Development and application of a volume penalization immersed boundary method for the computation of blood flow and shear stresses in cerebral vessels and aneurysms

A volume-penalizing immersed boundary method is presented for the simulation of laminar incompressible flow inside geometrically complex blood vessels in the human brain. We concentrate on cerebral aneurysms and compute flow in curved brain vessels with and without spherical aneurysm cavities attached. We approximate blood as an incompressible Newtonian fluid and simulate the flow with the use of a skew-symmetric finite-volume discretization and explicit time-stepping. A key element of the immersed boundary method is the so-called masking function. This is a binary function with which we identify at any location in the domain whether it is ‘solid’ or ‘fluid’, allowing to represent objects immersed in a Cartesian grid. We compare three definitions of the masking function for geometries that are non-aligned with the grid. In each case a ‘staircase’ representation is used in which a grid cell is either ‘solid’ or ‘fluid’. Reliable findings are obtained with our immersed boundary method, even at fairly coarse meshes with about 16 grid cells across a velocity profile. The validation of the immersed boundary method is provided on the basis of classical Poiseuille flow in a cylindrical pipe. We obtain first order convergence for the velocity and the shear stress, reflecting the fact that in our approach the solid-fluid interface is localized with an accuracy on the order of a grid cell. Simulations for curved vessels and aneurysms are done for different flow regimes, characterized by different values of the Reynolds number ( $Re$ ). The validation is performed for laminar flow at $Re=250$ , while the flow in more complex geometries is studied at $Re = 100$ and $Re = 250$ , as suggested by physiological conditions pertaining to flow of blood in the circle of Willis.     

A central location method for selecting random plots for vegetation surveys

A method is proposed for selecting points at random from a Cartesian grid and computing the distance and direction from the center of the field to the chosen point. This represents a convenient method for locating quadrats for vegetation surveys in the field. Two variations of this principle are discussed. One of these selects points from a square array, while the other selects points only from within a polygon of specified dimensions.     

Mapping Atmospheric Moisture Climatologies across the Conterminous United States

Spatial climate datasets of 1981–2010 long-term mean monthly average dew point and minimum and maximum vapor pressure deficit were developed for the conterminous United States at 30-arcsec (~800m) resolution. Interpolation of long-term averages (twelve monthly values per variable) was performed using PRISM (Parameter-elevation Relationships on Independent Slopes Model). Surface stations available for analysis numbered only 4,000 for dew point and 3,500 for vapor pressure deficit, compared to 16,000 for previously-developed grids of 1981–2010 long-term mean monthly minimum and maximum temperature. Therefore, a form of Climatologically-Aided Interpolation (CAI) was used, in which the 1981–2010 temperature grids were used as predictor grids. For each grid cell, PRISM calculated a local regression function between the interpolated climate variable and the predictor grid. Nearby stations entering the regression were assigned weights based on the physiographic similarity of the station to the grid cell that included the effects of distance, elevation, coastal proximity, vertical atmospheric layer, and topographic position. Interpolation uncertainties were estimated using cross-validation exercises. Given that CAI interpolation was used, a new method was developed to allow uncertainties in predictor grids to be accounted for in estimating the total interpolation error. Local land use/land cover properties had noticeable effects on the spatial patterns of atmospheric moisture content and deficit. An example of this was relatively high dew points and low vapor pressure deficits at stations located in or near irrigated fields. The new grids, in combination with existing temperature grids, enable the user to derive a full suite of atmospheric moisture variables, such as minimum and maximum relative humidity, vapor pressure, and dew point depression, with accompanying assumptions. All of these grids are available online at http://prism.oregonstate.edu, and include 800-m and 4-km resolution data, images, metadata, pedigree information, and station inventory files.     

Diversity, spatial arrangement, and endemism of Cactaceae in the Huizache area, a hot-spot in the Chihuahuan Desert

