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3582 Publications

Showing 3471-3480 of 3582 results
06/01/96 | Caste allometries in the soldier-producing aphidPseudoregma alexanderi (Hormaphididae: Aphidoidea)
D. L. Stern , A. Moon , C. Martinez del Rio
Insectes sociaux;43(2):137-147. doi: 10.1007/BF01242566

Colonies of the aphidPseudoregma alexanderi produce morphologically-specialized first-instar larvae, termed soldiers, that defend the colony from predators. The environmental cues and physiological mechanisms governing soldier production are currently unknown. Here we present a morphometric study of soldiers and normal first-instar larvae ofP. alexanderi. Several morphological features (fore-leg length and width, hind-leg length, and horn length) plotted against body length display relationship that are similar to a sigmoidal curve. We found further support for an earlier finding that soldiers fall into two size categories, majors and minors, although both types of soldiers appear to follow the same allometry. The patterns of allometry in the soldier-producing aphids are very different from those found in other social insects and do not easily fit into the traditional categorization of allometries. We present two simple alternative models of soldier development as a framework for guiding future studies of the mechanisms of soldier production.

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03/18/96 | Parametric generation of second sound by first sound in superfluid helium.
Rinberg D, Cherepanov V, Steinberg V
Physical Review Letters. 1996 Mar 18;76(12):2105-8. doi: 10.1523/JNEUROSCI.3613-08.2008

We report the experimental studies of a parametric excitation of a second sound (SS) by a first sound (FS) in a superfluid helium in a resonance cavity. The results on several topics in this system are presented: (i) The linear properties of the instability, namely, the threshold, its temperature and geometrical dependencies, and the spectra of SS just above the onset were measured. They were found to be in a good quantitative agreement with the theory. (ii) It was shown that the mechanism of SS amplitude saturation is due to the nonlinear attenuation of SS via three wave interactions between the SS waves. Strong low frequency amplitude fluctuations of SS above the threshold were observed. The spectra of these fluctuations had a universal shape with exponentially decaying tails. Furthermore, the spectral width grew continuously with the FS amplitude. The role of three and four wave interactions are discussed with respect to the nonlinear SS behavior. The first evidence of Gaussian statistics of the wave amplitudes for the parametrically generated wave ensemble was obtained. (iii) The experiments on simultaneous pumping of the FS and independent SS waves revealed new effects. Below the instability threshold, the SS phase conjugation as a result of three-wave interactions between the FS and SS waves was observed. Above the threshold two new effects were found: a giant amplification of the SS wave intensity and strong resonance oscillations of the SS wave amplitude as a function of the FS amplitude. Qualitative explanations of these effects are suggested.

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02/01/96 | The evolution of soldiers in aphids.
Stern DL, Foster WA
Biol Rev Camb Philos Soc. 1996 Feb;71(1):27-79

1. Defensive individuals, termed soldiers, have recently been discovered in aphids, Soldiers are typically early instar larvae, and in many species the soldiers are reproductively sterile and morphologically and behaviourally specialized. 2. Since aphids reproduce parthenogenetically, we might expect soldier production to be more widespread in aphids than it is. We suggest that a more useful way to think about these problems is to attempt to understand how a clone (rather than an individual) should invest in defence and reproduction. 3. Known soldiers are currently restricted to two families of aphids, the Pemphigidae and Hormaphididae, although they are distributed widely among genera within these families. We discuss the use of a phylogenetic perspective to aid comparative studies of soldier production and we demonstrate this approach using current estimates of phylogenetic affinities among aphids. We show that the distribution of soldier production requires a minimum of six to nine evolutionary origins plus at least one loss. 4. At least four main types of soldiers exist and we present and discuss this diversity of soldiers. 5. Most soldier-producing species produce soldiers within plant galls and we discuss the importance of galls for the evolution of soldiers. 6. We summarize the evidence on the interactions between soldiers and predators and between soldier-producing aphids and ants. 7. We present an optimality model for soldier investment strategies to help guide investigations of the ecological factors selecting for soldiers. 8. The proximate mechanisms of soldier production are currently very poorly understood and we suggest several avenues for further research.

