Optimized intermolecular potential for nitriles based on Anisotropic United Atoms model

An extension of the Anisotropic United Atoms intermolecular potential model is proposed for nitriles. The electrostatic part of the intermolecular potential is calculated using atomic charges obtained by a simple Mulliken population analysis. The repulsion-dispersion interaction parameters for methyl and methylene groups are taken from transferable AUA4 literature parameters [Ungerer et al., J. Chem. Phys., 2000, 112, 5499]. Non-bonding Lennard-Jones intermolecular potential parameters are regressed for the carbon and nitrogen atoms of the nitrile group (–C≡N) from experimental vapor-liquid equilibrium data of acetonitrile. Gibbs Ensemble Monte Carlo simulations and experimental data agreement is very good for acetonitrile, and better than previous molecular potential proposed by Hloucha et al. [J. Chem. Phys., 2000, 113, 5401]. The transferability of the resulting potential is then successfully tested, without any further readjustment, to predict vapor-liquid phase equilibrium of propionitrile and n-butyronitrile

Clutch Frequency Affects the Offspring Size-Number Trade-Off in Lizards

Background: Studies of lizards have shown that offspring size cannot be altered by manipulating clutch size in species with a high clutch frequency. This raises a question of whether clutch frequency has a key role in influencing the offspring sizenumber trade-off in lizards. Methodology/Principal Findings: To test the hypothesis that females reproducing more frequently are less likely to tradeoff offspring size against offspring number, we applied the follicle ablation technique to female Eremias argus (Lacertidae) from Handan (HD) and Gonghe (GH), the two populations that differ in clutch frequency. Follicle ablation resulted in enlargement of egg size in GH females, but not in HD females. GH females switched from producing a larger number of smaller eggs in the first clutch to a smaller number of larger eggs in the second clutch; HD females showed a similar pattern of seasonal shifts in egg size, but kept clutch size constant between the first two clutches. Thus, the egg sizenumber trade-off was evident in GH females, but not in HD females. Conclusions/Significance: As HD females (mean = 3.1 clutches per year) reproduce more frequently than do GH females (mean = 1.6 clutches per year), our data therefore validate the hypothesis tested. Our data also provide an inference that maximization of maternal fitness could be achieved in females by diverting a large enough, rather than a higher-than-usual

Support for a rare pattern of temperature-dependent sex determination in archaic reptiles: evidence from two species of tuatara (Sphenodon)

<p>Abstract</p> <p>Background</p> <p>The sex of many reptiles is determined by the temperature an embryo experiences during its development. Three patterns of temperature-dependent sex determination (TSD) have been defined, but one pattern where only males are produced above an upper temperature threshold (Type IB) is controversial. Here we report new data on the relationship between constant temperature incubation and sexual phenotype in two species of tuatara (<it>Sphenodon</it>), archaic reptiles of enormous zoological significance as the sole representatives of a once widespread reptilian order.</p> <p>Results</p> <p>In both species, the pattern observed with constant incubation temperatures from 18 to 23°C (or 24°C) supported a female→male (FM or Type IB) pattern of TSD: in <it>Sphenodon guntheri </it>males were produced above a pivotal temperature of 21.6°C, and in <it>S. punctatus </it>(unnamed subspecies on Stephens Island, Cook Strait), males were produced above a pivotal temperature of 22.0°C. The pivotal temperatures and scaling parameters differed between species (p < 0.001). The thermosensitive period (TSP), where temperature influences gonad morphogenesis, occurs between 0.25 and 0.55 of embryonic development. While it is possible that the more common female→male→female (FMF or Type II) pattern exists, with a second pivotal temperature above 23–24°C, we review several lines of evidence to the contrary. Most notably, we show that in <it>S. punctatus</it>, the warmest natural nests during the TSP produce predominantly males.</p> <p>Conclusion</p> <p>An FM pattern of TSD could be currently adaptive in promoting sexual size dimorphism in tuatara. However, an FM pattern has particularly serious consequences for <it>S. guntheri </it>because current patterns of global warming could exacerbate the male bias already present in the relic population.</p

Eggshell morphology and gekkotan life-history evolution

Eggshell structure is related to fundamental aspects of embryonic development (via water and gas exchange), adult ecology and behavior (via nest site selection), and demography (via effects on survival). We compared life-history characteristics between gekkotans that lay rigid- versus parchment- shelled eggs to determine if evolutionary shifts in eggshell structure are associated with life-history evolution. Ancestral gekkotans laid parchment-shelled eggs, with rigid-shelled eggs evolving later. Clutch size in oviparous gekkotans is fixed at one or two eggs, and this characteristic eliminates an egg size versus clutch size tradeoff as a life-history strategy. We found that species laying rigid-shelled eggs exhibit (1) smaller eggs relative to adult body size, (2) smaller hatchlings relative to the size of the egg, (3) earlier embryonic stage at oviposition, (4) longer incubation periods, and (5) smaller adult body sizes than species laying parchment-shelled eggs. These patterns hold when accounting for phylogenetic relatedness, and are not explained by geographic distributions of climate and habitat. In general, our data support the hypothesis that the spherical shapes of rigid-shelled eggs limit their size (volume), which in turn has restricted hatchling size and adult body size. In contrast, while parchment-shelled eggs are similarly constrained in width, elongate shapes allow egg sizes, and hence hatchling sizes, to increase relative to adult body sizes. Finally, the evolution of rigid-shelled eggs may have allowed gekkotans to become so successful; over 1,000 species lay rigid-shelled eggs, as compared to about 200 species that lay eggs exhibiting the ancestral parchment-shelled condition