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In This Issue
ENGINEERING
Turbulence muddles laminar flow scission
In strong fluid flows, the coupling of mechanical and atomic-scale chemical processes can shear long-chain polymers, such as DNA. This scission can adversely affect various applications, such as performance of gene therapy constructs, handling of polymeric fluids, and turbulent drag reduction in pipelines. Experimental results over the past four decades have not always agreed with laminar flow theory. In an attempt to reconcile discrepancies in flow dynamics, Siva Vanapalli et al. formulated a simple scaling theory for long-chain polymer scission that accounted for fluid turbulence as well as turbulent velocity fluctuations. The equation demonstrated that the smallest scales of turbulence and the largest scales of polymer dynamics dominated the relationship between the strength of the flow and the longest polymer chain that can remain unbroken in that flow. According to the theory, local stress at the Kolmogorov scale generates the molecular tension that leads to polymer covalent bond breakage. The authors' equation yields a universal scaling for polymer scission in turbulent flows, which can be applied to design flow that will break polymer chains into predictable sizes, useful in technologies such as the shotgun sequencing of DNA. — F.A.
Flow turbulence in differing geometries.
“Universal scaling for polymer chain scission in turbulence” by Siva A. Vanapalli, Steven L. Ceccio, and Michael J. Solomon (see pages 16660–16665)
AGRICULTURAL SCIENCES
Diversity loss in modern soybeans
Since crops were first domesticated several thousand years ago, human interventions have led to a few high-yielding cultivars dominating production, at the cost of genetic diversity. Such is the case with the American soybean, which is extremely uniform among cultivars because of three genetic bottlenecks: initial domestication in Asia to create numerous landraces (soybean types ideally suited to the land), introduction of only a few landraces to North America, and selective breeding over the last 75 years. Sequencing four soybean populations encompassing the entire domestication history, David Hyten et al. quantified this reduced genetic diversity. The largest impact occurred with Asian domestication, which halved the unusually low variability of wild soybean and resulted in the loss of 81% of rare alleles. Although recent human selection has further eroded diversity, modern cultivars still retain 72% of the sequence diversity found in Asian landraces (although 79% of rare alleles have been lost). The small number of introduced landraces, not recent breeding, was responsible for most of the loss. Because soybean diversity is inherently low, Hyten et al. suggest that these lost rare alleles may be the key to improving future production gains or protection against emerging pests and diseases. — N.Z.
Asian soybean landraces.
“Impacts of genetic bottlenecks on soybean genome diversity” by David L. Hyten, Qijian Song, Youlin Zhu, Ik-Young Choi, Randall L. Nelson, Jose M. Costa, James E. Specht, Randy C. Shoemaker, and Perry B. Cregan (see pages 16666–16671)
BIOPHYSICS
Male mosquito antennae hear female's signal
A male mosquito detects the weak wingbeat of a passing female through the Johnston's organ of its antennae, a sensitive acoustic receiver with nearly as many sensory cells as the human cochlea. Joseph Jackson and Daniel Robert report that the male antenna actively responds to the female's wingbeat, an evolutionary adaptation that amplifies the female's transient signal and enhances the male's hearing ability. To study the auditory mechanisms of the male mosquito, the authors modeled a stimulus that mimicked the temporal acoustic profile of a flying female. This stimulus was faithful to natural female sounds in both amplitude and time. Time-resolved analysis of the male antennae's mechanical responses revealed three distinct phases: an initial gain as the female approached, a compression at higher amplitudes when the female was closest to the male, and a final, large gain as the female departed. Jackson and Robert suggest that this auditory autofocus mechanism provides an unconventional type of selective attention allowing male mosquitoes to quickly identify females and potentially other organisms. — F.A.
Auditory organs of the male mosquito.
“Nonlinear auditory mechanism enhances female sounds for male mosquitoes” by Joseph C. Jackson and Daniel Robert (see pages 16734–16739)
DEVELOPMENTAL BIOLOGY
Embryonic stem cells form primary germ cell layers
To fully exploit the potential of embryonic stem (ES) cells in basic biology and regenerative medicine, recapitulating early embryonic cell lineage events in model systems is critical. Paul Gadue et al. demonstrate that ES cells give rise to a population similar to the primitive streak (PS), one of the first signs of gastrulation in development, and further develop into mesoderm and endoderm, prerequisite steps in the formation of many tissues. The authors established a mouse ES cell line that expressed the marker human CD4, which is present in the anterior PS; the anterior PS produces the endoderm and anterior mesoderm, including cardiac cells. Cells expressing high CD4 displayed characteristics of the anterior PS, whereas cells expressing low CD4 resembled the posterior streak, which gives rise to the hematopoietic cells of the yolk sac. Wnt and the TGF-β/nodal/activin signaling pathways, important in gastrulation, were simultaneously required for the generation of the PS cell populations. These findings uncover a use for ES cells in studies of developmental embryology and reveal insights into signaling pathways involved in the establishment of mesoderm and endoderm. — F.A.
Primitive streak gene expression.
“Wnt and TGF-β signaling are required for the induction of an in vitro model of primitive streak formation using embryonic stem cells” by Paul Gadue, Tara L. Huber, Patrick J. Paddison, and Gordon M. Keller (see pages 16806–16811)
GENETICS
Candidate gene for autism
The genetic underpinnings of autism are an active area of study. One gene, MET, is known to be involved in brain development, regulation of the immune system, and repair of the gastrointestinal system, all of which can be adversely affected in individuals with autism. Daniel Campbell et al. have identified a mutation in MET in children with autism that could provide insights into the mechanisms of the disorder. Although the coding region of the gene, which encodes for a receptor tyrosine kinase that mediates hepatocyte growth factor signaling, was not mutated, the promoter region controlling gene expression levels was altered. This mutation resulted in decreased MET expression. The mutation was common in children with autism, appearing more frequently in families with more than one child with autism. Overall, this mutation raised the relative risk of autism by 2.27 times. Given the gene's involvement in the development of the neocortex and cerebellum, Campbell et al. say their findings could provide leads in characterizing the brain abnormalities causing autism. These results may also lead to improved understanding of medical conditions associated with autism. — P.D.
MET locus genomic structure.
“A genetic variant that disrupts MET transcription is associated with autism” by Daniel B. Campbell, James S. Sutcliffe, Philip J. Ebert, Roberto Militerni, Carmela Bravaccio, Simona Trillo, Maurizio Elia, Cindy Schneider, Raun Melmed, Roberto Sacco, Antonio M. Persico, and Pat Levitt (see pages 16834–16839)