We studied several aspects of the cactus diversity in the Huizache, an area located in the southern extreme of the Chihuahuan Desert Region, in San Luis Potosí, Mexico. Fieldwork was conducted in a square-shaped polygon (22°30–23°00 N, 100°00–100°30 W), where a total of 80 systematically-chosen localities were sampled for cactus species. Results showed that the Huizache is an important focal point for the conservation of the Cactaceae, due to the remarkably high diversity of members of this plant family in the area. With 75 species recorded, this area has the highest concentration of cactus species in the American Continent, in comparison with other comparably-sized regions. The outstanding diversity of cactus species in the Huizache area may be explained by the relatively favorable climate of this desert area, its environmental heterogeneity, and its location in a transitional position between three regions with distinct cactus floras: Chihuahuan Desert Region, Queretaroan-Hidalgoan Arid Zone, and Tula-Jaumave Valleys. Species richness is not distributed evenly in the area; the zones of highest species concentration are primarily located in the lowland valleys, in the northwestern portion of the grid square, where typical Chihuahuan Desert conditions prevail. Of the 75 cactus species found in the area, 63% are endemic to the Chihuahuan Desert. Among these, ten species are outstanding for being endemic or nearly endemic to the Huizache area. The Huizache grid square is part of a recently declared natural reserve, The Real de Guadalcázar Natural Reserve.     

Interaction of the Aromatics Tyr-72/Trp-288 in the Interface of the Extracellular and Transmembrane Domains Is Essential for Proton Gating of Acid-sensing Ion Channels

Acid-sensing ion channels are proton-activated ion channels expressed in the nervous system. They belong to the family of ENaC/Degenerins whose members share a conserved structure but are activated by widely diverse stimuli. We show that interaction of two aromatic residues, Tyr-72, located immediately after the first transmembrane segment, and Trp-288, located at the tip of a loop of the extracellular domain directed toward the first transmembrane segment, is essential for proton activation of the acid-sensing ion channels. The subdomain containing Trp-288 is a module tethered to the rest of the extracellular domain by short linkers and intrasubunit interactions between residues in the putative “proton sensor.” Mutations in these two areas shift the apparent affinity of protons toward a more acidic range and change the kinetics of activation and desensitization. These results are consisting with displacement of the module relative to the rest of the extracellular domain to allow interaction of Trp-288 with Tyr-72 during gating. We propose that such interaction may provide functional coupling between the extracellular domain and the pore domain.The acid-sensing ion channels (ASICs)² are voltage-insensitive sodium channels turned on and off by extracellular protons. Four ASIC genes in the human genome, ASIC1 to ASIC4, give rise to at least six isoforms that associate in various combinations to form channels with different functional properties (1, 2). The ASICs constitute a distinct group in the large family of channels known as Degenerins characterized by a common structure but widely diverse gating stimuli: mechanical forces (3), neuropeptides (4, 5), protons (6), or no stimulus at all, such as ENaC, which exhibits constitutive activity (7). The structure shared by all Degenerins consists of two transmembrane segments, TM1 and TM2, a large extracellular domain, and short cytoplasmic amino and carboxyl termini. The recently published crystal structure of a truncated chicken ASIC1 (cASIC1) at a resolution of 1.9 Å (8) shows that ASIC1 is a trimer, and it provides detailed structure of the large extracellular domain that is crucial for understanding the gating mechanism of the ASICs. A feature revealed by the atomic structure is a cluster of negatively and one positively charged residue in the interface of subdomain D (Arg-191), subdomain E (Asp-238 and Glu-239), and subdomain F (Asp-346 and Asp-350) (see Fig. 1A) that was hypothesized to constitute the proton sensor. Furthermore, it was proposed that binding of protons to this site displaces subdomain F toward TM1 to open the pore (8).Open in a separate windowFIGURE 1.Ribbon representation of chicken ASIC1 structure. a, a single subunit is shown for simplicity with subdomains, A to F, indicated in different colors. The arrow points to the putative proton sensor with side chains of charged residues represented as sticks. Amino acids important for ASIC1 gating that were mutated in this study are also shown. The image was obtained with the molecular graphics program Chimera. b, amino acid sequence of subdomain F loop. Residues conserved in all ASIC proteins are in red.Although the solved atomic structure of cASIC1 provides a valuable tool to advance the understanding of how external protons activate the ASICs, it represents only a snapshot of the gating process thereby additional experimental evidence is needed to elucidate the gating mechanism. The general idea that conformational changes triggered by binding of the specific agonist to the extracellular domain of a ligand activated channel need to be transmitted to the transmembrane domain, where the pore gate is located, draws attention to the pair of closely located residues, Tyr-72 and Trp-288, as they provide a potential contact site between the extracellular and the transmembrane domains.This study examines the functional role of the conserved residues, Tyr-72 and Trp-288, that are located distantly in the primary sequence but are brought to close proximity (∼3.7 Å) by the folding of the extracellular domain (ECD). This arrangement could provide a contact site between the ECD and TM1 whereby a conformation change of the ECD is transmitted to the pore gate in the transmembrane domain.     

