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Urchin Species (urchin + species)
Selected AbstractsGastrulation in the sea urchin embryo: A model system for analyzing the morphogenesis of a monolayered epitheliumDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 4 2004Tetsuya Kominami Processes of gastrulation in the sea urchin embryo have been intensively studied to reveal the mechanisms involved in the invagination of a monolayered epithelium. It is widely accepted that the invagination proceeds in two steps (primary and secondary invagination) until the archenteron reaches the apical plate, and that the constituent cells of the resulting archenteron are exclusively derived from the veg2 tier of blastomeres formed at the 60-cell stage. However, recent studies have shown that the recruitment of the archenteron cells lasts as late as the late prism stage, and some descendants of veg1 blastomeres are also recruited into the archenteron. In this review, we first illustrate the current outline of sea urchin gastrulation. Second, several factors, such as cytoskeletons, cell contact and extracellular matrix, will be discussed in relation to the cellular and mechanical basis of gastrulation. Third, differences in the manner of gastrulation among sea urchin species will be described; in some species, the archenteron does not elongate stepwise but continuously. In those embryos, bottle cells are scarcely observed, and the archenteron cells are not rearranged during invagination unlike in typical sea urchins. Attention will be also paid to some other factors, such as the turgor pressure of blastocoele and the force generated by blastocoele wall. These factors, in spite of their significance, have been neglected in the analysis of sea urchin gastrulation. Lastly, we will discuss how behavior of pigment cells defines the manner of gastrulation, because pigment cells recently turned out to be the bottle cells that trigger the initial inward bending of the vegetal plate. [source] Size matters sometimes: wall height and the structure of subtidal benthic invertebrate assemblages in south-eastern Australia and Mediterranean SpainJOURNAL OF BIOGEOGRAPHY, Issue 12 2003A. R. Davis Abstract Aim, Variation in the structure of shallow subtidal invertebrate assemblages was examined over three spatial scales; within reef, between reef and between continents. We sought to provide a context from which to examine and interpret ecological processes between continents. In addition, we predicted that variation in pattern would increase as the scale of examination increased. Location, Reefs near Wollongong and within Jervis Bay in south-eastern Australia and Mediterranean reefs on the Costa Brava (Catalonia), north-eastern Spain. Methods, We compared assemblages on vertical rock walls of two heights , short (< 2 m) and tall (> 3 m) in two temperate regions over the same depth range. Specifically we examined the diversity and cover of invertebrates, the cover and biomass of foliose and crustose algae, the size of invertebrate colonies and the biomass of urchins on short and tall walls (n = 3) at each of two locations in each country. Results, Foliose algae dominated rock walls in Spain and although invertebrate cover was high, colonies were generally very small. Two urchin species were commonly encountered on rock walls in Spain, Arbacia lixula and Paracentrotus lividus; their biomass was relatively low and did not differ significantly between short and tall walls. These findings contrasted strongly with south-eastern Australia, where foliose algae were almost completely absent. A single urchin species, Centrostephanus rodgersii occurred with extremely high biomass on short walls, which were dominated by grazer-resistant crustose calcareous algae. In contrast, the biomass of this urchin was low on tall walls, which were dominated by invertebrates, usually exceeding 95% in cover. Invertebrate colonies were significantly larger on both short and tall walls in south-eastern Australia relative to the Mediterranean. Findings within a country were consistent between the replicate rock walls and between locations. In contrast to our prediction, however, there was significant variation among walls within a location, but not among locations within a continent. Temporal variation in the structure of these assemblages was not examined, but appears limited. Main conclusions, We conclude that submarine topography, i.e. the presence of short or tall rock walls, as a function of rock type and structure, has a marked impact on community structure in south-eastern Australia, but made little difference to the structure of the assemblage in Mediterranean Spain. The differences in structure we observed between walls of different heights in Australia were correlated with differences in the biomass of urchins and they appear to be major determinants of assemblage structure. Interactions among species are often reported from disparate parts of the globe with little or no reference to the structure of the assemblage of which they are a part; we contend that this will hinder interpretation. Our data are consistent with the organisms in these two regions experiencing distinct selection pressures; for example high levels of urchin grazing activity in south-eastern Australia, and shading and whiplash associated with an algal canopy in the Mediterranean. It may not be appropriate to contrast processes operating at very large (intercontinental) scales unless context can be established with a clear understanding of ecological pattern. Objectivos, En el presente trabajo hemos examinado la variación en estructura de comunidades de invertebrados de aguas someras a tres escalas espaciales: dentro de arrecifes rocosos, entre arrecifes y entre continentes. Nuestro objetivo era proveer un contexto para examinar y interpretar procesos ecológicos entre continentes. Además, realizamos la predicción de que el modelo de variación aumentaría conforme aumentaba la escala de observación. Localidad, Arrecifes rocosos cerca de Wollongong y en Jarvis Bay en la zona sudeste de Australia y arrecifes rocosos mediterráneos en la Costa Brava (Cataluña), en el nordeste de España. Metodos, Comparamos comunidades en paredes rocosas verticales de dos alturas , bajas (< 2 m) y altas (> 3 m) en dos