Developmental rules in the hematopoietic and immune systems of birds: how general are they?

Fundamental rules about the development of the hematopoietic and immune systems have been established in birds, some of which are reviewed here. The organ rudiments belonging to this system provide the stroma, that has to be seeded by extrinsic stem cells. This particular developmental pattern makes it possible to create chimeric organs with stromal cells and stem cells from different origins. Grafting the thymic epithelium alone in a young embryo is sufficient to induce tolerance to tissues of the same origin as that of epithelium, even in a xenogeneic combination. Stem cells that seed the rudiments during development are born in the embryo rather than in the yolk sac and are responsible for definitive hematopoiesis. Data in frog and mouse indicate that these conclusions may be valid in other classes of vertebrates.     

Plant hormone interactions: how complex are they?

Models describing plant hormone interactions are often complex and web-like. Here we assess several suggested interactions within one experimental system, elongating pea internodes. Results from this system indicate that at least some suggested interactions between auxin, gibberellins (GAs), brassinosteroids (BRs), abscisic acid (ABA) and ethylene do not occur in this system or occur in the reverse direction to that suggested. Furthermore, some of the interactions are relatively weak and may be of little physiological relevance. This is especially true if plant hormones are assumed to show a log-linear response curve as many empirical results suggest. Although there is strong evidence to support some interactions between hormones (e.g. auxin stimulating ethylene and bioactive GA levels), at least some of the web-like complexities do not appear to be justified or are overstated. Simpler and more targeted models may be developed by dissecting out key interactions with major physiological effects.     

Crystal structure of mouse RhoA:GTPγS complex in a centered lattice

RhoA, a member of the Rho sub-family of small GTPases, plays a significant signaling role in cell morphogenesis, migration, neuronal development, cell division and adhesion. So far, 4 structures of RhoA:GDP/GTP analogs and 14 structures of RhoA in complex with other proteins have been reported. All RhoA:GDP/GTP analog complexes have been crystallized in primitive lattices and RhoA is monomeric. This is the first time a RhoA:GTP analog complex has been crystallized as a dimer in a centered lattice. The present structure reveals structural differences in the switch-I (residues 28?C42) and switch-II (residues 61?C66) regions, which play important roles in interactions with downstream targets to transduce signals, when compared to the previously reported structures.     

Hybridization in the Daphnia galeata complex: are hybrids locally produced?

Within the species complex of Daphnia galeata,D. cucullata and D. hyalina variouscombinations of hybrids and parental taxa occur inlakes throughout Europe. Since daphnids are cyclicparthenogens and mostly reproduce asexually, hybridpopulations can be maintained by asexual reproductionand without recurrent hybridization events. Therefore,it is possible that hybridization events have beenrare, with range expansion occurring by dispersal ofhybrids.Allozyme data from seven European populations wereused to compare genetic variation within and betweenhybrid and parental taxa. An UPGMA cluster analysis ofgenetic distances showed that D. cucullata × galeatahybrids from different lakes grouped indifferent clusters according to the lake from whichthey were isolated, suggesting multiple hybridizationevents. Clonal diversity within hybrid taxa wascomparable to parental taxa. Furthermore, evidence wasfound for introgression of the Pgi-S allele fromD. cucullata to D. galeata in three lakes.These results indicate that multiple hybridizationevents within this species complex are likely, andthat hybrid taxa can reproduce sexually.     

How polymorphic are class II loci of the mouse H-2 complex?

DNA was isolated from 75 mouse strains carrying classical H-2 haplotypes as well as haplotypes derived from wild mice. The DNA was digested with three restriction endonucleases, Bst EII, Eco RI, and Bam HI, the digests hybridized, using the Southern blotting technique, with probes for the class II genes A , A, E, and E , and the restriction fragment length polymorphism at these loci determined. The analysis revealed that the most polymorphic of the four loci is A , followed by E , and, at a different level, by E and A . There is a large difference in the degree of polymorphism between the A and E genes, on the one hand, and the A and E genes, on the other hand. There is no difference in the degree of polymorphism between the A and E genes. These findings do not substantiate previous postulates of a high A polymorphism and they do not agree with the hypothesis that the class II region is divided into highly polymorphic centromeric and less polymorphic telomeric subregions. Rather, it appears that the differences in the degree of polymorphism of the different segments of the class II region are determined by the class II loci themselves. The polymorphism of the less polymorphic class II genes is, however, still greater than the polymorphism of certain other genes on chromosome 17, notably the 4-globin pseudogene. The distribution of polymorphisms at the A and E loci suggests that even populations occupying relatively small geographical regions differ in alleles at these loci. Sharing of A alleles between unrelated populations is yet to be detected. A certain degree of linkage disequilibrium exists among the A , A , and E loci; by contrast, the E locus appears to vary largely independently of the other class II loci.     

Embryonic cleavage cycles: how is a mouse like a fly?