Submerged plant survival strategies in relation to management and environmental pressures in drainage channel habitats

The abundance of submerged weeds, in relation to management regime and environmental factors, was surveyed during 1992 and 1993 in drainage channels located in four geographically-distinct areas of Britain. The aim of the study was to ascertain, using a multivariate approach, the degree to which species survival strategy and vegetation could be related to disturbance and stress pressures on plant survival. Indices of disturbance and stress were constructed from combined environmental data for each site. A species ordination using Canonical Correspondence Analysis showed that the combined disturbance variable explained more of the variability that did stress. Two main groups of species could be distinguished. The larger group scored low on the disturbance gradient and these species, with different tolerances to stress (especially light-limitation), appeared to be those better-adapted to habitats with low disturbance (e.g. Potamogeton pectinatus and Potamogeton lucens). The smaller group comprised species which tended to occur in sites with higher disturbance (e.g. regular cutting) such as Callitriche stagnalis. Using the terminology of strategy theory, most of the dominant species could be classed as competitive/disturbance tolerators (CD) or variants of this established-phase strategy. The limitations are discussed of applying the strategy approach at species level in a defined habitat-type which shows a high degree of uniformity between sites, such as artificial drainage channels.     

Mutagenesis and Functional Analysis of Ion Channels Heterologously Expressed in Mammalian Cells

We will demonstrate how to study the functional effects of introducing a point mutation in an ion channel. We study G protein-gated inwardly rectifying potassium (referred to as GIRK) channels, which are important for regulating the excitability of neurons. There are four different mammalian GIRK channel subunits (GIRK1-GIRK4) - we focus on GIRK2 because it forms a homotetramer. Stimulation of different types of G protein-coupled receptors (GPCRs), such as the muscarinic receptor (M2R), leads to activation of GIRK channels. Alcohol also directly activates GIRK channels. We will show how to mutate one amino acid by specifically changing one or more nucleotides in the cDNA for the GIRK channel. This mutated cDNA sequence will be amplified in bacteria, purified, and the presence of the point mutation will be confirmed by DNA sequencing. The cDNAs for the mutated and wild-type GIRK channels will be transfected into human embryonic kidney HEK293T cells cultured in vitro. Lastly, whole-cell patch-clamp electrophysiology will be used to study the macroscopic potassium currents through the ectopically expressed wild-type or mutated GIRK channels. In this experiment, we will examine the effect of a L257W mutation in GIRK2 channels on M2R-dependent and alcohol-dependent activation.Download video file.^{(94M, mov)}     

Watching small molecules move: Interrogating ionic channels using neutral solutes

Whether they are small enough to wriggle through the current-carrying part of an ionic channel or big enough to be kept outside and thus able to exert an osmotic stress on the channel space, polymers interact with channels in several instructive ways. The osmotic stress of excluded polymers allows one to measure the number of water molecules that come out of the channel in transitions between various open to closed states. The loss of osmotic activity, due to the partial or completely unrestricted admission of small polymers becomes a measure of the transfer probabilities of polymers from solution to small cavities; it provides an opportunity to study polymer conformation in a perfectly sieved preparation. Current fluctuations due to the partial blockage by a transient polymer are converted into estimates of times of passage and diffusion constants of polymers in channels. These estimates show how a channel whose functional states last for milliseconds is able to average over the interactions with polymers, interactions that last only microseconds. One sees clearly that in this averaging, the macromolecular channel is large enough to react like a macroscopic object to the chemical potentials of the species that modulate its activity.     

Structure and colour-production of the blue barbs of Agapornis roseicollis and Cotinga maynana

Summary The spongy structure in medullary cells responsible for the colour of blue barbs in rump feathers of Agapornis roseicollis and back feathers of Cotinga maynana is studied with the scanning and the transmission electron microscope. The Agapornis structure is an irregular three-dimensional network of connected keratin rods which in many places form rings with outer diameters 0.25–0.3 . The air-filled space likewise consists of an irregular network of connected channels. The Cotinga structure consists of spherical cavities fairly evenly distributed in a keratin matrix.Earlier spectrophotometric measurements on the Agapornis structure have shown that the colour production has to be ascribed to the interference of light, not to Rayleigh (Tyndall) scattering as previously held. The optical path lengths corresponding to the outer diameter of the Agapornis rings and to the Cotinga cavities including their horny walls both are c. 0.4 . It is suggested that the colours are produced by the interference of light reflected from the front and rear surfaces of the rings, respectively the spherical cavities.I wish to thank Miss Annelise Nørgaard Jensen, Institute of Historical Geology and Palaeontology, University of Copenhagen, for operating the scanning electron microscope for me.     

