Sex determination in Bonellia viridis (Echiura : Bonelliidae) : population dynamics and evolution

In the echiuran worm Bonellia viridis Rolando, the vast majority of sexually undifferentiated larvae metamorphose into dwarf males that live inside the female when exposed to females, but differentiate into females when developing in the absence of females. By means of a spatially explicit, individual-based model the authors examine how this specific form of environmental sex determination (ESD) affects dynamics of Bonellia populations and investigate the selective advantage of ESD over the more widespread genotypic sex determination (GSD). Population dynamics of Bonellia appear rather simple and not too sensitive to parameter changes around their measured values, or to changes in distribution and sizes of inhabitable patches. Starting even from low sizes, populations soon attain equilibrium densities. Explored aspects of population dynamics indicate an advantage of ESD over GSD. Moreover, simulated invasibility experiments show that while the maternal inheritance scenario allows for fixation of GSD under some limited conditions, both the classical and proportional inheritance scenarios always lead to fixation of ESD in the population. This article also shows that only the ability of ESD larvae to adapt their ultimate sex both in competition for empty burrows and for mating within females gives them a competitive edge over nonadaptive response to feminising and/or masculinising signals and generally leads to fixation of ESD by small step evolution. The original hypothesis of Charnov and Bull thus needs to be refined in the sense that along with females forming an unpredictable resource for males, empty burrows are an unpredictable resource for femalespeer-reviewe

Regeneration in the enteropneust hemichordate, Ptychodera flava, and its evolutionary implications

Hemichordates are marine invertebrates that are closely related to chordates, but while their body plans are comparable to those of chordates, they possess a remarkable capacity for regeneration, even as adults. A small fragment is sufficient to form a complete individual. Unlike echinoderms, their larvae transform directly into adults; therefore, hemichordate systems offer clear morphological and molecular parallels between regeneration and development. Morphological events in regeneration are generally similar to organogenesis in juveniles. Nonetheless, comparative analysis of gene expression in these two morphological phenomena suggests that hemichordate regeneration is regulated by regeneration-specific mechanisms, as well as by developmental mechanisms. Dependency upon resident pluripotent/multipotent stem cells is a significant difference in metazoan regeneration, and such stem cells are essential for regeneration in many lineages. Based on the present gene expression study, regeneration in acorn worms is more closely related to that in vertebrates, because it employs endogenous stem cell-independent transdifferentiation

Membranes from polycation-polyanion complexes

Membranes suitable for microfiltration, ultrafiltration, and dialysis consist of polycation-polyanion complexes prepd. by casting an aq. soln. of CM-cellulose (I) and cellulose sulfate (II) into a film and then treating with a cationic polyelectrolyte. Thus, 2.5% solns. of I with degree of substitution 1.0 and II were mixed in amts. to give I-II ratio 2:1, cast into a 2.0 mm layer on a glass plate, topped with a 2.0 mm layer of 20-5% polyethylenimine soln., dried, removed from the plate, and washed. The resulting membrane had throughput 16.0 L/h-m2 and sepn. efficiency 79.5% when used in a Millipore US cell at 0.2 MPa to sep. a 1 g/L aq. soln. of dextran with mol. wt. 500,000. [on SciFinder (R)