The stele – a developmental perspective on the diversity and evolution of primary vascular architecture

The stele concept is one of the oldest enduring concepts in plant biology. Here, I review the history of the concept and build an argument for an updated view of steles and their evolution. Studies of stelar organization have generated a widely ranging array of definitions that determine the way we classify steles and construct scenarios about the evolution of stelar architecture. Because at the organismal level biological evolution proceeds by changes in development, concepts of structure need to be grounded in development to be relevant in an evolutionary perspective. For the stele, most traditional definitions that incorporate development have viewed it as the totality of tissues that either originate from procambium – currently the prevailing view – or are bordered by a boundary layer (e.g. endodermis). Consensus between these two perspectives can be reached by recasting the stele as a structural entity of dual nature. Following a brief review of the history of the stele concept, basic terminology related to stelar organization, and traditional classifications of the steles, I revisit boundary layers from the perspective of histogenesis as a dynamic mosaic of developmental domains. I review anatomical and molecular data to explore and reaffirm the importance of boundary layers for stelar organization. Drawing on information from comparative anatomy, developmental regulation, and the fossil record, I propose a stele concept that integrates both the boundary layer and the procambial perspectives, consistent with a dual nature of the stele. This dual stele model posits that stelar architecture is determined at the apical meristem by two major cell fate specification events: a first one that specifies a provascular domain and its boundaries, and a second event that specifies a procambial domain (which will mature into conducting tissues) from cell subpopulations of the provascular domain. If the position and extent of the developmental domains defined by the two events are determined by different concentrations of the same morphogen (most likely auxin), then the distribution of this organizer factor in the shoot apical meristem, as modulated by changes in axis size and the effect of lateral organs, can explain the different stelar configurations documented among tracheophytes. This model provides working hypotheses that incorporate assumptions and generate implications that can be tested empirically. The model also offers criteria for an updated classification of steles in line with current understanding of plant development. In this classification, steles fall into two major categories determined by the configuration of boundary layers: boundary protosteles and boundary siphonosteles, each with subtypes defined by the architecture of the vascular tissues. Validation of the dual stele model and, more generally, in-depth understanding of the regulation of stelar architecture, will necessitate targeted efforts in two areas: (i) the regulation of procambium, vascular tissue, and boundary layer specification in all extant vascular plants, considering that most of the diversity in stelar architecture is hosted by seed-free plants, which are the least explored in terms of developmental regulation; (ii) the configuration of vascular tissues and, especially, boundary layers, in as many extinct lineages as possible.     

Les cétacés des gravures rupestres néolithiques de Bangu-dae (Corée du Sud) et les débuts de la chasse à la baleine dans le Pacifique nord-ouest

The Neolithic site of Bangu-dae (6000-1000 B.C.), south-east of the Korean peninsula, presents numerous rock carvings of cetaceans (n = 46, 19.9% of the figures), and especially large species. Among these two families of baleen whales (Balaenidae and Balaenopteridae) and the sperm whale can be identified. This site also presents whaling scenes suggesting that the Neolithic populations who lived along the coast of Korea were among the first to take advantage of coastal whales. All these carvings give exceptional evidence on the prehistoric beginning of whaling.     

La crosse,point d’interrogation ? Poursuite de l’analyse d’un signe néolithique,notamment à Locmariaquer (Morbihan)

In the European corpus of the recent Prehistory, the sign “crosse” or crook is most often limited to its esoteric sense (insignia of power) or in a exoteric sense (shepherd's stick, sickle) without true explicitation of choices, otherwise by an assumption relating to the sociopolitical and economic organisation of the Neolithic society. One will set out to show the complexity of the sign within the representations of Vth and IVth millenniums, notably in Brittany and precisely at La Table des Marchands monumental site. Its synchronous functioning with the hafted axe, itself distracted from its function admitted as a woodcutter's tool translating by habit an agricultural activity, dictates on the contrary a less quiet usage, far from a productive pastoral character. The weapon seems to be a more receivable hypothesis to understand the structural coherence of engravings on steles. The origin of words used since the XIXth century will be discussed; then will be proposed a european inventory of the different true objects included as throwing sticks or boomerangs; others representations out of Brittany will be evoked to help in the resolution of Neolithic signs. So, more cryptic than the stone or metal hafted axe which shows its first usage, more ambiguous than the right stick of the walker or the flail of the farmer, the object “crook” does not end up being reactivated throughout millenniums, as pictures reach us successively.     