Molecular Characterization of a Novel Family of Low Voltage-Activated, T-Type, Calcium Channels

Low voltage-activated, T-type, calcium channels are thought to be involved in pacemaker activity, low threshold Ca²⁺ spikes, neuronal oscillations and resonance, and rebound burst firing. Mutations in T-type channel genes may be a contributing factor to neurological and cardiovascular disorders, such as epilepsy, arrhythmia, and hypertension. Due to the lack of selective blockers, little is known about their structure or molecular biology. This review discusses our recent findings on the cloning, chromosomal localization, and functional expression, of two novel channels, 1G and 1H. The biophysical properties of these cloned channels (distinctive voltage dependence, kinetics, and single channel conductance) demonstrates that these channels are members of the T-type Ca²⁺ channel family.     

Empirical analysis of the removal rate of periphyton by grazers

To establish a general model for the removal rate of periphyton by grazers, we identified 27 publications in which removal rates could be estimated from grazer enclosure or exclosure experiments. When all the measurements obtained under different experimental conditions were extracted, these publications provided 107 data points. Multiple regression of these data showed that periphyton removal rate increased significantly with grazer body mass and food availability, and decreased with grazer crowding. Grazer body mass explained 65% of the variation, while crowding and food availability explained 7 and 6% respectively. Except for the significantly lower removal rate of amphibians, neither taxon of the grazer nor algal composition significantly affected removal rate. Experiments in the laboratory and in outdoor channels tended to give higher removal rates than experiments performed in streams or lakes. A comparison with previous allometric equations predicting the ingestion rate of other invertebrate guilds, and with experiments in which periphyton ingestion rate was measured as incorporation of labelled food, indicated that a large portion of periphyton is removed by the activity of the grazer rather than by direct ingestion. These results could be utilized to predict the impact of grazing on periphyton biomass.A contribution of the Groupe d'Écologie des eaux douces, Université de Montréal     

Lack of homogeneity of receptive fields of visual neurons in the cortical area 18 of the cat

The receptive fields of complex neurons within area 18 of the cerebral cortex of the cat were determined by a computer-assisted method using a moving light bar substantially shorter than the long diameter of the receptive field as a visual stimulus. The visual cells repeatedly generated nerve impulses when the stimulus crossed well-defined active points within their receptive fields. Outside of these active points, the cells remained silent. It is suggested that the receptive fields are formed by a discontinuous accumulation of such active points. When the electrical activities of two neighbouring visual neurons are recorded simultaneously, their active points do not coincide. In addition, some active points were located outside the most prominent excitatory part of the receptive field of the studied cells. Individual visual cells typically differ in the number and distribution of active points. Since these cells best respond to a stimulus moving in a certain direction, it is suggested that they may act as direction of movement and/or velocity detectors. Alternate firing of a number of neighboring cells connected to a distributed pattern of peripheral receptors may form a system which is able to code for velocity and direction of the moving stimulus.     

A simple simulation model of dispersal of animals among units of discrete habitats

Summary A simple simulation model of dispersal of animals among units of discrete habitats has been constructed. This model outputs the numbers of animals reaching each of twenty-five habitats at preselected loci in a grid under a variety of operating conditions. These conditions consist of variables controlling directionality, velocity and mortality during dispersal. The relationship between numbers of animals reaching a particular site and the distance of that site from the starting point was investigated. Similarly, the effects of directionality (as determined by the variance of the normal distribution used to assign directions of movement), mortality and velocity on the overall rate of success were studied. A negative exponential-type drop off in numbers of animals reaching a site was predicted with increasing distance from the starting point. The relationship between percent success, directionality and rate of mortality was predicted to be curvilinear; the percentage declining with decreased directionality and increased mortality. A positive linear relationship was indicated between the spatial displacement per iteration used and percent success.Wherever possible the assumptions and predictions of the model are compared with available data. The assumptions concerning initial and subsequent directions of movement were found to be borne out by the data examined as were the results showing the relationship between numbers at each site and distance from starting point. The other results of the simulations were more difficult to compare with available data and some of these remain to be tested.The advantages and disadvantages of the model and the prospects for its expansion are discussed. It is concluded that the model is simplistic but that this gives it a generality and versatility which make it a useful tool for further thinking and investigation on the process of dispersal.     