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01/10/96 | A phylogenetic reanalysis of allozyme variation among populations of Galapagos finches
David L. Stern , Peter R. Grant
Zoological Journal of the Linnean Society;118(2):119-134. doi: 10.1111/j.1096-3642.1996.tb00222.x

We reanalysed Yang & Pattern's allozyme data, published in Auk in 1981, of Darwin's finches with a variety of distance and cladistic methods to estimate the phylogeny of the group. Different methods yielded different results, nevertheless there was widespread agreement among the distance methods on several groupings. First, the two species of Camarhynchus grouped near one another, but not always as a monophyletic group. Second, Cactospiza pallida and Platyspiza crassirostris formed a monophyletic group. Finally, all the methods (including parsimony) supported the monophyly of the ground finches. The three distance methods also found close relationships generally between each of two populations of Geospiza scandens, G. difficilis and G. conirostris. There is evidence for inconstancy of evolutionary rates among species. Results from distance methods allowing for rate variation among lineages suggest three conclusions which differ from Yang and Patton's findings. First, the monophyletic ground finches arose from the paraphyletic tree finches. Yang and Patton found that the ground finches and tree finches were sister monophyletic taxa. Second, Geospiza scandens appears to be a recently derived species, and not the most basal ground finch. Third, G. fuliginosa is not a recently derived species of ground finch, but was derived from an older split from the remaining ground finches. Most of these conclusions should be considered tentative both because the parsimony trees disagreed sharply with the distance trees and because no clades were strongly supported by the results of bootstrapping and statistical tests of alternative hypotheses. Absence of strong support for clades was probably due to insufficient data. Future phylogenetic studies, preferably using DNA sequence data from several unlinked loci, should sample several populations of each species, and should attempt to assess the importance of hybridization in species phylogeny.

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12/29/95 | Drosophila homologs of baculovirus inhibitor of apoptosis proteins function to block cell death.
Hay BA, Wassarman DA, Rubin GM
Cell. 1995 Dec 29;83(7):1253-62. doi: 10.1186/gb-2007-8-7-r145

Apoptotic cell death is a mechanism by which organisms eliminate superfluous or harmful cells. Expression of the cell death regulatory protein REAPER (RPR) in the developing Drosophila eye results in a small eye owing to excess cell death. We show that mutations in thread (th) are dominant enhancers of RPR-induced cell death and that th encodes a protein homologous to baculovirus inhibitors of apoptosis (IAPs), which we call Drosophila IAP1 (DIAP1). Overexpression of DIAP1 or a related protein, DIAP2, in the eye suppresses normally occurring cell death as well as death due to overexpression of rpr or head involution defective. IAP death-preventing activity localizes to the N-terminal baculovirus IAP repeats, a motif found in both viral and cellular proteins associated with death prevention.

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12/01/95 | Regulation of nuclear genes encoding mitochondrial proteins in Saccharomyces cerevisiae.
Brown TA, Evangelista C, Trumpower BL
Journal of Bacteriology. 1995 Dec;177(23):6836-43

Selection for mutants which release glucose repression of the CYB2 gene was used to identify genes which regulate repression of mitochondrial biogenesis. We have identified two of these as the previously described GRR1/CAT80 and ROX3 genes. Mutations in these genes not only release glucose repression of CYB2 but also generally release respiration of the mutants from glucose repression. In addition, both mutants are partially defective in CYB2 expression when grown on nonfermentable carbon sources, indicating a positive regulatory role as well. ROX3 was cloned by complementation of a glucose-inducible flocculating phenotype of an amber mutant and has been mapped as a new leftmost marker on chromosome 2. The ROX3 mutant has only a modest defect in glucose repression of GAL1 but is substantially compromised in galactose induction of GAL1 expression. This mutant also has increased SUC2 expression on nonrepressing carbon sources. We have also characterized the regulation of CYB2 in strains carrying null mutation in two other glucose repression genes, HXK2 and SSN6, and show that HXK2 is a negative regulator of CYB2, whereas SSN6 appears to be a positive effector of CYB2 expression.

