Overproduction and selective abortion of ovules based on the order of fertilization revisited

Given that seeds fertilized by slowly growing pollen are of low quality genetically, we theoretically reanalyzed the hypothesis that plants selectively abort ovules fertilized later to enhance the mean quality of resulting seeds. We assumed that both superior and inferior pollen exist, the superior pollen growing faster to fertilize ovules, resulting in seeds of higher quality than those of ovules fertilized by inferior pollen. We developed two models to determine the conditions under which selective abortion is favored. In the first model, ovules in one flower are fertilized by pollen grains that arrive at different times, with each visit bringing both fast- and slow-growing pollen. In the second model, ovules in two flowers are fertilized by all pollen grains that arrive at the same time. In the first model, we found that selective abortion based on the order of fertilization is never advantageous irrespective of the duration of the time lag between the two visits. Rather, random abortion is possibly favored. In the second model, although selective abortion based on the order of fertilization can be advantageous, the parameter region favoring it is rather restricted. This is because overproduction can be advantageous only if the quantity of the superior pollen is not limited in one flower but is limited in the other flower. In addition, the degree of overproduction was very low, implying that the merit of overproduction (increase in the number of superior seeds) is low compared to the cost of overproducing ovules. These results suggest that selective abortion of ovules based on the order of fertilization is not as advantageous as previously considered.     

Toward convergence of experimental studies and theoretical modeling of the chromatin fiber

Understanding the structural organization of eukaryotic chromatin and its control of gene expression represents one of the most fundamental and open challenges in modern biology. Recent experimental advances have revealed important characteristics of chromatin in response to changes in external conditions and histone composition, such as the conformational complexity of linker DNA and histone tail domains upon compact folding of the fiber. In addition, modeling studies based on high-resolution nucleosome models have helped explain the conformational features of chromatin structural elements and their interactions in terms of chromatin fiber models. This minireview discusses recent progress and evidence supporting structural heterogeneity in chromatin fibers, reconciling apparently contradictory fiber models.     

Ranked Set Sampling: Ranking Error Models and Estimation of Visual Judgment Error Variance

Ranked set sampling where sampling is based on visual judgment of the differences between the sizes of pairs of units or on a concomitant variable is reviewed. An alternative model for judgment ranking based on ratios of sizes of pairs of units is presented. Computation of the variance of a visual ranked set sampling estimator of the mean of a distribution is enabled via maximum likelihood estimation of the visual judgment error variance. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Limited rotational motion of amphiphilic flavins in dipalmitoylphosphatidylcholine vesicles

The rotational motion of amphiphilic flavins in dipalmitoyl phospholipid bilayers was investigated with fluorescence anisotropy decay measurements. At temperatures between 10 and 50°C the rotation proved to be anisotropic, which indicated composite motion of both the aliphatic side-chain and the isoalloxazine moiety of the octadecyllumiflavin derivatives. Above the phase transition temperature (crystalline→liquid-crystalline state) the depolarization is complete within the average flavin fluorescence lifetime, implicating unrestricted motion and resulting in a non-ordered microenvironment. In the gel or crystalline state the flavin motion can best be characterized as a limited rotation or librational motion. The fluorescence decay of the flavins is heterogeneous at temperatures between 10 and 50°C, which is explained by assuming nanosecond relaxation of the polar phosphatidyl head-groups around the excited flavin. The lack of a significant cholesterol effect suggests that the isoalloxazine is located at the interphase of the bilayer and not in the hydrocarbon region. The microstructure is fluid-like, not in agreement with a preferred static localization of the flavins in the bilayer.     

Within-subject differences in degree of delay discounting as a function of order of presentation of hypothetical cash rewards

Procedural variants in estimating delay discounting (DD) have been shown to yield significant differences in estimated degree of DD as well as variations in individual patterns of choice. For example, a recent study found significantly different degrees of DD between groups assessed using either an ascending or descending order of presentation of the immediately available rewards. The purpose of this study was to test for within-subject effects of order of presentation of the immediate rewards in a DD task. In a single session, college students (N = 29) were asked to complete two DD tasks, one with the immediate rewards presented in ascending order and one in descending order. Consistent with previous results, significantly larger mean area under the discounting curve (AUC) was observed when the descending sequence was used compared to the ascending order of presentation; and the correlation between both measurements was moderate. These results suggest that some DD assessment tasks may be sensitive to contextual variables such as order and range of the reward and delay values.     

Origin of the First Cells on Earth: A Possible Scenario

A fundamental challenge in science is to understand the assembly of the first macromolecules necessary for cellular life and the order in which this occurred. The assembly events that led to the first minimal cell capable of growth and division are a highly debatable subject. Possibly, the formation of a primitive membrane or microsphere in a hydrophobic medium provided a suitable structure where subsequent biochemical self-replication and eventually enzyme catalysis, integrated biochemical pathways, and assembly of nucleic acids occurred. In this article, I examine a possible sequence of assembly for the first primitive cell(s) on the Earth.