The shared reward dilemma

One of the most direct human mechanisms of promoting cooperation is rewarding it. We study the effect of sharing a reward among cooperators in the most stringent form of social dilemma, namely the prisoner's dilemma (PD). Specifically, for a group of players that collect payoffs by playing a pairwise PD game with their partners, we consider an external entity that distributes a fixed reward equally among all cooperators. Thus, individuals confront a new dilemma: on the one hand, they may be inclined to choose the shared reward despite the possibility of being exploited by defectors; on the other hand, if too many players do that, cooperators will obtain a poor reward and defectors will outperform them. By appropriately tuning the amount to be shared a vast variety of scenarios arises, including the traditional ones in the study of cooperation as well as more complex situations where unexpected behavior can occur. We provide a complete classification of the equilibria of the n-player game as well as of its evolutionary dynamics.     

Complément d'inventaire des représentations magdaléniennes de l'Abri du Colombier, dans les gorges de l'Ardèche (commune de Vallon), par trois gravures inédites de Bouquetins

The discovery of four representations of Ibex in stratigraphy, each carved into rock found in fragments from the rock wall, raises the number of specimens of Ibex carvings found in l’Abri du Colombier to fifteen, of which at least two have been precisely dated. Absolute dating data obtained from the fragments, coupled with a meticulous technical and stylistic analysis of these new carvings, by means of a step-by-step comparative study of those, which already exist on the rock walls, allows for a good chronological approach to the decoration found in l’Abri du Colombier. The theory behind these fragmented carvings comes under the scrutiny of J. Combier, who explains that the representations remaining on the rock wall are: “…but remnants of a more widespread decoration, even if they did develop in the area around the cave itself” (1984). Moreover, the wealth and characteristics of the palaeolithic deposits, faunal remnants and technical evidence, together with the argument of a chronological approach to such artistic manifestations allows for an understanding of the human activities, which took place in l’Abri du Colombier.     

Gravures et abstraction: Les gravures non figuratives de Roc-la-Tour I, le schiste gravé des Beaux-Sarts et les progrès de l’abstraction durant la Préhistoire et l’Histoire

Beyond the 75 animal and human pictures, the 491 Magdalenian engraved plaquettes bear 410 abstract signs, in 14 different types, among which eight were unknown up to now in South-Western France, (more than 130 cases). Some examples of all those signs are shown. Besides there are 213 groups of parallel lines. At “Les Beaux-Sarts”, less than 2 m from the hut described in 2003, another human excavation (maybe a tomb?) delivered a schist plaquette with 94 parallel lines carved on its surface, the lines appear as three groups of 14, 7 (or 8?) and 24 (or 28?) lines and one group of 17 (or 18?) oblique lines, with various secondary lines. Five thousand years before the cerebral center of precise calculation was at the beginning of its formation, this piece, out of the bases of which calculation might be subdivided, cannot be considered as the evidence that they were able to count. The author analyses the affective mental possibilities proved by those elements and puts them apart from the rational progress in the abstraction possibilities of the human brain, which occurred later all through ten thousand years.     

Why weight?

