PNAS | November 28, 2006 | vol. 103 | no. 48 | 18027-18028
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THIS WEEK IN PNAS
In This Issue
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INAUGURAL ARTICLE, CHEMISTRY
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Comparing atmospheric haze of Titan and early Earth
One of the most alluring features of Saturn's moon Titan is its thick layer of organic haze, which is formed from photochemistry between the methane and nitrogen gas in the high atmosphere. Although the recent Cassini/Huygens probe documented some of the organic constituents of Titan's atmosphere, the exact composition remains shrouded in the haze. Melissa Trainer et al. mimicked Titan's chemistry by using a deuterium lamp to irradiate various simulated atmospheres. In analyzing particle production, the authors observed the formation of multiple long-chain hydrocarbons and aromatics principally ranging from 28 to 109 atomic mass units. Overall, the aerosol properties matched well with documented data. Because Earth may have had a similar haze in its infancy, Trainer et al. next performed experiments within atmospheres that might have resembled early Earth (methane and carbon dioxide). The authors found that a significant haze layer could indeed have been produced, containing biotic precursors such as aldehydes and carboxylic acids. Earth's aerosol production may have totaled a calculated 1014 g per year. Such a magnitude suggests that these haze particles could have served as a primary source of surface organic material, allowing life to flourish globally and not just in localized environments. — N.Z.
"Organic haze on Titan and the early Earth" by Melissa G. Trainer, Alexander A. Pavlov, H. Langley DeWitt, Jose L. Jimenez, Christopher P. McKay, Owen B. Toon, and Margaret A. Tolbert (see pages 18035–18042)
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APPLIED BIOLOGICAL SCIENCES
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RNAi-engineered cottonseed for potential food use
Annually, the global harvest of cotton fibers produces 44 million metric tons of cottonseed as a byproduct. Cottonseeds are a source of high-quality protein, but the presence of the terpenoid compound gossypol makes cottonseeds unsafe for consumption by humans and other nonruminant animals. In the 1950s, breeding techniques produced "glandless cotton," but these varieties were commercial failures because their lack of protective terpenoids (normally stored in the glands) made them susceptible to insect pests. In an attempt to unlock the nutritional potential of cottonseed, Ganesan Sunilkumar et al. generated cotton plants with seeds containing gossypol levels well below the threshold for safe human consumption. Gossypol and related terpenoids are derived from 
-cadinene, and the authors used RNAi to target the -cadinene synthase gene in the engineered seeds. This RNAi-mediated gene silencing was targeted enough not to wipe out terpenoid production systemically, and thus the engineered plants appeared to escape the drawbacks of glandless cotton. Sunilkumar et al. note that in addition to possibly providing enough protein for 500 million people each year via gossypol-free cottonseed, targeted RNAi could also improve food supplies by modifying other crops with harmful components, such as cassava, grass pea, or fava beans. — N.Z.
"Engineering cottonseed for use in human nutrition by tissue-specific reduction of toxic gossypol" by Ganesan Sunilkumar, LeAnne M. Campbell, Lorraine Puckhaber, Robert D. Stipanovic, and Keerti S. Rathore (see pages 18054–18059)
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EVOLUTION
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Genetic admixture between archaic human lineages
The current view of Homo sapiens evolution leans toward a model where modern humans replaced archaic Homo lineages rather than interbred with them, although opposing views exist. Identifying traces of admixture within genetic loci could settle this debate, but under conditions of neutrality, genetic drift would make admixture detection almost impossible. However, if admixture resulted in transfer of an adaptive allele, it would rise in frequency and escape genetic drift. Patrick Evans et al. examined one candidate allele, the microcephalin D haplogroup. Microcephalin is a key regulator of brain size, and the D allele is present at high frequency despite a young coalescence age (37,000 years), indicating a rapid rise due to positive selection. Evans et al. found that this allele likely originated from a Homo lineage separated from modern humans 
1.1 million years ago. By 37,000 years ago, copies of this allele had introgressed into modern humans. Because this allele exists in highest frequency outside of Africa, it also suggests admixture with an ancient Eurasian population. This combination of dates and locations raises the intriguing possibility that the D allele originated in Neandertals, who may have contributed to rapid Homo sapiens evolution before being displaced. — N.Z.
"Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage" by Patrick D. Evans, Nitzan Mekel-Bobrov, Eric J. Vallender, Richard R. Hudson, and Bruce T. Lahn (see pages 18178–18183)
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GENETICS
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Origin and evolution of snake sex chromosomes
Researchers have found that the sex chromosomes of snakes, birds, and mammals origi- nated from different auto- somes (non-sex chromosomes) of a common ancestor. Kazumi Matsubara et al. constructed a cytogenetic map of the Japanese four-striped rat snake to examine its genetic relationships to humans and chickens. Eleven of the 109 clones identified were located on the Z sex chromosome. When the 11 clones were compared with their chicken and human homologs, the chicken and human genes were all found to be on autosomes, not sex chromosomes. This finding shows that human sex chromosomes evolved from different autosomes of a common ancestor, the authors say. Matsubara et al. also compared the positions of the 11 rat snake clones with those of the Burmese python and the habu (a venomous Japanese snake) to investigate the process of W chromosome differentiation. The sex chromosomes were only slightly differentiated in the python, somewhat differentiated in the rat snake, and fully differentiated in the habu. The differentiation of sex chromosomes may have begun at the end of the proto-sex chromosome of the snakes' common ancestor, and subsequent rearrangements and deletions progressed on the sex-specific chromosome from primitive to advanced species. — P.D.
"Evidence for different origin of sex chromosomes in snakes, birds, and mammals and step-wise differentiation of snake sex chromosomes" by Kazumi Matsubara, Hiroshi Tarui, Michihisa Toriba, Kazuhiko Yamada, Chizuko Nishida-Umehara, Kiyokazu Agata, and Yoichi Matsuda (see pages 18190–18195)
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MEDICAL SCIENCES
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Anxiety and stress as targets for Rett syndrome therapies
Mutations in the methyl-CpG binding protein 2 (Mecp2) gene account for most cases of Rett syndrome, a disorder characterized by Enhanced Crh expression in mouse brain.mental retardation, hand wringing, and behavioral abnormalities similar to autism. The MeCP2 protein acts as a transcriptional repressor, silencing target genes, but the target genes contributing to the symptoms of Rett syndrome remain elusive. Based on the observation that anxiety is also a common feature of Rett syndrome, Bryan McGill et al. assessed functioning of the hypothalamic–pituitary–adrenal axis, a molecular pathway that mediates stress and anxiety, in mice with a mutation in Mecp2. The authors found that Mecp2-mutant mice displayed increased anxiety, enhanced physiological stress responses, and an increased expression of corticotropin-releasing hormone (Crh), which coordinates behavioral and physiological stress responses. The results demonstrate that Crh is a target of MeCP2 and that impaired regulation of Crh by MeCP2 likely underlies the anxiety- and stress-related symptoms of Rett syndrome, suggesting that the Crh pathway could be targeted to relieve such symptoms. — M.M.
"Enhanced anxiety and stress-induced corticosterone release are associated with increased Crh expression in a mouse model of Rett syndrome" by Bryan E. McGill, Sharyl F. Bundle, Murat B. Yaylaoglu, James P. Carson, Christina Thaller, and Huda Y. Zoghbi (see pages 18267–18272)
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Related articles in PNAS:
- Inaugural Article: Organic haze on Titan and the early Earth
- Melissa G. Trainer, Alexander A. Pavlov, H. Langley DeWitt, Jose L. Jimenez, Christopher P. McKay, Owen B. Toon, and Margaret A. Tolbert
PNAS 2006 103: 18035-18042.
[Abstract]
[Full Text]
- From the Cover: Engineering cottonseed for use in human nutrition by tissue-specific reduction of toxic gossypol
- Ganesan Sunilkumar, LeAnne M. Campbell, Lorraine Puckhaber, Robert D. Stipanovic, and Keerti S. Rathore
PNAS 2006 103: 18054-18059.
[Abstract]
[Full Text]
- Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage
- Patrick D. Evans, Nitzan Mekel-Bobrov, Eric J. Vallender, Richard R. Hudson, and Bruce T. Lahn
PNAS 2006 103: 18178-18183.
[Abstract]
[Full Text]
- From the Cover: Evidence for different origin of sex chromosomes in snakes, birds, and mammals and step-wise differentiation of snake sex chromosomes
- Kazumi Matsubara, Hiroshi Tarui, Michihisa Toriba, Kazuhiko Yamada, Chizuko Nishida-Umehara, Kiyokazu Agata, and Yoichi Matsuda
PNAS 2006 103: 18190-18195.
[Abstract]
[Full Text]
- From the Cover: Enhanced anxiety and stress-induced corticosterone release are associated with increased Crh expression in a mouse model of Rett syndrome
- Bryan E. McGill, Sharyl F. Bundle, Murat B. Yaylaoglu, James P. Carson, Christina Thaller, and Huda Y. Zoghbi
PNAS 2006 103: 18267-18272.
[Abstract]
[Full Text]
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INAUGURAL ARTICLE, CHEMISTRY
APPLIED BIOLOGICAL SCIENCES
