This Week in Science

Science  18 Jun 2010:
Vol. 328, Issue 5985, pp. 1449
  1. Date with the Pharaohs


      Ancient Egypt dominated the Mediterranean world for several thousand years. However, the absolute chronology of this civilization has been uncertain, even though the sequence of rulers is well documented. Bronk Ramsey et al. (p. 1554; see the Perspective by Bruins) now provide a detailed radiocarbon-based record using more than 200 samples that spans much of this time and reduces uncertainties in some cases to less than 20 years. To avoid artifacts, the authors dated only short-lived plant remains from known contexts (i.e., that were associated with specific reigns). They then used the known reign lengths as a further constraint to obtain a final chronology. The final dates agree most closely with the previous older chronology but force some revisions to the timing of events in the Old Kingdom, the period in the third millennium B.C.E. when Egypt attained its first continuous peak of civilization.

    1. Birth of the Cool

        Over the past 4 million years or so, tropical sea surface temperatures have experienced a cooling trend (see the Perspective by Philander). Herbert et al. (p. 1530) analyzed sea surface temperature records of the past 3.5 million years from low-latitude sites spanning the world's major ocean basins in order to determine the timing and magnitude of the cooling that has accompanied the intensification of Northern Hemisphere ice ages since the Pliocene. Martínez-Garcia et al. (p. 1550) found that the enigmatic eastern equatorial Pacific cold tongue, a feature one might not expect to find in such a warm region receiving so much sunlight, first appeared between 1.8 and 1.2 million years ago. Its appearance was probably in response to a general shrinking of the tropical warm water pool caused by general climate cooling driven by changes in Earth's orbit.

      1. Close, But Not Too Close

          MicroRNAs (miRNAs) in plants are generally highly complementary to their target RNAs, yet, in most animal miRNAs, only the ∼8-nucleotide “seeds” sequence bases pair fully with the target, with few base pairs between the remainder of the miRNA and target. Plant miRNAs are methylated at their 3′ ends, whereas animals' miRNAs are not. Ameres et al. (p. 1534; see the Perspective by Pasquinelli) noticed that, in fruit flies, miRNAs engineered to have high complementarity to target RNAs were present at reduced levels. These miRNAs were trimmed and uridylated at their 3′ ends, features involved in RNA degradation. Fly small interfering RNAs, all of which are methylated at their 3′ ends, were unaffected, unless the methylating enzyme, Hen1, was mutated. Thus, 3′-methylation may prevent complementarity-driven remodeling and degradation of small RNAs.

        1. Hot on the Trail

            CREDIT: TISDALE ET AL.

            Solar cells essentially operate by absorbing light, which needs to be above a certain energy threshold. The absorbed light then liberates charges within the solar cell to carry electrical current. Unfortunately, the liberated charges behave the same way whether they are excited right at the threshold (e.g., by visible light) or well above it (by ultraviolet light), which leads to any excess energy being dissipated as waste heat. Tisdale et al. (p. 1543) have documented a potential first step toward resolving this inefficiency. Specifically, electrons excited by light absorption in lead selenide nanocrystals were able to migrate to an adjacent titanium dioxide surface without releasing their excess energy to heat. The next step will be to devise a means of harnessing the stored energy in a circuit.

          1. Going Down the Tube

              Two pillars of modern physics are quantum mechanics and general relativity. So far, both have remained apart with no quantum mechanical description of gravity available. Van Zoest et al. (p. 1540; see the Perspective by Nussenzveig and Barata) present work with a macroscopic quantum mechanical system—a Bose-Einstein condensate (BEC) of rubidium atoms in which the cloud of atoms is cooled into a collective quantum state—in microgravity. By dropping the BEC down a 146-meter-long drop chamber and monitoring the expansion of the quantum gas under these microgravity conditions, the authors provide a proof-of-principle demonstration of a technique that can probe the boundary of quantum mechanics and general relativity and perhaps offer the opportunity to reconcile the two experimentally.