Whether phylogenetic data should be differentially or equally weighted is currently debated. Further, if differential weighting is to be explored, there is no consensus among investigators as to which weighting scheme is most appropriate. Mitochondrial genome data offer a powerful tool in assessment of differential weighting schemes because taxa can be selected from which a highly corroborated phylogeny is available (so that accuracy can be assessed), and it can be assumed that different data partitions share the same history (so that gene-sorting issues are not so problematic). Using mitochondrial data from 17 mammalian genomes, we evaluated the most commonly used weighting schemes, such as successive weighting, transversion weighting, codon-based weighting, and amino acid coding, and compared them to more complex weighting schemes including a 6-parameter weighting, pseudoreplicate reweighting, and tri-level weighting. We found that the most commonly used weighting schemes perform the worst with these data. Some of the more complex schemes perform well, however, none of them is consistently superior. These results support ones biases; if one has a predilection to avoid differential weighting, these data support equally weighted parsimony and maximum likelihood. Others might be encouraged by these results to try weighting as a form of data exploration.     

The genetic network of greater sage‐grouse: Range‐wide identification of keystone hubs of connectivity

Genetic networks can characterize complex genetic relationships among groups of individuals, which can be used to rank nodes most important to the overall connectivity of the system. Ranking allows scarce resources to be guided toward nodes integral to connectivity. The greater sage‐grouse (Centrocercus urophasianus) is a species of conservation concern that breeds on spatially discrete leks that must remain connected by genetic exchange for population persistence. We genotyped 5,950 individuals from 1,200 greater sage‐grouse leks distributed across the entire species’ geographic range. We found a small‐world network composed of 458 nodes connected by 14,481 edges. This network was composed of hubs—that is, nodes facilitating gene flow across the network—and spokes—that is, nodes where connectivity is served by hubs. It is within these hubs that the greatest genetic diversity was housed. Using indices of network centrality, we identified hub nodes of greatest conservation importance. We also identified keystone nodes with elevated centrality despite low local population size. Hub and keystone nodes were found across the entire species’ contiguous range, although nodes with elevated importance to network‐wide connectivity were found more central: especially in northeastern, central, and southwestern Wyoming and eastern Idaho. Nodes among which genes are most readily exchanged were mostly located in Montana and northern Wyoming, as well as Utah and eastern Nevada. The loss of hub or keystone nodes could lead to the disintegration of the network into smaller, isolated subnetworks. Protecting both hub nodes and keystone nodes will conserve genetic diversity and should maintain network connections to ensure a resilient and viable population over time. Our analysis shows that network models can be used to model gene flow, offering insights into its pattern and process, with application to prioritizing landscapes for conservation.     

The Radial Exchange of Labelled Water in Isolated Steles of Maize Roots

The kinetics of the efflux of labelled water from isolated stelesof maize roots have been studied.Analysis of the efflux kineticsindicated that the rate-limiting step was possibly permeationacross cell ‘membranes‘. On this basis we have computedfrom the efflux curve that the ‘typical‘ membranepermeability of stelar cells to water is about 1.8x10^-5 cm s^-1;thus, the cells in the stele are about four times more permeableto water than the cortical cells. The water permeability ofthe stelar cells, however, may have been substantially alteredby the extraction of steles from the cortical sleeves. Similarefflux studies were performed on ‘dead‘ steles preparedby boiling the normal steles for a short time. These experimentsdemonstrated that diffusion within the tissue and external ‘unstirredlayer‘ was rate-controlling the initial stage of the effluxfrom ‘dead‘ steles.A common feature of both effluxcurves for normal and ‘dead‘ steles was that eachdeviated from theoretical cylindrical diffusion curves for largevalues of time. Certain experimental evidence suggested thatthis deviation resulted from the existence of a slowly exchangingunidentified compartment occupying at least 8 per cent of thestelar volume.     

