Under Strict Control
- Stephen J. Simpson
In many species of animals, males and females are strikingly (and visibly) different. Three evolutionary genetic pathways can produce such sexual dimorphism. From the outset, the expression of the alleles for dimorphic traits could be limited to one sex. Or the alleles could be expressed initially in both sexes but subsequently promoted only in one sex or repressed only in the other. Darwin believed that the bright colors and ornaments of males were male-limited from their first appearance, whereas Fisher held that they had been lost from females through the accumulation of suppressors. Coyne et al. weigh these alternatives by examining female hybrids between bird species [such as Costa's (left) and black-chinned (right) hummingbirds] whose males show distinct ornamental traits (for example, plumage patterns and the presence or absence of elongated feathers). Unless the suppressors are completely dominant, these females should show some expression of the male traits. In none of the 13 hybrid cases (from four orders and eight families) that met their search criteria did the females express a male-limited trait. This suggests that male ornaments were sex-limited at their first appearance (perhaps by hormonal or physiological differences between the sexes) or are now controlled by completely dominant modifier alleles or regulatory elements. — SJS
Evolution 62, 214 (2008).
- PLANETARY SCIENCE
Telling Time on Mars
- Brooks Hanson
Our primary information on the surface age and geologic history of Mars comes from crater densities on different parts of the planet and from dating of Martian meteorites (though their source region on Mars is not known with certainty). Crater ages imply that most of the crust is over 2 billion years old. In contrast, dating of one distinct group of Martian meteorites, by a variety of isotopic systems, has generally indicated young ages, from a few hundred thousand to about 1 billion years. These numbers imply that Mars had enough recent magmatic activity to be sampled by a significant fraction of the available Martian meteorites. A problem, however, is that all of these rocks show evidence of alteration and impact events (required for but not limited to their excavation from Mars), as well as hints of the very early geochemical history of Mars, so interpreting their ages is not generally straightforward. Bouvier et al. present additional analyses of two of the purportedly young meteorites using several isotopic systems and evaluate the ages of all of the Martian meteorites. In general, the Sm-Nd and Lu-Hf ages derived from minerals were young, but the Pb-Pb whole-rock ages were much older (∼4 billion years). In contrast to some other recent analyses, the authors interpret this difference, and ultimately the other young ages, as indicating later alteration that has re-initialized mineral-based isochrons. — BH
Earth Planet. Sci. Lett. 266, 105 (2008).
- APPLIED PHYSICS
Plasmons on a Wire
- Ian S. Osborne
The miniaturization of optical elements such as waveguides is limited by the wavelength of light, typically from a few hundred nanometers up to the micrometer scale. By comparison, electrical circuits can be patterned down to a few tens of nanometers. A promising route for integrating these two scales is direct conversion of an optical signal into an electrical one via surface plasmons created at the surface of metal dielectric interfaces. Schmidt et al. use a photonic crystal fiber as the optical carrier and show that they can fill in the air holes that run the length of the fiber with gold or silver, introduced in molten form, to create an array of metal nanowires embedded within the silica matrix. Characterization of the hybrid structure shows that plasmon resonances are excited in the nanowires, thereby demonstrating the direct integration of optics and nanoelectronics. — ISO
Phys. Rev. B 77, 33417 (2008).
- PLANT SCIENCE
- Laura M. Zahn
Bacillus thuringiensis (Bt) toxins are bacterial proteins that kill some insects and are widely used in crops for pest control. Plants expressing the Bt toxins Cry1Ac and Cry1Ab make up the majority of insect-resistant transgenic corn and cotton, respectively. A major concern has been that the pests will develop resistance; refuges, where non-transgenic plants grow next to transgenic plants, have been hypothesized to delay the evolution of resistance because resistant individuals will mate with susceptible individuals from the refuge and hence not confer resistance (if inheritance is recessive) to their offspring.
