As geologically relevant models of prebiotic environments, systems consisting of clay, water, and amino acids were subjected to cyclic variations in temperature and water content. Fluctuations of both variables produced longer oligopeptides in higher yields than were produced by temperature fluctuations alone. The results suggest that fluctuating environments provided a favorable geological setting in which the rate and extent of chemical evolution would have been determined by the number and frequency of cycles.

Abstraet--A stable porous system consisting of montmorillonite cross-linked by Al-hydroxide oligomers was synthesized by reacting at room temperature an aqueous solution of such oligomers with a unit-layer dispersion of montmorillonite. The resulting cross-linked montmorillonite (AI-CLM) is a nonswelling material, showing basal spacings of 14.4 to 18.8/~, after air drying and between 14.2 to 18.0/~ after treatment at 119~ The basal spacing is found to depend on the age and OH/AI ratio of the Al-hydroxide solution, as well as on the relative amounts of the two reactants. A specific surface area of 160 m~/g and a diffraction pattern with a dominant basal spacing of 17.5 ]~ is obtained by using Al-hydroxide with OH/ AI = 1.85, aged for at least 5 days, and by applying an Al/montmorillonite ratio greater than 1.5 in the cross-linking process. The basal spacing of AI-CLM remains essentially unchanged after heating at 220~ while the specific surface area is not affected by heat treatment up to 480~Two possible configurations of Al-hydroxide oligomers, homogeneously distributed between parallel montmorillonite unit-layers, were considered in order to account for the basal spacing of 17.5-18.8/~, viz. (a) stacking of two oligomeric ring units in parallel orientation relative to the clay lamellae and (b) perpendicular orientation of individual oligomeric units.

Published data on adsorption and condensation of amino acids, purine and pyrimidine bases, sugars, nucleosides, and nucleotides are analyzed in connection with Bernal's hypothesis that clays and other minerals may have provided the most likely surface for adsorption and condensation of these molecules in prebiotic times. Using surface concentration and reaction rate as the main criteria for the feasibility of condensation reactions, four types of prebiotic environments were analyzed: (1) an ocean-sediment system, (2) a dehydrated lagoon bed produced by evaporation, (3) the surface of a frozen sediment, and (4) a fluctuating system where hydration (rainstorms, tidal variations, flooding) and dehysration (evaporation) take place in a cyclic manner. With the possible exception of nucleotides, low adsorption of organomonomers on sediment surfaces of a prebiotic ocean (pH 8) is expected, and significant condensation is considered unlikely. In dehydrated and frozen systems, high surface concentrations are probable and condensation is more likely. In fluctuating environments, condensation rates will be enhanced and the size distribution of the oligomers formed during dehydration may be influenced by a "redistribution mechanism" in which adsorbed oligomers and monomers are desorbed and redistributed on the solid surface during the next hydration-dehydration cycle.

The degree of reproductive isolation between the B and Q biotypes of the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is currently not clear. Laboratory experiments have shown that the two biotypes are capable of producing viable F1 hybrids but that these females are sterile as their F2 generation failed to develop, indicating, most likely, a post-zygotic reproductive barrier. Here, we confirm, by molecular and ecological tools, that the B and Q biotypes of Israel are genetically isolated and provide two independent lines of evidence that support the existence of a pre-zygotic reproductive barrier between them. Firstly, monitoring of mating behaviors in homogeneous and heterogeneous couples indicated no copulation events in heterogeneous couples compared to approximately 50% in homogeneous B and Q couples. Secondly, we could not detect the presence of sperm in the spermathecae of females from heterogeneous couples, compared to 50% detection in intra-B biotype crosses and 15% detection in intra-Q biotype crosses. The existence of pre-zygotic reproductive barriers in Israeli B and Q colonies may indicate a reinforcement process in which mating discrimination is strengthened between sympatric taxa that were formerly allopatric, to avoid maladaptive hybridization. As the two biotypes continued to perform all courtship stages prior to copulation, we also conducted mixed cultures experiments in order to test the reproductive consequences of inter-biotype courtship attempts. In mixed cultures, a significant reduction in female fecundity was observed for the Q biotype but not for the B biotype, suggesting an asymmetric reproductive interference effect in favour of the B biotype. The long-term outcome of this effect is yet to be determined since additional environmental forces may reduce the probability of demographic displacement of one biotype by the other in overlapping niches.

The light absorption spectra of monoionic montmorillonite suspensions with Li, Na, K, Rb, NH4, Cs, Mg, Ca and Ba as adsorbed ions were studied in the wavelength range 200–800 mμ. Theoretical analysis of the spectra showed that in the visible range, only scattering is responsible for light intensity attenuation. The scattering was found to depend on the type of adsorbed ion and to increase systematically along the series from Li to Ba. This was attributed to increased particle size of the clay caused by parallel plate condensation to form tactoids. A geometrical model of a montmorillonite tactoid was assumed. The general dependence of the properties of such tactoids, and of their suspensions, on the number of plates per tactoid was formulated. Using the measured values of light absorbance index, and the theoretical model, the relative sizes, measured by the number of plates per tactoid, in the different ionic forms were calculated. The relative sizes thus estimated were found to agree favorably with relative sizes calculated from negative adsorption surface area measurements. A typical peak of specific light absorption was observed at 240–245 mμ, for both the suspensions and their clay‐free supernatant solutions. Though the compound responsible for this peak is unidentified yet, its quantity in the various suspensions could be roughly estimated. The quantity was found to be negatively correlated with the relative size of the tactoid. It was concluded that the average particle size of montmorillonite in suspension markedly and systematically depends on the type of adsorbed ion. The implications of this finding are discussed.