Cartographie des restes osseux et conservation différentielle dans l’amas coquillier d’Ouessant « Mez Notariou » (du Bronze moyen à la période romaine)

This article addresses the impacts of differential preservation in an alkaline shell midden overlying an acid soil which does not allow bone to survive. This midden accumulated over an extended period and displays unusual traits attributable to practices of a ritual nature (Le Bihan and Méniel, 2002). Excavation extended to approximately 1000 m². Bone was collected on a quarter metre grid by individual context. Some 35,000 animal and bird bones were recovered; fish bones, which were very numerous, are presently still under study. Consideration of the entire bone assemblage indicates that the principal deposition occurred along the SW/NE axis of the midden, with a lesser dump on the NW (zone XLI). The sector with the greatest concentration of bones covered some 50 m²; there, fragile remains, notably those of birds and fish, were at their most frequent. The distribution of ovicaprid bones is more uniform. Cattle and pigs are more frequent on the edges of the midden. Overall, there was a noticeable gradient in the quality of bone preservation between the core and the periphery of the deposit. The principal dump of bones displays two remarkable characteristics: a great quantity of shoulder blades and a preponderance of right-side remains over their left-side equivalents. The chronology of the accumulation of the midden shows four major phases: Bronze Age, Hallstatt Iron Age, La Tène Iron Age and Gallo-Roman. Plotting the bones by phase shows that the prime area of the deposit moves from NE to SW over time. It is thus possible to track the evolving rules of selection of species, individuals or animal parts from their distributions on the ground. This is particularly true for domestic pig, which increases in importance over time, and is encountered especially towards the south of the midden. Some bones show clear signs of alteration attributable to their exposure to the weather, making it plain that the midden was not covered with soil during its accumulation. The impact of such degradation on the bones can be very visible. The fact that some bones display signs of weather-related damage on more than one facet indicates that there was considerable secondary manipulation of this material. This study demonstrates the value of coupling a standardized but rapid method of data collection (by quarter metre squares) with detailed cartographic analysis of the material recovered. Both the taphonomic effects on a deposit built up over a very long time scale and continuity in certain practices (preferential selection of right shoulder blades) are clear from an initial consideration of the data.     

Ambrosiodmus rubricollis (Eichhoff) (Coleoptera; Curculionidae; Scolytinae) associated with young Tasmanian blue gum trees

Eucalypts are among the most widely planted forest trees in the world, and outside their native Australian range, the main arthropod pests are sap‐sucking insects, defoliators, gall‐making insects and xylophagous beetles. We report on a new association between a polyphagous wood‐boring beetle and Tasmanian blue gum (Eucalyptus globulus Labill.) in Central Portugal. Unidentified wood‐boring insects were found attacking two three‐year‐old E. globulus trees showing signs of decline among otherwise healthy trees in a commercial plantation, in June 2018. Declining trees presented dead twigs and branches, and recently developed epicormic sprouts evident on the trunks. Insects emerging from logs were identified as Ambrosiodmus rubricollis (Eichhoff), a species native to eastern and southern Asia, with the taxonomic identification validated by molecular analysis. To our knowledge, this is the first record of A. rubricollis associated with a eucalypt species worldwide. It is not clear whether the beetles played any significant role on the decline of the trees, but Ambrosiodmus may be potential pests for several tree and shrub species in Europe, as these beetles can transport pathogenic fungi.     

Alpha male chimpanzee grooming patterns: implications for dominance “style”

In social primates, individuals use various tactics to compete for dominance rank. Grooming, displays and contact aggression are common components of a male chimpanzee's dominance repertoire. The optimal combination of these behaviors is likely to differ among males with individuals exhibiting a dominance “style” that reflects their tendency to use cooperative and/or agonistic dominance tactics. Here, we examine the grooming behavior of three alpha male chimpanzees at Gombe National Park, Tanzania. We found that (1) these males differed significantly in their tendency to groom with other males; (2) each male's grooming patterns remained consistent before, during and after his tenure as alpha, and (3) the three males tended to groom with high‐ middle‐ and low‐ranking partners equally. We suggest that body mass may be one possible determinant of differences in grooming behavior. The largest male exhibited the lowest overall grooming rates, whereas the smallest male spent the most time grooming others. This is probably because large males are more effective at physically intimidating subordinates. To achieve alpha status, a small male may need to compensate for reduced size by investing more time and energy in grooming, thereby ensuring coalitionary support from others. Rates of contact aggression and charging displays conformed to this prediction, suggesting that each male exhibited a different dominance “style.” Am. J. Primatol. 71:136–144, 2009. © 2008 Wiley‐Liss, Inc.     