Role of cell-cycle in regulating neuroepithelial cell shape during bending of the chick neural plate

Summary Neuroepithelial cells transform from spindle-shaped to wedge-shaped within the median and paired dorsolateral hinge points of the bending neural plate, but the mechanisms underlying these localized changes are unclear. This study was designed to evaluate further the hypothesis that localized wedging of neuroepithelial cells during bending involves basal cellular expansion resulting from alteration of the cell-cycle. Neurulating chick embryos were treated with tritiated thymidine, and transverse sections through the midbrain were examined autoradiographically. Parameters of the cell-cycle as well as nuclear position and size were assessed in the median hinge point, which contains predominantly wedge-shaped cells, and in adjacent lateral areas of the neural plate, which contain predominantly spindle-shaped cells. Both the DNA-synthetic phase and non-DNA synthetic portion of the cell-cycle were significantly longer in the median hinge point than in lateral neuroepithelial areas, some nuclei in both regions were located basally during these phases, and virtually all basal nuclei in the median hinge point were large. Additionally, the mitotic phase was significantly shorter in the median hinge point than in lateral areas. We present a model to explain how alteration of the cell-cycle in the median hinge point could generate wedging of cells in this region.     

Quantifying the relative importance of experimental data points in parameter estimation

Background

Ordinary differential equations (ODEs) are often used to understand biological processes. Since ODE-based models usually contain many unknown parameters, parameter estimation is an important step toward deeper understanding of the process. Parameter estimation is often formulated as a least squares optimization problem, where all experimental data points are considered as equally important. However, this equal-weight formulation ignores the possibility of existence of relative importance among different data points, and may lead to misleading parameter estimation results. Therefore, we propose to introduce weights to account for the relative importance of different data points when formulating the least squares optimization problem. Each weight is defined by the uncertainty of one data point given the other data points. If one data point can be accurately inferred given the other data, the uncertainty of this data point is low and the importance of this data point is low. Whereas, if inferring one data point from the other data is almost impossible, it contains a huge uncertainty and carries more information for estimating parameters.

Results

G1/S transition model with 6 parameters and 12 parameters, and MAPK module with 14 parameters were used to test the weighted formulation. In each case, evenly spaced experimental data points were used. Weights calculated in these models showed similar patterns: high weights for data points in dynamic regions and low weights for data points in flat regions. We developed a sampling algorithm to evaluate the weighted formulation, and demonstrated that the weighted formulation reduced the redundancy in the data. For G1/S transition model with 12 parameters, we examined unevenly spaced experimental data points, strategically sampled to have more measurement points where the weights were relatively high, and fewer measurement points where the weights were relatively low. This analysis showed that the proposed weights can be used for designing measurement time points.

Conclusions

Giving a different weight to each data point according to its relative importance compared to other data points is an effective method for improving robustness of parameter estimation by reducing the redundancy in the experimental data.

     

A site accessible to extracellular TEA+ and K+ influences intracellular Mg2+ block of cloned potassium channels

The members of the RCK family of cloned voltage-dependent K⁺ channels are quite homologous in primary structure, but they are highly diverse in functional properties. RCK4 channels differ from RCK1 and RCK2 channels in inactivation and permeation properties, the sensitivity to external TEA, and to current modulation by external K⁺ ions. Here we show several other interesting differences: While RCK1 and RCK2 are blocked in a voltage and concentration dependent manner by internal Mg²⁺ ions, RCK4 is only weakly blocked at very high potentials. The single-channel current-voltage relations of RCK4 are rather linear while RCK2 exhibits an inwardly rectifying single-channel current in symmetrical K⁺ solutions. The deactivation of the channels, measured by tail current protocols, is faster in RCK4 by a factor of two compared with RCK2. In a search for the structural motif responsible for these differences, point mutants creating homology between RCK2 and RCK4 in the pore region were tested. The single-point mutant K533Y in the background of RCK4 conferred the properties of Mg²⁺ block, tail current kinetics, and inward ion permeation of RCK2 to RCK4. This mutant was previously shown to be responsible for the alterations in external TEA sensitivity and channel regulation by external K⁺ ions. Thus, this residue is expected to be located at the external side of the pore entrance. The data are consistent with the idea that the mutation alters the channel occupancy by K⁺ and thereby indirectly affects internal Mg²⁺ block and channel closing.Abbreviations TEA tetraethylammonium - EGTA Ethylene glycol-bis (-aminoethyl ether) N,N,N,N-tetraacetic acid - 2S3B model 2-site 3-barrier model Correspondence to: S. H. Heinemann     

