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  • 1
    Electronic Resource
    Electronic Resource
    Scanning motion, ocellar morphology and orientation mechanisms in the phototaxis of the nematode Mermis nigrescens (1990)
    Springer
    Journal of comparative physiology 167 (1990), S. 257-268 
    Details
    ISSN: 1432-1351
    Keywords: Scanning motion ; Shadowing ; Ocellus ; Mechanism ; Orientation ; Phototaxis ; Klinotaxis ; Photomovement ; Transverse phototaxis ; Nematode
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The putative ocellus of Mermis females consists of a hollow cylinder of dense hemoglobin pigmentation located in the anterior tip. The exact location of the photoreceptive nerve endings, however, is unknown. During phototaxis a continual bending or scanning motion of the head (anterior 2 mm) causes the orientation of the tip to swing about the direction of the source. By turning off (shuttering) the light source whenever the tip orientation was to one side of the source direction, the average orientation of the base of the head, and eventually the body orientation, was caused to be biased about 28° to the opposite side. Because the shuttering was synchronized with the scanning motion, the scanning motion must be involved in the maintenance of orientation to light. The direction of the bias rules out a two-signal comparison mechanism of orientation and demonstrates that a deviation of the tip from the source direction must decrease, rather than increase, the illumination of the photoreceptors. These findings, and the ocellar morphology, require that the photoreceptors be located inside the hollow tube of pigmentation where they can be shadowed by the pigment during deviations of the tip. Focusing by the curved anterior end should cause a similar modulation of the illumination at this location. The occasional episodes of transverse phototaxis can be explained by the leakiness of the pigment walls to transverse illumination. Analysis of the motion of the anterior in the presence and absence of shuttering indicates that the orientation of the base of the head, due to the motion of the neck, is controlled by the signals generated during one or more cycles of the scanning motion of the head. The orientation may be regulated by the phase relationship between the photoreceptor signal and putative proprioceptive signals that indicate the bending in the head.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00188118
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  • 2
    Electronic Resource
    Electronic Resource
    Components of phototaxis of the nematode Mermis nigrescens (1990)
    Springer
    Journal of comparative physiology 167 (1990), S. 245-255 
    Details
    ISSN: 1432-1351
    Keywords: Undulatory locomotion ; Scanning motion ; Directed turn ; Orientation ; Phototaxis ; Klinotaxis ; Photomovement ; Nematode
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The gravid females of Mermis are positively phototaxic at the time of their migration to egglaying sites in vegetation on which their grasshopper hosts feed. On a horizontal felt surface, segments of the path traced by the tail are oriented approximately towards a source of monochromatic light in the 350–540 nm region, but are not oriented at longer wavelengths and in the dark. The components of this phototaxis include locomotion by the posterior 4/5 of the body, orientational bending of the neck region while the anterior is held above the substrate, and a scanning motion (bending) of the head region (anterior 2 mm). Like other nematodes and snakes, propulsion is associated with posteriorly propagated body waves, but unlike other animals known, the waves tend to lie perpendicular to a felt surface, and unlike other nematodes, contact with the surface is on the female's ventral surface. The body waves are initiated by the motion of the anterior 1/5 (15 mm) of the body, the average orientation of which determines the path of the following 4/5. During phototaxis, the anterior tip is swung both sideways and vertically about the direction towards the light source. The tip motion is a result of a scanning motion of the head and a slower orientational bending of the neck. The base of the head appears to be actively directed towards the source by the bending of the neck. This behavior can resolve two light sources positioned 120° apart but not 90° apart. The scanning motion of the head is independent of neck orientation and appears to enhance the probability of discovering the direction of a new source. Discovery is followed by a directed turn of the base of the head towards the source which is initiated by the bending of the neck. Locomotion of the body follows the path of the anterior through the turn and phototaxis is thus initiated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00188117
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  • 3
    Electronic Resource
    Electronic Resource
    Sequential up-regulation of thyroid hormone β receptor, ornithine transcarbamylase, and carbamyl phosphate synthetase mRNAs in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis (1992)
    Chichester [u.a.] : Wiley-Blackwell
    Developmental Genetics 13 (1992), S. 289-301 
    Details
    ISSN: 0192-253X
    Keywords: Thyroid hormone ; carbamyl phosphate synthetase ; Rana catesbeiana ; metamorphosis ; Life and Medical Sciences ; Genetics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology
    Notes: During both spontaneous and thyroid hormone (TH)-induced metamorphosis, the Rana catesbeiana tadpole undergoes postembryonic developmental changes in its liver which are necessary for its transition from an ammonotelic larva to a ureotelic adult. Although this transition ultimately results from marked increases in the activities and/or de novo synthesis of the urea cycle enzymes, the precise molecular means by which TH exerts this tissue-specific response are presently unknown. Recent reports, using RNA from whole Xenopus laevis tadpole homogenates and indirect means of measuring TH receptor (TR) mRNAs, suggest a correlation between the up-regulation of TRβ-mRNAs and the general morphological changes occurring during amphibian metamorphosis. To assess whether or not this same relationship exists in a TH-responsive tissue, such as liver, we isolated and characterized a cDNA clone containing the complete nucleotide sequence for a R. catesbeiana urea cycle enzyme, ornithine transcarbamylase (OTC), as well as a genomic clone containing a portion of the hormone-binding domain of a R. catesbeiana TRβ gene. Through use of these homologous sequences and a heterologous cDNA fragment encoding rat carbamyl phosphate synthetase (CPS), we directly determined the relative levels of the TRβ, OTC, and CPS mRNAs in liver from spontaneous and TH-induced tadpoles. Our results establish that TH affects an up-regulation of mRNAs for its own receptor prior to up-regulating CPS and OTC mRNAs. Moreover, results with cultured tadpole liver demonstrate that TH, in the absence of any other hormonal influence, can affect an up-regulation of both the TRβ and OTC mRNAs. © 1992 Wiley-Liss, Inc.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/dvg.1020130406
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  • 4
    Electronic Resource
    Electronic Resource
    The independent stage-specific expression of the 18-kDa heat shock protein genes during microsporogenesis in Zea mays L (1993)
    Chichester [u.a.] : Wiley-Blackwell
    Developmental Genetics 14 (1993), S. 15-26 
    Details
    ISSN: 0192-253X
    Keywords: Heat shock protein ; maize ; mi-crosporogenesis ; gametogenesis ; Life and Medical Sciences ; Genetics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology
    Notes: The small (18-kDa) heat shock proteins (hsps) of maize are encoded by a complex multigene family. In a previous report, we described the genetic information from cDNAs encoding two different members of the family. In this communication, we report the isolation and characterization of cDNA and genomic clones encoding information for a third member of this hsp family (c/gMHSP18-1). DNA fragments containing nucleotide sequences common to, or specific for, each of these characterized 18-kDa genes were prepared and used as probes to assess the expression of these genes during microsporogenesis and development of the gametophyte in an inbred line of maize (Oh43). Our results demonstrate (1) that mRNA transcripts encoding the 18-kDa hsps are expressed and/or accumulate during microsporogenesis, and (2) that genes encoding two of the characterized 18-kDa hsps are expressed and/or accumulate independently, in a stage-specific manner during microsporogenesis. These observations imply that the stage-specific expression of particular 18-kDa hsp genes results from gene-specific regulation during microsporogenesis and gametophyte development rather than from an overall activation of the heat shock or stress response. © 1993Wiley-Liss, Inc.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/dvg.1020140104
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  • 5
    Electronic Resource
    Electronic Resource
    Metamorphosis: Model systems for studying gene expression in postembryonic development (1994)
    Chichester [u.a.] : Wiley-Blackwell
    Developmental Genetics 15 (1994), S. 313-319 
    Details
    ISSN: 0192-253X
    Keywords: Metamorphosis ; postembryonic development ; thyroid hormones ; ecdysteroids ; amphibians ; insects ; fish ; Life and Medical Sciences ; Genetics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/dvg.1020150402
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  • 6
    Electronic Resource
    Electronic Resource
    Characterization and expression of C/EBP-like genes in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis (1994)
    Chichester [u.a.] : Wiley-Blackwell
    Developmental Genetics 15 (1994), S. 366-377 
    Details
    ISSN: 0192-253X
    Keywords: C/EBP ; thyroid hormone ; metamorphosis ; gene expression ; Rana cafesbeiana ; bZlP proteins ; Life and Medical Sciences ; Genetics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology
    Notes: Tissue-specific changes in gene expression occur in the liver of Rana cafesbeiana tadpoles undergoing metamorphosis. Many of these changes can be induced precociously by administration of thyroid hormone (TH) to a tadpole or to cultured tadpole liver. While the precise molecular means by which TH exerts a tissue-specific response is unknown, recent studies suggest that the expression of genes which are liver-specific and characteristic of the adult liver phenotype may rely on TH-induction of tissue-specific transcription factors, as well as the thyroid hormone receptor proteins. Guided by this notion, we screened our Rana catesbeiana liver cDNA library and isolated clones, RcC/EBP-1 and -2, encoding Rana homologues of a mammalian transcription factor, C/EBP (CCAAT/enhancer core binding protein), implicated in the expression of liver-specific genes and terminal differentiation of hepatocytes. Gel mobility shift assays demonstrate that the proteins synthesized from these cDNAs bind specifically to the consensus binding site for C/EBP-related proteins. Characterization of the amino acid sequence in the bZlP DNA-binding domains of these proteins suggests that RcC/EBP-1 and -2 encode Rana homologues of C/EBPα and δ, respectively. Hybridization analyses demonstrate that the amount of RcC/EBP-2 mRNAs in tadpole liver remains constant throughout metamorphosis, whereas RcC/EBP-1 mRNAs are up-regulated during both spontaneous and TH-induced metamorphosis. The TH-induced up-regulation of RcC/ EBP-1 mRNAs precedes the up-regulation of liver-specific urea cycle enzyme mRNAs by 6 to 12 hours. These results, coupled with in situ hybridization studies, suggest that RcC/EBP-1 mRNAs encode a transcription factor which may play an early role(s) in the terminal differentiation and/or reprogramming of gene expression in this tadpole's liver cells during both spontaneous and TH-induced metamorphosis. ©1994 WiIey-Liss, Inc.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/dvg.1020150408
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