The evolutionary advent of uterine support of embryonic growth in mammals is relatively recent. Nonetheless, striking differences in the earliest steps of embryogenesis make it difficult to draw parallels even with other chordates. We suggest that use of fertilization as a reference point misaligns the earliest stages and masks parallels that are evident when development is aligned at conserved stages surrounding gastrulation. In externally deposited eggs from representatives of all the major phyla, gastrulation is preceded by specialized extremely rapid cleavage cell cycles. Mammals also exhibit remarkably fast cell cycles in close association with gastrulation, but instead of beginning development with these rapid cycles, the mammalian egg first devotes itself to the production of extraembryonic structures. Previous attempts to identify common features of cleavage cycles focused on post-fertilization divisions of the mammalian egg. We propose that comparison to the rapid peri-gastrulation cycles is more appropriate and suggest that these cycles are related by evolutionary descent to the early cleavage stages of embryos such as those of frog and fly. The deferral of events in mammalian embryogenesis might be due to an evolutionary shift in the timing of fertilization.     

Just how complex is the Brassica S-receptor complex?

Of the plant self-incompatibility (SI) systems investigated to date, that possessed by members of the Brassicaceae is currently the best understood. Whilst the recent demonstrations of interactions between the male determinant (S-locus cysteine rich protein, SCR) and the female determinant (S-locus receptor kinase, SRK) indicate the minimal requirement for SI in Brassica, no consensus exists as to the nature of these molecules in vivo and the potential involvement of accessory molecules in establishing the active S-receptor complex. Variation between S haplotypes appears to be present in the molecular composition of the receptor complex, the regulation of downstream signalling and the requirement for accessory molecules. This review discusses what constitutes an active receptor complex and highlights potential differences between haplotypes. The role of accessory molecules, in particular SLG (S-locus glycoprotein) and low molecular weight pollen coat proteins (PCPs), in pollination are discussed, as is the link between SI and unilateral incompatibility (UI).     

Cranial growth in Homo erectus: how credible are the Ngandong juveniles?

Confusion exists regarding the developmental ages of numerous Asian and southeast Asian Homo erectus fossils because of Weidenreich's contention that Pithecanthropus fused its sutures prematurely relative to H. sapiens. I reevaluate the cranial developmental ages of the Ngandong “juveniles” (2, 5, 8, 9) based on a series of indicators of youth (superstructure development, suture development/fusion, and cranial thickness) and cranial contours. The Ngandong juveniles are compared with H. sapiens adults (n = 281) and subadults (n = 81) and with Ngandong and other H. erectus adults (n = 20) and subadults (n = 4). Cranial contours are assessed using bivariate plots of arc vs. chord measurements. All indicators suggest that Ngandong 5 and 9 are adults, whereas Ngandong 8 is an older juvenile or young adult and Ngandong 2 is a juvenile with a developmental age range of greater than 6 and less than 11 years. In addition, adult cranial contours and the pattern of contour development are similar between Ngandong adults and other H. erectus adults. There is nothing in the cranial contour data to suggest that Ngandong is, despite a relatively large brain, transitional in vault shape between H. erectus and H. sapiens. Am J Phys Anthropol 108:223–236, 1999. © 1999 Wiley-Liss, Inc.     

After the ribosome structures: how are the subunits assembled?

The recent structures of the ribosome and the ribosomal subunits only heighten the intrigue of trying to understand how the ribosome is assembled. Biochemical and mechanistic studies have mapped out the basic series of protein binding events that occur, but we do not yet have a clear picture of the RNA conformational changes that must accompany the protein binding. Recent studies point to roles of protein folding chaperones and RNA helicases as facilitators of ribosome assembly, but the basic process of assembly seems to be encoded in the RNA sequences and can occur for the most part spontaneously in vitro, and quite possibly in vivo as well.     

Toxocara in the mouse: a model for parasite-altered host behaviour?

The objective of this paper is to critically evaluate the significance of parasite-altered host behaviour in the Toxocara mouse model particularly in the light of the Manipulation Hypothesis. Murine behaviours were examined in both outbred and inbred strains of mice infected with different doses of Toxocara canis ova. Behaviours investigated included activity, exploration, response to novelty, anxiety, learning, memory and social behaviour. Subsequent modifications to the behaviour of infected mice were investigated with respect to dose administered and larval accumulation in the brain. There was substantial variation in the number of larvae recovered from brains of individual mice, which received similar doses of Toxocara ova. Furthermore, the numbers of larvae recovered at different doses differed significantly between an outbred and inbred strain of mouse. Alterations in infected host behaviour occurred and were related to the number of larvae recovered from the brain. For social behaviour in outbred mice, a high infection in the brain reduced levels of aggressive behaviour and increased levels of flight and defensive behaviours. In contrast, outbred mice with a low infection in the brain displayed a greater level of risk behaviour in respect of predator odour and the light/dark box compared to control or high infection mice. Post-infection, outbred mice were more immobile whereas inbred mice showed reduced immobility and increased digging and climbing. Impaired learning ability was observed in outbred mice with moderate and high levels of infection in the brain compared to control and low infection mice. Toxocara infection has an impact upon a diverse range of murine behaviours with little evidence for a specific and hence an adaptive alteration. Many of the effects on murine host behaviour by Toxocara are likely to be pathological side effects of infection rather than as a consequence of adaptive host-manipulation. Observed changes in murine behaviour may be relevant to human toxocariasis.     

