51 research outputs found
Stationary Black Holes: Uniqueness and Beyond
The spectrum of known black-hole solutions to the stationary Einstein
equations has been steadily increasing, sometimes in unexpected ways. In
particular, it has turned out that not all black-hole-equilibrium
configurations are characterized by their mass, angular momentum and global
charges. Moreover, the high degree of symmetry displayed by vacuum and
electro-vacuum black-hole spacetimes ceases to exist in self-gravitating
non-linear field theories. This text aims to review some developments in the
subject and to discuss them in light of the uniqueness theorem for the
Einstein-Maxwell system.Comment: Major update of the original version by Markus Heusler from 1998.
Piotr T. Chru\'sciel and Jo\~ao Lopes Costa succeeded to this review's
authorship. Significantly restructured and updated all sections; changes are
too numerous to be usefully described here. The number of references
increased from 186 to 32
Comparative genomics of the tardigrades <i>Hypsibius dujardini</i> and <i>Ramazzottius varieornatus</i>
Tardigrada, a phylum of meiofaunal organisms, have been at the center of discussions of the evolution of Metazoa, the biology of survival in extreme environments, and the role of horizontal gene transfer in animal evolution. Tardigrada are placed as sisters to Arthropoda and Onychophora (velvet worms) in the superphylum Panarthropoda by morphological analyses, but many molecular phylogenies fail to recover this relationship. This tension between molecular and morphological understanding may be very revealing of the mode and patterns of evolution of major groups. Limnoterrestrial tardigrades display extreme cryptobiotic abilities, including anhydrobiosis and cryobiosis, as do bdelloid rotifers, nematodes, and other animals of the water film. These extremophile behaviors challenge understanding of normal, aqueous physiology: how does a multicellular organism avoid lethal cellular collapse in the absence of liquid water? Meiofaunal species have been reported to have elevated levels of horizontal gene transfer (HGT) events, but how important this is in evolution, and particularly in the evolution of extremophile physiology, is unclear. To address these questions, we resequenced and reassembled the genome of H. dujardini, a limnoterrestrial tardigrade that can undergo anhydrobiosis only after extensive pre-exposure to drying conditions, and compared it to the genome of R. varieornatus, a related species with tolerance to rapid desiccation. The 2 species had contrasting gene expression responses to anhydrobiosis, with major transcriptional change in H. dujardini but limited regulation in R. varieornatus. We identified few horizontally transferred genes, but some of these were shown to be involved in entry into anhydrobiosis. Whole-genome molecular phylogenies supported a Tardigrada+Nematoda relationship over Tardigrada+Arthropoda, but rare genomic changes tended to support Tardigrada+Arthropoda
Human Chorionic-Gonadotropin Beta-Subunit Is Encoded by at Least Eight Genes Arranged in Tandem and Inverted Pairs
Human chorionic gonadotropin β-subunit is encoded by at least eight genes arranged in tandem and inverted pairs
Cytology and Cytogenetics
Several cytological aspects have been considered in tardigrades. Firstly,
the cell constancy which is not a true eutely being several mitoses present even after
hatching, even though some organs, such epidermis and nervous ganglia, have the
same cell number in juveniles and adults. The total number of these cells is speciesspecific.
Then the ultrastructure of cuticle, epidermis, feeding and digestive apparatus,
excretory and osmoregulatory organs, muscles, nerve cells, sensory cells and
storage cells has been considered. Instead, the ultrastructure of the germ cells has
been considered in the chapter on reproduction. With regard to chromosome number
and shape, it has been observed that generally there is little difference among the
species (n ¼ 5 or n ¼ 6), but several cases of polyploid populations exist, often very
similar to diploid populations from a morphological point of view. In most cases the
polyploid populations do not have males and reproduce by apomixis. Studies on the
genome size have confirmed the presence of polyploid populations, as well as the
presence of nuclei with multiple amounts of DNA within the same specimen. The
genome size of the tardigrades is always relatively small and does not seem related to
phylogenetic lineages. Studies on tardigrade genomes have placed this phylum at the
centre of discussions on the evolution of Metazoa and have considered the role of
horizontal gene transfer in animal evolution with contrasting results
Least-squares regression with unitary constraints for network behaviour classification
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