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12/01/95 | Role of the morphogenetic furrow in establishing polarity in the Drosophila eye.
Chanut F, Heberlein U
Development. 1995 Dec;121(12):4085-94

The Drosophila retina is a crystalline array of 800 ommatidia whose organization and assembly suggest polarization of the retinal epithelium along anteroposterior and dorsoventral axes. The retina develops by a stepwise process following the posterior-to-anterior progression of the morphogenetic furrow across the eye disc. Ectopic expression of hedgehog or local removal of patched function generates ectopic furrows that can progress in any direction across the disc leaving in their wake differentiating fields of ectopic ommatidia. We have studied the effect of these ectopic furrows on the polarity of ommatidial assembly and rotation. We find that the anteroposterior asymmetry of ommatidial assembly parallels the progression of ectopic furrows, regardless of their direction. In addition, ommatidia developing behind ectopic furrows rotate coordinately, forming equators in various regions of the disc. Interestingly, the expression of a marker normally restricted to the equator is induced in ectopic ommatidial fields. Ectopic equators are stable as they persist to adulthood, where they can coexist with the normal equator. Our results suggest that ectopic furrows can impart polarity to the disc epithelium, regarding the direction of both assembly and rotation of ommatidia. We propose that these processes are polarized as a consequence of furrow propagation, while more global determinants of dorsoventral and anteroposterior polarity may act less directly by determining the site of furrow initiation.

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Egnor Lab
12/01/95 | The uncertain response in the bottlenosed dolphin (Tursiops truncatus).
Smith JD, Schull J, Strote J, McGee K, Egnor R, Erb L
Journal of Experimental Psychology. 1995 Dec;124(4):391-408

Humans respond adaptively to uncertainty by escaping or seeking additional information. To foster a comparative study of uncertainty processes, we asked whether humans and a bottlenosed dolphin (Tursiops truncatus) would use similarly a psychophysical uncertain response. Human observers and the dolphin were given 2 primary discrimination responses and a way to escape chosen trials into easier ones. Humans escaped sparingly from the most difficult trials near threshold that left them demonstrably uncertain of the stimulus. The dolphin performed nearly identically. The behavior of both species is considered from the perspectives of signal detection theory and optimality theory, and its appropriate interpretation is discussed. Human and dolphin uncertain responses seem to be interesting cognitive analogs and may depend on cognitive or controlled decisional mechanisms. The capacity to monitor ongoing cognition, and use uncertainty appropriately, would be a valuable adaptation for animal minds. This recommends uncertainty processes as an important but neglected area for future comparative research.

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Baker Lab

In Drosophila dosage compensation increases the rate of transcription of the male's X chromosome and depends on four autosomal male-specific lethal genes. We have cloned the msl-2 gene and shown that MSL-2 protein is co-localized with the other three MSL proteins at hundreds of sites along the male polytene X chromosome and that this binding requires the other three MSL proteins. msl-2 encodes a protein with a putative DNA-binding domain: the RING finger. MSL-2 protein is not produced in females and sequences in both the 5' and 3' UTRs are important for this sex-specific regulation. Furthermore, msl-2 pre-mRNA is alternatively spliced in a Sex-lethal-dependent fashion in its 5' UTR.

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07/01/95 | Toward simplifying and accurately formulating fragment assembly.
Myers EW
Journal of Computational Biology: A Journal of Computational Molecular Cell Biology. 1995 Summer;2(2):275-90

The fragment assembly problem is that of reconstructing a DNA sequence from a collection of randomly sampled fragments. Traditionally, the objective of this problem has been to produce the shortest string that contains all the fragments as substrings, but in the case of repetitive target sequences this objective produces answers that are overcompressed. In this paper, the problem is reformulated as one of finding a maximum-likelihood reconstruction with respect to the two-sided Kolmogorov-Smirnov statistic, and it is argued that this is a better formulation of the problem. Next the fragment assembly problem is recast in graph-theoretic terms as one of finding a noncyclic subgraph with certain properties and the objectives of being shortest or maximally likely are also recast in this framework. Finally, a series of graph reduction transformations are given that dramatically reduce the size of the graph to be explored in practical instances of the problem. This reduction is very important as the underlying problems are NP-hard. In practice, the transformed problems are so small that simple branch-and-bound algorithms successfully solve them, thus permitting auxiliary experimental information to be taken into account in the form of overlap, orientation, and distance constraints.

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