The paradox of seed size and adaptation

Correlations between habitat and seed size suggest that this character is adaptive. Mean seed size is a relatively invariant species characteristic and seed size has marked effects upon fitness. These observations have previously led to the conclusion that seed size is under stabilizing selection. This conclusion, originally based mainly on evidence from crop plants grown in controlled environments, is questioned here on the grounds that recent studies of wild plants show marked phenotypic plasticity and low heritability of seed size. If seed size is not readily altered by natural selection in the wild, then its effects on fitness are evidence that this character is a constraint on habitat distribution. Constancy of mean seed size may be due to developmental canalization to a size set by previous selection, rather than a continuing process of stabilizing selection.     

A new model of counterion condensation in polyelectrolyte solutions. III. Theoretical predictions of competitive condensation between counterions of different valences

Our model has been extended for theoretical estimation of competitive condensation of counterions of different valences onto polyelectrolytes in solution. The estimations are compared with those obtained from Manning theory and with experimental data on counterion activity coefficients. The agreement with the data for sodium polystyrenesulfonate/MgCl2, CaCl2 is satisfactory.     

Herbicide Resistance in Datura innoxia: Cross-Resistance of Sulfonylurea-Resistant Cell Lines to Imidazolinones

Cells resistant to the sulfonylurea herbicides chlorsulfuron and sulfometuron methyl were isolated from a predominantly haploid cell suspension culture of Datura innoxia P. Mill. Exponentially growing cell colonies (aggregates of about 40 cells) were mutagenized with ethyl methane sulfonate, subcultured for 10 days to allow growth recovery and plated on a medium containing either chlorsulfuron or sulfometuron methyl at a concentration (10⁻⁸ molar) which killed wild type cells. Surviving clones were picked up after 3 to 4 weeks, further proliferated as callus or cell suspension cultures, and tested for their resistance to both the sulfonylureas and imidazolinones, a chemically different class of herbicides. The variants were stable and showed high (100- to 1000-fold) resistance to the sulfonylureas. While some also exhibited cross resistance to imidazolinones, others showed no cross-resistance at all or, as in one case, greater sensitivity than wild type cells to the imidazolinones. Both classes of herbicides tested inhibited acetolactate synthase activity isolated from wild type cells. The acetolactate synthase of the resistant variants, however, was found to be resistant to the sulfonylureas and also to the imidazolinone(s) in those cells showing cross-resistance to the latter. The lack of cross-resistance observed in some cases provides evidence that the two groups of herbicides have slightly different sites on the acetolactate synthase molecule.     

The flea ovary: ultrastructure and analysis of cell clusters

Panoistic ovarioles are found in the order of fleas (Siphonaptera). Only in some species of the Hystrichopsylloidea do polytrophic meroistic ovaries occur. No stem cells and no dividing cystocytes are found in female imagines of Hystrichopsylla talpae. However, each germ cell cluster consists of 32 cells which are generated by five mitotic cycles during the pupal stage. One of the cells containing five intercellular bridges becomes the oocyte, the others serve as nurse cells. Thus, germ cell cluster formation follows the 2(n)-rule. However, no polyfusome is found and nurse cells do not form a rosette. Furthermore, nurse cells remain small and show the same ultrastructural characters as the oocytes, which became distinguishable from nurse cells only by their enhanced growth during pre-vitellogenesis. The first phase of pre-vitellogenesis is dominated by the production of an unknown cytoplasmatic component, consisting of spherical particles, clearly distinguishable from ribosomes by diameter and contrast. The next phase is characterized by a tremendous increase in the production of ribosomes. During this second phase another cytoplasmic component consisting of ball-like structures becomes prominent. During pre-vitellogenesis, germ cell nuclei undergo a pronounced structural change in which, finally, numerous extranucleolar particles predominate. Thus, H. talpae has a polytrophic meroistic ovary, but its oocyte genomes behave panoistically.     

苏芸金杆菌浓缩液的保存和不用二甲苯的防腐剂配方

在苏芸金杆菌浓缩商品制剂中,通常含有3—4％的二甲苯以防止芽孢的萌发。除此以外,二甲苯还能防止各种腐生微生物污染,尤其是肠杆菌和真菌。最近,一种用苏芸金杆菌血清型H3a3b制备的称为Futufa的超浓缩液已经问世,其中不含二甲苯,用于防治枞色卷蛾(属鳞