Thyroid Hormone Receptor Genes of Neotenic Amphibians

Since thyroid hormones play a pivotal role in amphibian metamorphosis we used PCR to amplify DNA fragments corresponding to a portion of the ligand-binding domain of the thyroid hormone receptor (TR) genes in several neotenic amphibians: the obligatory neotenic members of the family Proteidea the mudpuppy Necturus maculosus and Proteus anguinus as well as two members of the facultative neotenic Ambystoma genus: the axolotl Ambystoma mexicanum and the tiger salamander Ambystoma tigrinum. In addition, we looked for TR genes in the genome of an apode Typhlonectes compressicaudus. TR genes were found in all these species including the obligatory neotenic ones. The PCR fragments obtained encompass both the C and E domains and correspond to α and β genes. Their sequences appear to be normal, suggesting that there is no acceleration of evolutionary rates in the TR genes of neotenic amphibians. This result is not surprising for Ambystomatidae, which are known to respond to T3 (3,3′,5-triiodothyronine) but is not in agreement with biochemical and biological data showing that Proteidea cannot respond to thyroid hormones. Interestingly, by RT-PCR analysis we observed a high expression levels of TRα in gills, intestine, and muscles of Necturus as well as in the liver of Ambystoma mexicanum, whereas TRβ expression was only detected in Ambystoma mexicanum but not in Necturus. Such a differential expression pattern of TRα and TRβ may explain the neoteny in Proteidea. The cloning of thyroid-hormone-receptor gene fragments from these species will allow the molecular study of their failure to undergo metamorphosis. Received: 23 April 1996 / Accepted: 20 January 1997     

Hypoxia- and normoxia-induced reversibility of autonomic control in Andean guinea pig heart

León-Velarde, Fabiola, Jean-Paul Richalet, Juan-CarlosChavez, Rachid Kacimi, Maria Rivera-Chira, José-Antonio Palacios, and Daniel Clark. Hypoxia- and normoxia-induced reversibility ofautonomic control in Andean guinea pig heart. J. Appl.Physiol. 81(5): 2229-2234, 1996.We hereindescribe the regulation of cardiac receptors in a typical high-altitudenative animal. Heart rate response to isoproterenol(HR_Iso)(beats ^· min^{1 ·} mgIso ^· kg¹)and atropine, the density of -adrenergic(_AR) and muscarinic (M₂) receptors, and theventricular content of norepinephrine (NE) and dopamine (DA) werestudied in guinea pigs (Caviaporcellus). Animals native to Lima, Peru (150 m) werestudied at sea level (SL) and after 5 wk at 4,300-m altitude (SL-HA).Animals native to Rancas [Pasco, Peru (4,300 m)] werestudied at high altitude (HA) and after 5 wk at SL (HA-SL). HA animalshad a lower HR_Iso, maximum numberof _AR binding sites(B_max),_AR dissociation constant (K_d), NE, andDA (P < 0.05) and a higherM₂B_max(P < 0.001) when compared with theSL group. HA-SL showed an increase of theHR_Iso, _ARK_d, and NE(P < 0.05) and a decrease of theM₂B_max andK_d (P < 0.0001) when compared with theHA group. The present study demonstrates the differential regulationand reversibility of the autonomic control in the guinea pig heart.

     