DEVELOPMENTAL DYNAMICS OF ARBORESCENT LYCOPHYTES—APICAL AND LATERAL GROWTH IN STIGMARIA FICOIDES

Meristematic activity in Stigmaria ftcoides is interpreted from a combination of historical information and newly developed data on stelar diameter, wood production, and morphology of the apex. Many specimens of Stigmaria preserved as mold/casts display little or no recognizable change in the width of the stelar groove between successive forks. Thickness of the xylem cylinder in permineralized axes of similar diameters also is relatively uniform. Over a distance of 1.5 m along one axis the mean number of tracheids per radial row diminishes from 56 to 46 toward the apex. While this is a statistically significant decrease, in biological terms it is consistent with the reduction in primary growth that one would expect in a zone of apoxogenesis. Coupled with our knowledge of the morphology of the apex, these data allow us to interpret radially aligned tracheids in the steles of most stigmarian axes as having been produced largely from the action of a primary thickening meristem, rather than from a secondary vascular cambium as commonly believed.     

Éléments d'une analyse sémiologique des gravures rupestres du Haut Atlas (vallée de l'Ourika, Maroc)

The semiological study of a set of rock engravings such as the one in the Haut Atlas would consist in studying the graphic elements within their context to determine their mutual relations, because such relations are meaningful. Some details could be signs to which we could attach a conventional meaning. But, being unable to identify these conventional signs by their meaning, we must look for similarities between the figures themselves. Indeed, the repetition of some relations will provide constitutive elements of the global meaning. Unfortunately, ignoring the relative positions of the figures, I can only give some indications about what such a study should entail. So, I focus on one hand on the attributes of the figures, which are, in any case, identity marks, and on the other hand, on the context of the figures, that is the graphics around the figure, in order to outline the likely semantic field and the actions, actual or symbolic, in which they are involved.     

Equally Parsimonious Pathways Through an RNA Sequence Space Are Not Equally Likely

An experimental system for determining the potential ability of sequences resembling 5S ribosomal RNA (rRNA) to perform as functional 5S rRNAs in vivo in the Escherichia coli cellular environment was devised previously. Presumably, the only 5S rRNA sequences that would have been fixed by ancestral populations are ones that were functionally valid, and hence the actual historical paths taken through RNA sequence space during 5S rRNA evolution would have most likely utilized valid sequences. Herein, we examine the potential validity of all sequence intermediates along alternative equally parsimonious trajectories through RNA sequence space which connect two pairs of sequences that had previously been shown to behave as valid 5S rRNAs in E. coli. The first trajectory requires a total of four changes. The 14 sequence intermediates provide 24 apparently equally parsimonious paths by which the transition could occur. The second trajectory involves three changes, six intermediate sequences, and six potentially equally parsimonious paths. In total, only eight of the 20 sequence intermediates were found to be clearly invalid. As a consequence of the position of these invalid intermediates in the sequence space, seven of the 30 possible paths consisted of exclusively valid sequences. In several cases, the apparent validity/invalidity of the intermediate sequences could not be anticipated on the basis of current knowledge of the 5S rRNA structure. This suggests that the interdependencies in RNA sequence space may be more complex than currently appreciated. If ancestral sequences predicted by parsimony are to be regarded as actual historical sequences, then the present results would suggest that they should also satisfy a validity requirement and that, in at least limited cases, this conjecture can be tested experimentally. Received: 27 August 1996 / Accepted: 14 April 1997     