regiones templadas y dentro del mismo rango de profundidades. Específicamente, examinamos la diversidad y el recubrimiento de invertebrados, el recubrimiento y la biomasa de algas foliosas e incrustantes, la medida de las colonias de invertebrados y la biomasa de erizos en paredes bajas y altas (n = 3) en dos localidades de cada país. Resultados, Las algas foliosas dominaban las paredes rocosas en España y, aunque el recubrimiento de invertebrados era alto, las colonias eran generalmente muy pequeñas. Dos especies de erizos eran comunes en las paredes rocosas en España: Arbacia lixula y Paracentrotus lividus. Su biomasa total era relativamente pequeña y no mostraba diferencias significativas entre paredes bajas y altas. Estos resultados contrastan con los hallados en el sudeste de Australia, donde las algas foliosas eran prácticamente ausentes. Una única especie de erizo, Centrostephanus rodgersii, se encontraba en las paredes bajas, donde presentaba elevadas biomasas. Estas paredes estaban dominadas por algas incrustantes y carbonatadas resistentes al ramoneo por erizos. En cambio, la biomasa de este erizo era baja en paredes altas, que estaban dominadas por invertebrados, con recubrimientos normalmente por encima del 95%. Las colonias de invertebrados eran significativamente más grandes tanto en las paredes altas como bajas en el sudeste de Australia en comparación con el Mediterráneo. Los resultados dentro de cada país eran consistentes entre las réplicas de paredes estudiadas y entre localidades. Contrariamente a nuestra predicción, sin embargo, había una variación significativa entre paredes dentro de una localidad pero no entre localidades dentro de un continente. La variación temporal en estructura en estas comunidades no fue estudiada, pero parece ser limitada. Conclusiones Principales, Concluimos que la topografía submarina, esto es, la presencia de paredes bajas o altas en función del tipo de roca y estructura, tiene un efecto importante en la estructura de las comunidades en el sudeste de Australia. Este factor, sin embargo, tiene un efecto muy limitado en el Mediterráneo español. Las diferencias en estructura observadas entre paredes de diferentes alturas en Australia se correlacionan con diferencias en la biomasa de erizos, los cuales parecen ser determinantes en la estructura de las comunidades. Frecuentemente se estudian interacciones entre especies en zonas alejadas del globo con escasa o nula referencia a la estructura de la comunidad de la que forman parte. En nuestra opinión este hecho impide una correcta interpretación. Nuestros datos son consistentes con la idea de que los organismos en las dos regiones estudiadas experimentan diversas presiones selectivas, por ejemplo, niveles altos de ramoneo por erizos en el sudeste de Australia, y oscurecimiento y disturbancia mecánica asociadas a la cobertura de algas en el Mediterráneo. No parece apropiado comparar procesos que operan a escalas muy amplias (intercontinentales) sin establecer antes un contexto con una comprensión clara de los parámetros ecológicos. [source] Morphological evolution in sea urchin development: hybrids provide insights into the pace of evolutionBIOESSAYS, Issue 4 2004Maria Byrne Hybridisations between related species with divergent ontogenies can provide insights into the bases for evolutionary change in development. One example of such hybridisations involves sea urchin species that exhibit either standard larval (pluteal) stages or those that develop directly from embryo to adult without an intervening feeding larval stage. In such crosses, pluteal features were found to be restored in fertilisations of the eggs of some direct developing sea urchins (Heliocidaris erythrogramma) with the sperm of closely (Heliocidaris tuberculata) and distantly (Pseudoboletia maculata) related species with feeding larvae. Such results can be argued to support the punctuated equilibrium model,conservation in pluteal regulatory systems and a comparatively rapid switch to direct development in evolution.1,2 Generation of hybrids between distantly related direct developers may, however, indicate evolutionary convergence. The ,rescue' of pluteal features by paternal genomes may require maternal factors from H. erythrogramma because the larva of this species has pluteal features. In contrast, pluteal features were not restored in hybridisations with the eggs of Holopneustes purpurescens, which lacks pluteal features. How much of pluteal development can be lost before it cannot be rescued in such crosses? The answer awaits hybridisations among indirect and direct developing sea urchins differing in developmental phenotype, in parallel with investigations of the genetic programs involved. BioEssays 26:343,347, 2004. © 2004 Wiley Periodicals, Inc. [source] The modulator is a constitutive enhancer of a developmentally regulated sea urchin histone H2A geneBIOESSAYS, Issue 9 2002Giovanni Spinelli Going back to the late 1970s and early 1980s, we trace the Xenopus oocyte microinjection experiments that led to the emergence of the concept of "modulator". The finding that the modulator could transactivate transcription from far upstream and in either orientation suggested that a new genetic element, different from the classical prokaryotic promoter sequences, had been discovered. This particular enhancer transactivates transcription of the sea urchin early (,) histone H2A gene which is regulated in early sea urchin development. We summarise the data from sea urchin microinjection experiments that confirm and extend the results obtained with Xenopus oocytes. We conclude that the H2A enhancer is bipartite, is located approx. 100 bp upstream of the TATAAATA box in the H2A gene of two sea urchin species and enhances transcription when placed at a position far upstream or far downstream of the gene unless an insulator intervenes between enhancer and promoter. Evidence from microinjection experiments with sea urchin embryos suggests that the developmental control of H2A expression resides not with the enhancer, which is constitutively active, but with a striking chromatin structure with two positioned nucleosomes near the 3, end of the gene. Within this structure, there is an insulator element. BioEssays 24:850,857, 2002. © 2002 Wiley Periodicals, Inc. [source] | ||||||||||