The oral-aboral axis of a sea urchin embryo is specified by first cleavage

Several lines of evidence suggest that the oral-aboral axis in Strongylocentrotus purpuratus embryos is specified at or before the 8-cell stage. Were the oral-aboral axis specified independently of the first cleavage plane, then a random association of this plane with the blastomeres of the four embryo quadrants in the oral-aboral plane (viz. oral, aboral, right and left) would be expected. Lineage tracer dye injection into one blastomere at the 2-cell stage and observation of the resultant labeling patterns demonstrates instead a strongly nonrandom association. In at least ninety percent of cases, the progeny of the aboral blastomeres are associated with those of the left lateral blastomeres and the progeny of the oral blastomeres with the right lateral ones, respectively. Thus, ninety percent of the time the oral pole of the future oral-aboral axis lies 45 degrees clockwise from the first cleavage plane as viewed from the animal pole. The nonrandom association of blastomeres after labeling of the 2-cell stage implies that there is a mechanistic relation between axis specification and the positioning of the first cleavage plane

Macromere cell fates during sea urchin development

This paper examines the cell lineage relationships and cell fates in embryos of the sea urchin Strongylocentrotus purpuratus leading to the various cell types derived from the definitive vegetal plate territory or the veg_2 tier of cells. These cell types are gut, pigment cells, basal cells and coelomic pouches. They are cell types that constitute embryonic structures through cellular migration or rearrangement unlike the relatively non-motile ectoderm cell types. For this analysis, we use previous knowledge of lineage to assign macromeres to one of four types: VOM, the oral macromere; VAM, the aboral macromere, right and left VLM, the lateral macromeres. Each of the four macromeres contributes progeny to all of the cell types that descend from the definitive vegetal plate. Thus in the gut each macromere contributes to the esophagus, stomach and intestine, and the stripe of labeled cells descendant from a macromere reflects the re-arrangement of cells that occurs during archenteron elongation. Pigment cell contributions exhibit no consistent pattern among the four macromeres, and are haphazardly distributed throughout the ectoderm. Gut and pigment cell contributions are thus radially symmetrical. In contrast, the VOM blastomere contributes to both of the coelomic pouches while the other three macromeres contribute to only one or the other pouch. The total of the macromere contribution amounts to 60% of the cells constituting the coelomic pouches

A comparative molecular approach to mesodermal patterning in basal deuterostomes: the expression pattern of Brachyury in the enteropneust hemichordate Ptychodera flava

This work concerns the formation of mesoderm in the development of an enteropneust hemichordate, Ptychodera flava, and the expression of the Brachyury gene during this process. Brachyury expression occurs in two distinct phases. In the embryo, Brachyury is transcribed during gastrulation in the future oral and anal regions of the gut, but transcripts are no longer detected by 2 weeks of development. Brachyury expression is not detected during the 5 months of larval planktonic existence. During this time, the adult coeloms begin to develop, originating as coalescences of cells that appear to delaminate from the wall of the gut. Brachyury expression cannot be detected again until metamorphosis, when transcripts appear in the mesoderm of the adult proboscis, collar and the very posterior region of the trunk. It is also expressed in the posterior end of the gut. At no time is Brachyury expressed in the stomochord, the putative homologue of the chordate notochord. These observations illuminate the process of maximal indirect development in Ptychodera and, by comparison with patterns of Brachyury expression in the indirect development of echinoderms, their sister group, they reveal the evolutionary history of Brachyury utilization in deuterostomes

Influence of springtime phenology on the ratio of soil respiration to total ecosystem respiration in a mixed temperate forest

Total ecosystem (Reco) and soil (Rs) respiration are important CO2 fluxes in the carbon balance of forests. Typically Rs accounts for between 30-80% of Reco, although variation in this ratio has been shown to occur, particularly at seasonal time scales. The objective of this study was to relate changes in Rs/Reco ratio to changing springtime phenological conditions in forest ecosystems. We used one year (2003) of automated and twelve years (1995-2006) of manual chamber-based measurements of Rs. Reco was determined using tower-based eddy covariance measurements for an oak-dominated mixed temperate forest at Harvard Forest, Petersham, MA, USA. Phenological data were obtained from field observations and the JRC fAPAR remote sensing product. The automated and eddy covariance data showed that springtime phenological events do influence the ratio of soil to total ecosystem respiration. During canopy development, Reco rose strongly, mainly the aboveground component, due to the formation of an increasing amount of respiring leaf tissue. An increase in Rs was observed after most of the canopy development, which is probably the consequence of a shift in allocation of photosynthate products from above- to belowground. This hypothesized allocation shift was also confirmed by the results of the twelve year manual chamber-based measurements

The role of WASP family members in Dictyostelium discoideum cell migration

The WASP family of proteins are nucleation-promoting factors that dictate the temporal and spatial dynamics of Arp2/3 complex recruitment, and hence actin polymerisation. Consequently, members of the WASP family, such as SCAR/WAVE and WASP, drive processes such as pseudopod formation and clathrin-mediated endocytosis, respectively. However, the nature of functional specificity or overlap of WASP family members is controversial and also appears to be contextual. For example, some WASP family members appear capable of assuming each other’s roles in cells that are mutant for certain family members. How the activity of each WASP family member is normally limited to promoting the formation of a specific subset of actin-based structures and how they are able to escape these constraints in order to substitute for one another, remain unanswered questions. Furthermore, how the WASP family members collectively contribute to complex processes such as cell migration is yet to be addressed. To examine these concepts in an experimentally and genetically tractable system we have used the single celled amoeba Dictyostelium discoideum. The regulation of SCAR via its regulatory complex was investigated by dissecting the Abi subunit. Abi was found to be essential for complex stability but not for its recruitment to the cell cortex or its role in pseudopod formation. The roles of WASP A were examined by generating a wasA null strain. Our results contradicted previous findings suggesting that WASP A was essential for pseudopod formation and instead demonstrated that WASP A was required for clathrin-mediated endocytosis. Unexpectedly, WASP A – driven clathrin-mediated endocytosis was found to be necessary for efficient uropod retraction during cell migration and furrowing during cytokinesis. Finally, we created a double scrA/wasA mutant, and found that it was unable to generate pseudopodia. Therefore, we were able to confirm that SCAR is the predominant driver of pseudopod formation in wild-type Dictyostelium cells, and that only WASP A can assume its role in the scrA null. Surprisingly, the double mutant was also deficient in bleb formation, showing that these proteins are also necessary for this alternative, Arp2/3 complex-independent mode of motility. This implies that there exists interplay between the different types of actin-based protrusions and the molecular pathways that underlie their formation

A l'origine des grands animaux, un petit ver tout nu

Les premiers multicellulaires, marins et invértebrés, sont apparus avant le début du Cambrien, il y a 544 millions d'années. Quarante millions d'années plus tard, les branches principales du règne animal étaient probablement déjà présentes, à l’exception des espèces terrestres. Durant « l'explosion cambrienne » apparaissent done tous les plans de base des animaux actuels. Comment expliquer l'émergence de cette extraordinaire diversité? La clé du mystère se cache sans doute dans les processus génétiques qui contrôlent le développement embryonnaire