In This Issue
Evolution
Fly memories linked to gene variants
Even with brains no bigger than a punctuation mark, fruit flies not only show a capacity to learn but also heritable variation in how good they are at it. Before research by Frederic Mery et al., little was known about genes that contribute to this inherited variation in learning ability. The authors investigated the foraging gene (for), which codes for an enzyme, PKG, which is likely active in neurotransmission. This gene occurs in two variants, or alleles. The rover allele (forR) produces more enzyme and results in restless flies, whereas the sitter allele (forS) results in more sedentary flies. Mery et al. found that flies with the rover allele were quicker to associate certain smells with a mechanical shock. However, the sitter flies were better at maintaining a long-term memory of this association. Selectively increasing the levels of the enzyme in the mushroom bodies, which are the centers of olfactory learning, in sitter flies led to improvements in short-term memory but a reduction in long-term memory. This change identifies the mushroom bodies as the spatial focus of the gene's action on learning. Continued work to identify genes that contribute to natural variation in learning may lead to a better understanding of the evolution of cognitive traits. — T.H.D.
Expression of FOR in mushroom bodies of the Drosophila brain.
“Natural polymorphism affecting learning and memory in Drosophila” by Frederic Mery, Amsale T. Belay, Anthony K.-C. So, Marla B. Sokolowski, and Tadeusz J. Kawecki (see pages 13051–13055)
Medical Sciences
Noninvasive fetal diagnosis for Down's syndrome
For human chromosomes, three is a dangerous crowd. Trisomy 21, having an extra copy of chromosome 21, results in Down's syndrome. Current prenatal testing methods are invasive and carry a risk to fetus and mother. Noninvasive tests, like ultrasound, exist, although they detect only the manifestations of and not the chromosomal abnormality itself. To address these limitations, Dennis Lo et al. have developed a noninvasive test to analyze fetal DNA and RNA. During pregnancy, fetal nucleic acids circulate in the mother's bloodstream, but because most of these are not contained within cells, analyzing the relative number of chromosomes is difficult. To detect these tiny differences, Lo et al. used a sensitive type of PCR, called digital PCR, to count individual molecules. By determining relative levels of chromosome-21-derived nucleic acids, the authors could determine whether elevated amounts were present, indicating the abnormality. The work provides a set of benchmarks that are needed to correctly classify the abnormality >97% of the time. Further development of the technique could provide a noninvasive test for Down's syndrome. — T.H.D.
Sequential probability analysis of euploid or aneuploid fetus.
“Digital PCR for the molecular detection of fetal chromosomal aneuploidy” by Y. M. Dennis Lo, Fiona M. F. Lun, K. C. Allen Chan, Nancy B. Y. Tsui, Ka C. Chong, Tze K. Lau, Tak Y. Leung, Benny C. Y. Zee, Charles R. Cantor, and Rossa W. K. Chiu (see pages 13116–13121)
Neuroscience
LIM-homeodomain transcription factors direct Purkinje cell differentiation
The cerebellum controls movement and posture by integrating sensory input with motor output. Granule and Purkinje cells are the two main types of neurons that form the cerebellum, but the development of these cell types and the formation of the cerebellum are not well understood at the molecular level. Yangu Zhao et al. found that two closely related LIM-homeodomain proteins, Lhx1 and Lhx5, control Purkinje cell differentiation. LIM homeodomain proteins are transcription factors that play essential roles in many types of neurons throughout the developing central nervous system; they require a key cofactor called Ldb1. The researchers found that Lhx1, Lhx5, and Ldb1 mRNAs were expressed in developing Purkinje cells soon after they migrated out of the cerebellar ventricular zone. Mice that lacked both of the LIM homeodomain proteins displayed a severe reduction in number of Purkinje cells, and the inactivation of Ldb1 resulted in a similar phenotype. The authors' research provides insight into the molecular mechanisms at work in the developing mammalian cerebellum. — F.A.
Development of cerebellum Ldb1 conditional mutant.
“LIM-homeodomain proteins Lhx1 and Lhx5, and their cofactor Ldb1, control Purkinje cell differentiation in the developing cerebellum” by Yangu Zhao, Kin-Ming Kwan, Christina M. Mailloux, Woon-Kyu Lee, Alexander Grinberg, Wolfgang Wurst, Richard R. Behringer, and Heiner Westphal (see pages 13182–13186)
Plant Biology
Geometry of vascular systems governs plant forms
In comparative biology, the West, Brown, and Enquist (WBE) theory describes the relationship between an organism's phenotypic traits and its size, stating that the governing factor is the geometry of its vascular network. This core assumption has been combined with optimization assumptions: the branching network must obey biomechanical constraints, minimize transport costs, and maximally fill space to acquire nutrients and resources. Together with these secondary assumptions, the WBE theory predicts a “quarter-power” scaling rule that defines allometric variation. However, many plants do not conform to this mathematical relationship. Charles Price et al. have found that, by relaxing the secondary assumptions, the WBE theory does, in fact, predict the variation in botanical scaling relationships, demonstrating that the theory works for plants. The authors compiled botanical databases with measures of allometric and morphological traits and observed that selection has primarily acted to minimize hydrodynamic resistance in plant's vasculature. Their relaxed version of the theory quantitatively predicts mathematical functions that describe continuous covariation in the scaling of many phenotypic traits, which has not yet been shown for animals. — F.A.
Vascular network of a Calycanthus floridus leaf.
“A general model for allometric covariation in botanical form and function” by Charles A. Price, Brian J. Enquist, and Van M. Savage (see pages 13204–13209)
