acer peudoplatanus, Fagus sylvatica, Quercus petraea, Abies alba, Picea abies, Pinus sylvestris). Drought resulted in significant reduction of in-situ soil N-uptake in deciduous trees accompanied by reduced carbon allocation to roots and by a reduction in root biomass available for N-uptake. While physiological root N-uptake capacity was not affected by drought in deciduous saplings, reduced maximum ammonium but not nitrate uptake was observed for A.alba and P.abies. Our results indicate that drought has species-specific effects on N-uptake. Even water limitations of only 5 weeks as assessed here can decrease whole plant inorganic N-uptake independent of whether the physiological N-uptake capacity is affected or not.
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