After the ribosome structure: how does translocation work?

Structures of the ribosomal large and small subunits have been solved to atomic resolution by X-ray crystallography. These structures provide a new foundation to address the complex process of protein biosynthesis by the ribosome. Translocation of the tRNA-mRNA complex is one of the most fascinating tasks performed by the ribosome. The impact of the crystal structures in understanding the molecular mechanism of translocation is highlighted in this review.     

The koala-forest-management interactive game: are there lessons in modelling a complex system?

[First paragraph]Koalas are considered to be overabundant in some areas of their natural range. This is the case at Mount Eccles National Park, Victoria, Australia. The concern at Mount Eccles National Park is that an overabundant koala population will have deleterious impacts on the unique forest community as well as leading to a koala population collapse. Due to social and political constraints, the only available management technique is to sterilise adult female koalas. Two questions of interest to management are: (1) Can sterilisation reduce the likelihood of both a decline in the forest community and a severe collapse in the koala population? (2) Can management sterilise enough koalas such that the likelihood is sufficiently reduced?     

Measuring acoustic complexity in continuously varying signals: how complex is a wolf howl?

Communicative complexity is a key behavioural and ecological indicator in the study of animal cognition. Much attention has been given to measures such as repertoire size and syntactic structure in both bird and mammal vocalizations, as large repertoires and complex call combinations may give an indication of the cognitive abilities both of the sender and receiver. However, many animals communicate using a continuous vocal signal that does not easily lend itself to be described by concepts such as ‘repertoire’. For example, dolphin whistles and wolf howls both have complex patterns of frequency modulation, so that no two howls or whistles are quite the same. Is there a sense in which some of these vocalizations may be more ‘complex’ than others? Can we arrive at a quantitative metric for complexity in a continuously varying signal? Such a metric would allow us to extend familiar analyses of communicative complexity to those species where vocal behaviour is not restricted to sequences of stereotyped syllables. We present four measures of complexity in continuous signals (Wiener Entropy, Autocorrelation, Inflection Point Count, and Parsons Entropy), and examine their relevance using example data from members of the genus Canis. We show that each metric can lead to different conclusions regarding which howls could be considered complex or not. Ultimately, complexity is poorly defined and researchers must compare metrics to ensure that they reflect the properties for which the hypothesis is being tested.     

Discrimination of the elderly: how guilty are you?

In the Netherlands a recent discussion on ‘the right to die’ and premature ending of life for people over the age of 70 has only highlighted one side of the story. The tale of a situation of bad health, depression and physical ailments, which progresses to, worst of all, a painful and loveless ending of life, has often been told and scares us literally to death. It may distract us from a far bigger and more complex problem.     

Lipid microdomains in model and biological membranes: how strong are the connections?

The concept of 'lipid rafts' and related liquid-ordered membrane microdomains has attracted great attention in the field of membrane biology, both as a novel paradigm in models of membrane organization and for the potential importance of such domains in phenomena such as membrane signaling and the differential trafficking of various membrane components. Studies of biological and of model membranes have gone hand in hand in shaping our current picture of the possible organization and functions of liquid-ordered lipid microdomains in membranes. This essay discusses some important current questions concerning the existence and functional importance of lipid microdomains in mammalian cell membranes, and the potential as well as the limitations of using model systems to help to address such questions.     

Detection of short pure-tone stimuli in the noctuid ear: what are temporal integration and integration time all about?

The interception of a pheromone filament induces flying moths to surge briefly nearly straight upwind; in the absence of pheromone moths cease upwind progress and zigzag crosswind. We tested males of the almond moth, Cadra cautella (Lepidoptera, Pyralidae), in a low-turbulence wind tunnel in wind velocities of 20, 40 and 80 cm s⁻¹. A mechanical pulse generator was set to produce plumes either with same pheromone pulse frequency (pulse generation frequency of 2.9 Hz, interpulse distances from 7 cm to 28 cm) or plumes with same interpulse distance across the three wind velocities (interpulse distance of 14 ± 2 cm, pulse generation frequency of 1.7–5.0 Hz). In plumes of similar pulse frequency, the faster the speed of the wind the slower the ground speed of flight. However, in plumes of similar interpulse distance, ground speed remained relatively constant independent of the wind speed. A `realized' frequency of pulse interception for males flying along the various combinations of pulse frequencies and wind velocities was calculated using the males' average airspeed and the spatial distribution of pheromone pulses in the plume. Realized frequency of pulse interception ranged from 1.3- to 3.0-fold higher than the frequency of pulse generation. The flight tracks of males reflected the regime of realized pulse interception. These results suggest that upwind flight orientation of male C. cautella to pheromone in different wind velocities is determined by the flux of filament encounter. Accepted: 3 September 1997     

How biologically relevant are interaction-based modules in protein networks?

By applying a graph-based algorithm to yeast protein-interaction networks we have extracted modular structures and show that they can be validated using information from the phylogenetic conservation of the network components. We show that the module cores, the parts with the highest intramodular connectivity, are biologically relevant components of the networks. These constituents correlate only weakly with other levels of organization. We also discuss how such structures could be used for finding targets for antimicrobial drugs.