Organic Input to Titan's Subsurface Ocean Through Impact Cratering.

Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a habitable environment. In this work, we determined the amount of organic material that can be delivered from Titan's surface to its ocean through impact cratering. We assumed that Titan's craters produce impact melt deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 1011 mol/Gyr for HCN hydrolysis and from 0 to 1014 mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass productivity of ∼103 kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than recent estimates of the hypothetical biomass production supported by Enceladus's subsurface ocean. Unless biologically available compounds can be sourced from Titan's interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere.

Conserved chromatin and repetitive patterns reveal slow genome evolution in frogs.

Frogs are an ecologically diverse and phylogenetically ancient group of anuran amphibians that include important vertebrate cell and developmental model systems, notably the genus Xenopus. Here we report a high-quality reference genome sequence for the western clawed frog, Xenopus tropicalis, along with draft chromosome-scale sequences of three distantly related emerging model frog species, Eleutherodactylus coqui, Engystomops pustulosus, and Hymenochirus boettgeri. Frog chromosomes have remained remarkably stable since the Mesozoic Era, with limited Robertsonian (i.e., arm-preserving) translocations and end-to-end fusions found among the smaller chromosomes. Conservation of synteny includes conservation of centromere locations, marked by centromeric tandem repeats associated with Cenp-a binding surrounded by pericentromeric LINE/L1 elements. This work explores the structure of chromosomes across frogs, using a dense meiotic linkage map for X. tropicalis and chromatin conformation capture (Hi-C) data for all species. Abundant satellite repeats occupy the unusually long (~20 megabase) terminal regions of each chromosome that coincide with high rates of recombination. Both embryonic and differentiated cells show reproducible associations of centromeric chromatin and of telomeres, reflecting a Rabl-like configuration. Our comparative analyses reveal 13 conserved ancestral anuran chromosomes from which contemporary frog genomes were constructed.

Dodecaploid Xenopus longipes provides insight into the emergence of size scaling relationships during development.

Genome and cell size are strongly correlated across species1,2,3,4,5,6 and influence physiological traits like developmental rate.7,8,9,10,11,12 Although size scaling features such as the nuclear-cytoplasmic (N/C) ratio are precisely maintained in adult tissues,13 it is unclear when during embryonic development size scaling relationships are established. Frogs of the genus Xenopus provide a model to investigate this question, since 29 extant Xenopus species vary in ploidy from 2 to 12 copies (N) of the ancestral frog genome, ranging from 20 to 108 chromosomes.14,15 The most widely studied species, X. laevis (4N = 36) and X. tropicalis (2N = 20), scale at all levels, from body size to cellular and subcellular levels.16 Paradoxically, the rare, critically endangered dodecaploid (12N = 108) Xenopus longipes (X. longipes) is a small frog.15,17 We observed that despite some morphological differences, X. longipes and X. laevis embryogenesis occurred with similar timing, with genome to cell size scaling emerging at the swimming tadpole stage. Across the three species, cell size was determined primarily by egg size, whereas nuclear size correlated with genome size during embryogenesis, resulting in different N/C ratios in blastulae prior to gastrulation. At the subcellular level, nuclear size correlated more strongly with genome size, whereas mitotic spindle size scaled with cell size. Our cross-species study indicates that scaling of cell size to ploidy is not due to abrupt changes in cell division timing, that different size scaling regimes occur during embryogenesis, and that the developmental program of Xenopus is remarkably consistent across a wide range of genome and egg sizes.

The power of amphibians to elucidate mechanisms of size control and scaling.

Size is a fundamental feature of biology that affects physiology at all levels, from the organism to organs and tissues to cells and subcellular structures. How size is determined at these different levels, and how biological structures scale to fit together and function properly are important open questions. Historically, amphibian systems have been extremely valuable to describe scaling phenomena, as they occupy some of the extremes in biological size and are amenable to manipulations that alter genome and cell size. More recently, the application of biochemical, biophysical, and embryological techniques to amphibians has provided insight into the molecular mechanisms underlying scaling of subcellular structures to cell size, as well as how perturbation of normal size scaling impacts other aspects of cell and organism physiology.

Kif2a Scales Meiotic Spindle Size in Hymenochirus boettgeri.

Size is a fundamental feature of biological systems that affects physiology at all levels. For example, the dynamic, microtubule-based spindle that mediates chromosome segregation scales to a wide range of cell sizes across different organisms and cell types. Xenopus frog species possess a variety of egg and meiotic spindle sizes, and differences in activities or levels of microtubule-associated proteins in the egg cytoplasm between Xenopus laevis and Xenopus tropicalis have been shown to account for spindle scaling [1]. Increased activity of the microtubule severing protein katanin scales the X. tropicalis spindle smaller compared to X. laevis [2], as do elevated levels of TPX2, a protein that enriches the cross-linking kinesin-5 motor Eg5 at spindle poles [3]. To examine the conservation of spindle scaling mechanisms more broadly across frog species, we have utilized the tiny, distantly related Pipid frog Hymenochirus boettgeri. We find that egg extracts from H. boettgeri form meiotic spindles similar in size to X. tropicalis but that TPX2 and katanin-mediated scaling is not conserved. Instead, the microtubule depolymerizing motor protein kif2a functions to modulate spindle size. H. boettgeri kif2a possesses an activating phosphorylation site that is absent from X. laevis. Comparison of katanin and kif2a phosphorylation sites across a variety of species revealed strong evolutionary conservation, with X. laevis and X. tropicalis possessing distinct and unique alterations. Our study highlights the diversity and complexity of spindle assembly and scaling mechanisms, indicating that there is more than one way to assemble a spindle of a particular size.

Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes.

Saturn's moon Enceladus has an ice-covered ocean; a plume of material erupts from cracks in the ice. The plume contains chemical signatures of water-rock interaction between the ocean and a rocky core. We used the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft to detect molecular hydrogen in the plume. By using the instrument's open-source mode, background processes of hydrogen production in the instrument were minimized and quantified, enabling the identification of a statistically significant signal of hydrogen native to Enceladus. We find that the most plausible source of this hydrogen is ongoing hydrothermal reactions of rock containing reduced minerals and organic materials. The relatively high hydrogen abundance in the plume signals thermodynamic disequilibrium that favors the formation of methane from CO2 in Enceladus' ocean.

Glutamylation of Nap1 modulates histone H1 dynamics and chromosome condensation in Xenopus.

Linker histone H1 is required for mitotic chromosome architecture in Xenopus laevis egg extracts and, unlike core histones, exhibits rapid turnover on chromatin. Mechanisms regulating the recruitment, deposition, and dynamics of linker histones in mitosis are largely unknown. We found that the cytoplasmic histone chaperone nucleosome assembly protein 1 (Nap1) associates with the embryonic isoform of linker histone H1 (H1M) in egg extracts. Immunodepletion of Nap1 decreased H1M binding to mitotic chromosomes by nearly 50%, reduced H1M dynamics as measured by fluorescence recovery after photobleaching and caused chromosome decondensation similar to the effects of H1M depletion. Defects in H1M dynamics and chromosome condensation were rescued by adding back wild-type Nap1 but not a mutant lacking sites subject to posttranslational modification by glutamylation. Nap1 glutamylation increased the deposition of H1M on sperm nuclei and chromatin-coated beads, indicating that charge-shifting posttranslational modification of Nap1 contributes to H1M dynamics that are essential for higher order chromosome architecture.

Histone H1 compacts DNA under force and during chromatin assembly.

Histone H1 binds to linker DNA between nucleosomes, but the dynamics and biological ramifications of this interaction remain poorly understood. We performed single-molecule experiments using magnetic tweezers to determine the effects of H1 on naked DNA in buffer or during chromatin assembly in Xenopus egg extracts. In buffer, nanomolar concentrations of H1 induce bending and looping of naked DNA at stretching forces below 0.6 pN, effects that can be reversed with 2.7-pN force or in 200 mM monovalent salt concentrations. Consecutive tens-of-nanometer bending events suggest that H1 binds to naked DNA in buffer at high stoichiometries. In egg extracts, single DNA molecules assemble into nucleosomes and undergo rapid compaction. Histone H1 at endogenous physiological concentrations increases the DNA compaction rate during chromatin assembly under 2-pN force and decreases it during disassembly under 5-pN force. In egg cytoplasm, histone H1 protects sperm nuclei undergoing genome-wide decondensation and chromatin assembly from becoming abnormally stretched or fragmented due to astral microtubule pulling forces. These results reveal functional ramifications of H1 binding to DNA at the single-molecule level and suggest an important physiological role for H1 in compacting DNA under force and during chromatin assembly.

Xenopus egg extracts increase dynamics of histone H1 on sperm chromatin.

BACKGROUND: Linker histone H1 has been studied in vivo and using reconstituted chromatin, but there have been few systematic studies of the effects of the cellular environment on its function. Due to the presence of many other chromatin factors and specific chaperones such as RanBP7/importin beta that regulate histone H1, linker histones likely function differently in vivo than in purified systems. METHODOLOGY/PRINCIPAL FINDINGS: We have directly compared H1 binding to sperm nuclei in buffer versus Xenopus egg extract cytoplasm, and monitored the effects of adding nuclear import chaperones. In buffer, RanBP7 decondenses sperm nuclei, while H1 binds tightly to the chromatin and rescues RanBP7-mediated decondensation. H1 binding is reduced in cytoplasm, and H1 exhibits rapid FRAP dynamics in cytoplasm but not in buffer. RanBP7 decreases H1 binding to chromatin in both buffer and extract but does not significantly affect H1 dynamics in either condition. Importin beta has a lesser effect than RanBP7 on sperm chromatin decondensation and H1 binding, while a combination of RanBP7/importin beta is no more effective than RanBP7 alone. In extracts supplemented with RanBP7, H1 localizes to chromosomal foci, which increase after DNA damage. Unlike somatic H1, the embryonic linker histone H1M binds equally well to chromatin in cytoplasm compared to buffer. Amino-globular and carboxyl terminal domains of H1M bind chromatin comparably to the full-length protein in buffer, but are inhibited ∼10-fold in cytoplasm. High levels of H1 or its truncations distort mitotic chromosomes and block their segregation during anaphase. CONCLUSION/SIGNIFICANCE: RanBP7 can decondense sperm nuclei and decrease H1 binding, but the rapid dynamics of H1 on chromatin depend on other cytoplasmic factors. Cytoplasm greatly impairs the activity of individual H1 domains, and only the full-length protein can condense chromatin properly. Our findings begin to bridge the gap between purified and in vivo chromatin systems.