On Error Minimization in a Sequential Origin of the Standard Genetic Code

Distances between amino acids were derived from the polar requirement measure of amino acid polarity and Benner and co-workers' (1994) 74-100 PAM matrix. These distances were used to examine the average effects of amino acid substitutions due to single-base errors in the standard genetic code and equally degenerate randomized variants of the standard code. Second-position transitions conserved all distances on average, an order of magnitude more than did second-position transversions. In contrast, first-position transitions and transversions were about equally conservative. In comparison with randomized codes, second-position transitions in the standard code significantly conserved mean square differences in polar requirement and mean Benner matrix-based distances, but mean absolute value differences in polar requirement were not significantly conserved. The discrepancy suggests that these commonly used distance measures may be insufficient for strict hypothesis testing without more information. The translational consequences of single-base errors were then examined in different codon contexts, and similarities between these contexts explored with a hierarchical cluster analysis. In one cluster of codon contexts corresponding to the RNY and GNR codons, second-position transversions between C and G and transitions between C and U were most conservative of both polar requirement and the matrix-based distance. In another cluster of codon contexts, second-position transitions between A and G were most conservative. Despite the claims of previous authors to the contrary, it is shown theoretically that the standard code may have been shaped by position-invariant forces such as mutation and base content. These forces may have left heterogeneous signatures in the code because of differences in translational fidelity by codon position. A scenario for the origin of the code is presented wherein selection for error minimization could have occurred multiple times in disjoint parts of the code through a phyletic process of competition between lineages. This process permits error minimization without the disruption of previously useful messages, and does not predict that the code is optimally error-minimizing with respect to modern error. Instead, the code may be a record of genetic process and patterns of mutation before the radiation of modern organisms and organelles. Received: 28 July 1997 / Accepted: 23 January 1998     

Rapid Evolution of the Ribonuclease A Superfamily: Adaptive Expansion of Independent Gene Clusters in Rats and Mice

The two eosinophil ribonucleases, eosinophil-derived neurotoxin (EDN/RNase 2) and eosinophil cationic protein (ECP/RNase 3), are among the most rapidly evolving coding sequences known among primates. The eight mouse genes identified as orthologs of EDN and ECP form a highly divergent, species-limited cluster. We present here the rat ribonuclease cluster, a group of eight distinct ribonuclease A superfamily genes that are more closely related to one another than they are to their murine counterparts. The existence of independent gene clusters suggests that numerous duplications and diversification events have occurred at these loci recently, sometime after the divergence of these two rodent species (∼10–15 million years ago). Nonsynonymous substitutions per site (d _N) calculated for the 64 mouse/rat gene pairs indicate that these ribonucleases are incorporating nonsilent mutations at accelerated rates, and comparisons of nonsynonymous to synonymous substitution (d _N / d _S) suggest that diversity in the mouse ribonuclease cluster is promoted by positive (Darwinian) selection. Although the pressures promoting similar but clearly independent styles of rapid diversification among these primate and rodent genes remain uncertain, our recent findings regarding the function of human EDN suggest a role for these ribonucleases in antiviral host defense. Received: 8 April 1999 / Accepted: 22 June 1999     

Multi-unit recording of antennal mechano-sensitive units in the central complex of the cockroach, <Emphasis Type="Italic">Blaberus discoidalis</Emphasis>

The central complex (CC) is a group of midline neuropils in the protocerebrum of all insects (Williams, J Zool, 176:67–86, 1975; Strausfeld, Prog Brain Res, 123:273–284, 1999). Its columnar organization coupled with the anatomical tracts to and from this region suggests that the CC may supervise various forms of locomotion. In cockroach, lesions of the CC affect turning and controlled climbing over blocks (Ridgel et al., J Comp Physiol A, 193:385–402, 2007). Since these behaviors are largely directed by tactile cues detected by antennae, we predicted that some neurons in the CC respond to mechanical antennal stimulation. We used 16-channel probes to record from broad regions within the CC, while mechanically stimulating one or the other antenna. Using cluster cutting procedures, we examined 277 units in 31 preparations. Many of these units responded to mechanical stimulation of the antennae, and of these, most responded equally well to medial or lateral stimulation of either antenna. However, several units either responded to only one antenna or responded significantly more strongly to one of them. Most of the units responding to antennal stimulation were sensitive to changes in the velocity as well as changes in light. Our data reveal a large population of mult-sensory neurons in the CC that could contribute to locomotion control.