Tabashnik et al. have analyzed field studies from around the world to determine the effectiveness of the refuge strategy. They found that resistance has evolved in populations of the corn earworm at some U.S. locations, but not elsewhere nor in five other pests. Furthermore they conclude that the refuge strategy has been successful in slowing the evolution of resistance and that expressing multiple Bt toxins in single plants may further slow the appearance of resistance. — LMZ
Nat. Biotechnol. 26, 199 (2008).
Air to the Rescue
- Jake Yeston
Chemists generally take pains to ensure the purity of their reagents, lest a highly reactive contaminant doom the anticipated outcome. Once in a while, though, an adventitious impurity proves more help than hindrance. The reported synthesis of quinine by Woodward and Doering in 1944 was a landmark achievement in the history of organic chemistry, but in the ensuing years, controversy swelled over the report's reliance on a final reduction step published 26 years earlier by Rabe and Kindler. Was the chemistry in this last step really viable? Had Woodward taken his synthesis all the way to fruition, or actually stopped at the penultimate step? In a meticulous effort to reproduce the Rabe study, Smith and Williams now find that modern concerns probably arose on account of excessively pure materials. They show that the reduction proceeds most efficiently when the aluminum powder called for in the published procedure has been partially oxidized by exposure to air, a reasonably likely circumstance in 1918. — JSY
Angew. Chem. Int. Ed. 47, 1736 (2008).
- Stephen J. Simpson
In 1960, Medawar and Burnet were awarded the Nobel Prize for Physiology or Medicine for showing that immunological tolerance to foreign grafts can be acquired. What this and many studies in animal models have shown since is that it is possible to “re-educate” the immune system of the recipient to the foreign antigens carried by the donor cells and grafts. The induction of full immune tolerance to transplanted organs has also been pursued in the clinic because this would dispense with the need for extensive immunosuppressive regimes currently used to prevent organ rejection.
Scandling et al. and Kawai et al. report cases of human kidney transplantation in which hematopoietic cells from the donor were co-transplanted with the organ, followed by distinct protocols of conditioning involving depletion of host cells. In the first study, this resulted in ongoing mixed chimerism; meaning that there were comparable proportions of donor and recipient hematopoietic cells circulating in the body. In the second study, the extent of chimerism was considerably less and was also transient. Nevertheless, in both clinical settings it was possible to discontinue immunosuppressive drugs without significant adverse responses to the grafted organs; furthermore, cellular evidence of immunological tolerance was detected. In a third study, Alexander et al. describe a female liver transplant patient in which a spontaneous switch occurred in the blood group and—because the donor was male—the sex of the circulating leukocytes. Impressively, the removal of immunosuppressive drugs, which was undertaken to resolve hemolytic anemia in the patient, resulted in complete chimerism (meaning that all hematopoietic cells were of donor origin) and long-term acceptance of the grafted liver. These three studies reveal that immunological acceptance of transplanted organs is a realistic goal worthy of further exploration. — SJS
N. Engl. J. Med. 358, 362; 353; 369 (2008).
We're Never Alone
- Katrina L. Kelner
Each of us carries a community of microorganisms in our intestinal tract that is necessary for optimal health but varies in composition from person to person. To learn which gut microbes most influence human physiology, Li et al. surveyed these symbionts in seven members of a four-generation Chinese family. By sequencing rRNA genes, they assembled a phylogenetic picture of the resident microbes-mainly Bacteroidetes and Firmicutes spp. The Chinese microbiomes showed individual and male/female differences in their composition, although all differed markedly from those seen in Americans. A parallel analysis of urine samples from the same family allowed the authors to correlate variations in resident microbes with variations in excreted metabolites, which served as a proxy for the individuals' metabolic state. They found several microbial species that appeared to significantly affect their hosts: for instance, Faecalibacterium prausnitzii was associated with the presence of dimethylamine, suggested to be an indicator of metabolic syndrome and diabetes. This functional metagenomics approach can be extended by combining full genomic sequencing of the microbes with measurements of metabolic or clinical features of interest. Ultimately, the hope is to identify which gut inhabitants are associated with disease states and which with human well-being. — KK
Proc. Natl. Acad. Sci. U.S.A. 105, 2117 (2008).