Direct current electric field in some teleost species: Effect of medium salinity

Summary A direct current electric field up to 3 mV/ cm was recorded in 33 sea water around the fishMyoxocephalus brandti, Hexogrammos octogrammos, Enophrys diceraus, Pleuronectes stellatus, Bathimaste r derjugini, Sebastes scorpaeniformis. The body surface potentials were positive in relation to the external and internal media; they attained 10 mV and slowly varied near the mean value at every point. The potentials at the surface points of individual skin sections adjoining the oral and branchial cavities, the anal orifice and peripheral fin sections were normally characterized by polarities opposite to those of body surface potentials (in sea water they were negative in relation to the external medium).When placed in sea water during their fresh water cycle, the salmonOncorhynchus keta and the fresh water fishSalvelinus alpinus andMisgurnus fossilis had no d.c. field.In fresh water containing less than 0.03 salt, a d.c. field up to 25 mV/cm was recorded around all the above mentioned species. The potentials had an opposite polarity to that recorded in sea water.The distribution of potentials over the fish surface depends on the species. The potentials at some points of the body surfaces were found to vary when other fish or metal objects were placed in the aquarium.The parameters of the direct current electric field generated by a whole fish and by isolated skin pieces were identical and varied by the same law with changed medium salinity. Thus it may be assumed that the d.c. electric field around the fish is produced by active electrogenic ion transport mechanisms localized in the skin.     

Identifying dramatic selection shifts in phylogenetic trees

Background

The rate of evolution varies spatially along genomes and temporally in time. The presence of evolutionary rate variation is an informative signal that often marks functional regions of genomes and historical selection events. There exist many tests for temporal rate variation, or heterotachy, that start by partitioning sampled sequences into two or more groups and testing rate homogeneity among the groups. I develop a Bayesian method to infer phylogenetic trees with a divergence point, or dramatic temporal shifts in selection pressure that affect many nucleotide sites simultaneously, located at an unknown position in the tree.

Results

Simulation demonstrates that the method is most able to detect divergence points when rate variation and the number of affected sites is high, but not beyond biologically relevant values. The method is applied to two viral data sets. A divergence point is identified separating the B and C subtypes, two genetically distinct variants of HIV that have spread into different human populations with the AIDS epidemic. In contrast, no strong signal of temporal rate variation is found in a sample of F and H genotypes, two genetic variants of HBV that have likely evolved with humans during their immigration and expansion into the Americas.

Conclusion

Temporal shifts in evolutionary rate of sufficient magnitude are detectable in the history of sampled sequences. The ability to detect such divergence points without the need to specify a prior hypothesis about the location or timing of the divergence point should help scientists identify historically important selection events and decipher mechanisms of evolution.

     

Intercalary heterochromatin in Drosophila

Sites of intercalary heterochromatin (IH) in the complete set of Drosophila melanogaster polytene chromosomes were localized and studied according to the following criteria: tendency to break (weak points), ectopic pairing and late replication, the existence of repeats (in X and 2R) including those enriched with A-T bases. Correlation between these features investigated, the highest correlation coefficients found between weak point behavior, late replication, and ectopic pairing. The frequency of breaks in weak points in some IH bands was shown to be different in different tissues, strains and closely related Drosophila species. Sexual differences in morphology and manifestation of IH features were found in bands of the X chromosome: weak point behavior and participation in ectopic pairing of IH bands are an order of magnitude less frequent in male X chromosomes than in female X chromosomes. In autosomes such differences have not been observed. IH bands in male X chromosomes look more massive than the homologous ones in female X chromosomes: the DNA content of the 11A6-9 region is four times less in females than in males. The hypothesis is proposed that the specific features of intercalary heterochromatin bands are determined by tandem repetitiveness and late replication. The latter, if it occurs in a cluster of repetitions, could cause incomplete polytenization of the region and, as a consequence, breaks (or weak points) and the appearance of adhesive ends which may take part either in realization of ectopic contacts or in fixation of those occurring previously. Breaks caused by chromosome aberrations in regions with repeats may not result in a sharp decline of viability, so that break points of chromosome rearrangements in intercalary heterochromatin may be more frequent than in other regions.