Sunlight Induced Movement of Planktonic Organisms and Their Relationships to Water Movements

Previous studies have shown the planktonic organisms of various natural water bodies to be quite sensitive to the levels of solar ultraviolet radiation incident at the water surface in summer. It has been assumed that the majority of the phytoplankton are simply carried about in the mass of the water which they inhabit with little control of their position or sunlight exposure. Photosynthetic microorganisms must strike a delicate balance with regard to solar radiation; they must obtain enough visible light for adequate photosynthesis while avoiding an excessive exposure to the injurious solar UV-B radiation. It is proposed that the circulation in natural waters is such that organisms can ride the current to obtain radiation exposures approximating the average radiation level of the euphotic zone. If exposure tends to become excessive, the plankton drops out of the circulation into the relatively immobile thermocline region until conditions are :favorable for a return to the highly illuminated surface waters. Observations with two species of aquatic plants generally confirm the proposed model. A radiation regime where the entire population receives essentially the same exposure to sunlight could be substantially more efficient in using the available light for photosynthesis than the static model where it is assumed that part of the population is overly exposed and another segment continuously dwells deep in the water in very dim light. The uniform dosage raises the potential problem that, should the average UV-B dose exceed the tolerance of the individual, virtually the entire population would be wiped out in a very short time. Perhaps such a species might gradually recover its prominence from the limited survivors or return the next season after being supplanted in its niche by other more resistant organisms

Genome evolution in the allotetraploid frog Xenopus laevis

To explore the origins and consequences of tetraploidy in the African clawed frog, we sequenced the Xenopus laevis genome and compared it to the related diploid X. tropicalis genome. We characterize the allotetraploid origin of X. laevis by partitioning its genome into two homoeologous subgenomes, marked by distinct families of ???fossil??? transposable elements. On the basis of the activity of these elements and the age of hundreds of unitary pseudogenes, we estimate that the two diploid progenitor species diverged around 34 million years ago (Ma) and combined to form an allotetraploid around 17-18 Ma. More than 56% of all genes were retained in two homoeologous copies. Protein function, gene expression, and the amount of conserved flanking sequence all correlate with retention rates. The subgenomes have evolved asymmetrically, with one chromosome set more often preserving the ancestral state and the other experiencing more gene loss, deletion, rearrangement, and reduced gene expression.ope

Moisture absorption characteristics of food thickeners used for the management of swallowing dysfunctions

Six commercial food thickeners, based on starch, and guar and xanthan gums, were investigated for their moisture absorption properties. The thickeners were exposed to 100% relative humidity as well as soaked in water for selected time periods. The gum-based thickeners dissolved in water to yield 100% water solubility index. The water absorption index of the starch-based thickeners ranged from 14.7 to 18.0 g/g dry solids of thickener. The starch-based thickeners sorbed less moisture than the gum-based ones, with the xanthan gum-based thickeners having the highest sorptive ability on mass basis. The Peleg model was the most suitable (r(2)=0.966-0.997) of the sorption models investigated to describe the sorption data. The diffusion rate constant of the thickeners ranged from 8.7 to 13.5 day%(-1), while the equilibrium moisture content was between 35.7 and 46.8%. The sorptive ability explained viscosity development in some of the thickeners, but was unsuitable in fully describing comparisons of the solutions viscosities of starch-based and xanthan gum-based thickeners

Lighting up developmental mechanisms: how fluorescence imaging heralded a new era

International audienc

Genome evolution in the allotetraploid frog Xenopus laevis.

To explore the origins and consequences of tetraploidy in the African clawed frog, we sequenced the Xenopus laevis genome and compared it to the related diploid X. tropicalis genome. We characterize the allotetraploid origin of X. laevis by partitioning its genome into two homoeologous subgenomes, marked by distinct families of 'fossil' transposable elements. On the basis of the activity of these elements and the age of hundreds of unitary pseudogenes, we estimate that the two diploid progenitor species diverged around 34 million years ago (Ma) and combined to form an allotetraploid around 17-18 Ma. More than 56% of all genes were retained in two homoeologous copies. Protein function, gene expression, and the amount of conserved flanking sequence all correlate with retention rates. The subgenomes have evolved asymmetrically, with one chromosome set more often preserving the ancestral state and the other experiencing more gene loss, deletion, rearrangement, and reduced gene expression