            1. Tracking Triplet-State Transfers

                In devices such as organic transistors and photovoltaic cells, energy flow from donor to acceptor sites can occur via electrons that have been excited into higher electronic levels, which create a triplet state with two unpaired spins. At short distances between donor and acceptor, the transfer occurs through direct tunneling, but at longer distances, the electron “hops” in a multistep process. Vura-Weis et al. (p. 1547) used femtosecond transient absorption spectroscopy to observe this crossover in transfer mechanism directly in a series of molecules with varying bridge lengths between the donor and acceptor.

              1. Don't Forget the Edges

                  Reef-building corals are highly diverse, and many are threatened with extinction. In order to make predictions about coral survival, Budd and Pandolfi (p. 1558) examined morphological changes occurring over the evolutionary history of Caribbean corals. Long-term evolutionary patterns, including hybridization between species and diversification into new species (so-called lineage fusion and splitting) differed depending on the geographical location within the colony, with higher change occurring at a species' geographic margin relative to those in central locations. Thus, edge zones, which often experience limited gene flow, are responsible for the predominance of evolutionary innovation. If conservation strategies are biased toward biodiversity hotspots, which represent centers of high species richness, they may miss important sources of evolutionary novelty during global change.

                1. Keeping Egg Production Going

                    Whether oogenesis ceases around birth in vertebrate ovaries has been a topic of long-standing interest and considerable debate. Nakamura et al. (p. 1561, published online 20 May) now identify germline stem cells in the ovary of the teleost fish medaka. The stem cells are found as clusters of germ cells (termed germinal cradles) in a cord-like structure that expresses sox9, a gene critical for testis formation in mammals. The cords are buried within the ovary within the germinal epithelium. This work in fish shows that there can indeed be continuing egg production from vertebrate germline stem cells.

                  1. The Space in Your Head

                      CREDIT: LANGSTON ET AL.

                      Space, and events associated with places and spaces, are represented in the brain by a circuitry made of place cells, head direction cells, grid cells, and border cells. These cell types form a collective dynamic representation of our position as we move through the environment. How this representation is formed has remained a mystery. Is it acquired, or are we born with the ability to represent external space (see the Perspective by Palmer and Lynch)? Langston et al. (p. 1576) and Wills et al. (p. 1573) investigated the early development of spatial activity in the hippocampal formation and the entorhinal cortex of rat pups when they first began to explore their environment. Rudiments of place cells, head direction cells, and grid cells already existed when the pups made their first movements out of the nest. A neural representation of external space at this early time points to strong innate components for perception of space. These findings provide experimental support for Kant's 200-year-old concept of space as an a priori faculty of the mind.

                    1. Herpes Virus MiRNA Modulation

                        Viruses use a number of strategies to manipulate the cells of their host to ensure a successful infection. Herpesvirus saimiri (HVS) generates highly conserved small noncoding RNAs HSUR 1 and HSUR 2, which modulate expression of a number of proteins in infected primate T cells. Cazalla et al. (p. 1563; see the Perspective by Pasquinelli) observed complementarity between HSUR sequences and the seed regions of three different miRNAs—miR-142-3p, miR-27, and miR-16—and found that these HSURs could bind to the miRNAs. Furthermore, the level of mature miR-27 was modulated by binding to HSUR 1, which targeted the miRNA for degradation.

                      1. miR-33 in Cholesterol Control

                          With the well-established link between serum cholesterol levels and cardiovascular disease and the availability of effective cholesterol-lowering drugs, cholesterol screening has rapidly become a routine part of health care. Yet, much remains to be learned about how cholesterol levels are regulated at the cellular level (see the Perspective by Brown et al.). Now, Najafi-Shoushtari et al. (p. 1566, published online 13 May) and Rayner et al. (p. 1570, published online 13 May) have discovered a new molecular player in cholesterol control—a small noncoding RNA that, intriguingly, is embedded within the genes coding for sterol regulatory element-binding proteins (SREBPs), transcription factors already known to regulate cholesterol levels. This microRNA, called miR-33, represses expression of the adenosine triphosphate–binding cassette transporter A1, a protein that regulates synthesis of high-density lipoprotein (HDL, or “good” cholesterol) and that helps to remove “bad” cholesterol from the blood. Reducing the levels of miR-33 in mice boosted serum HDL levels, suggesting that manipulation of this regulatory circuit might be therapeutically useful.