Involvement of Ureides in Nitrogen Fixation Inhibition in Soybean

The sensitivity of N₂ fixation to drought stress in soybean (Glycine max Merr.) has been shown to be associated with high ureide accumulation in the shoots, which has led to the hypothesis that N₂ fixation during drought is decreased by a feedback mechanism. The ureide feedback hypothesis was tested directly by measuring the effect of 10 mm ureide applied by stem infusion or in the nutrient solution of hydroponically grown plants on acetylene reduction activity (ARA). An almost complete inhibition of ARA was observed within 4 to 7 d after treatment, accompanied by an increase in ureide concentration in the shoot but not in the nodules. The inhibition of ARA resulting from ureide treatments was dependent on the concentration of applied ureide. Urea also inhibited ARA but asparagine resulted in the greatest inhibition of nodule activity. Because ureides did not accumulate in the nodule upon ureide treatment, it was concluded that they were not directly inhibitory to the nodules but that their influence mediated through a derivative compound, with asparagine being a potential candidate. Ureide treatment resulted in a continual decrease in nodule permeability to O₂ simultaneous with the inhibition of nitrogenase activity during a 5-d treatment period, although it was not clear whether the latter phenomenon was a consequence or a cause of the decrease in the nodule permeability to O₂.The physiological basis of N₂ fixation inhibition by water deficits in legume nodules is not clearly understood. A potential physiological basis for this water-deficit sensitivity may be that drought stress decreases the P_o (Weisz et al., 1985), as has been shown with other stresses such as temperature, salinity, or nitrate (Hunt and Layzell, 1993; Serraj et al., 1994; Denison and Harter, 1995). The role of O₂ limitation in the response of nitrogenase activity to drought stress has been discussed extensively (Diaz del Castillo and Layzell, 1995; Purcell and Sinclair, 1995; Serraj and Sinclair, 1996b; Serraj et al., 1999). However, the mechanisms by which drought affects P_o have not been elucidated. It is not clear whether drought stress has a direct effect on P_o, or whether the decrease in P_o is a consequence of a decrease in nodule activity.An alternative explanation for the decrease in nitrogenase activity under drought could be a feedback mechanism involving the accumulation of N compounds. Pate et al. (1969) proposed that lower rates of water movement out of the nodule during drought stress may restrict export of products of N₂ fixation, and the accumulation of these products would inhibit nitrogenase activity. Others have suggested that N₂ fixation in legumes might be regulated by a feedback mechanism involving N metabolism and the pool of reduced N in the plant (Silsbury et al., 1986; Parsons et al., 1993; Hartwig et al., 1994). Oti-Boateng and Silsbury (1993) reported an inhibition of nitrogenase activity in fava bean after plant uptake of Asn or Gln.Soybean (Glycine max Merr.) usually exports more than 80% of the N compounds out of the nodules in the form of the ureides Aln and Alac. They are transported in the xylem to the shoots, where they are catabolized (Winkler et al., 1987). High accumulation of petiole ureides has been measured during the inhibition of N₂ fixation by drought in both controlled (de Silva et al., 1996; Serraj and Sinclair, 1996a) and field (Purcell et al., 1998) environments. Furthermore, in a comparison of grain legume species, Sinclair and Serraj (1995) reported that those species exporting ureides from the nodules had N₂ fixation that was drought sensitive. Those species that exported little or no ureide had N₂ fixation that was relatively drought tolerant.An important possibility is that the accumulation of ureides in soybean nodules under soil-water deficits might trigger a feedback mechanism that results in decreased N₂ fixation activity (Sinclair and Serraj, 1995; Serraj et al., 1999). This paper reports a series of experiments to investigate the hypothesis of a ureide feedback inhibition of N₂ fixation in soybean. First, ureide levels were measured in plant tissue (nodules, roots, and shoots) upon the imposition of water deficits to confirm that ureide levels increased in the nodules themselves, and not just in the shoot. Second, the influence of ureides on nodule activity was examined by introducing ureides, along with other compounds, into soybean plants. These experiments were designed to examine the time course of the response and to determine the concentration response. Third, data were collected to determine if P_o and the response of N₂ fixation to pO₂ were also sensitive to introduced ureides.     

Spatial analyses of ecological count data: A density map comparison approach

Analysing spatial patterns of population distributions may help to infer the decisive underlying ecological processes. Here we propose a method adapted to the spatial analysis of count data. Named MAPCOMP (MAP COMParison), it is based on the calculation of a formal distance, the Hellinger distance, between the density map of counts and the density map of sampling effort. Statistical tests of spatial homogeneity are based on count permutations across sampling sites and on valuable properties of the Hellinger distance. We assessed the efficiency of MAPCOMP by simulating different types and locations of clusters of individuals and compared its performance to the classical red–blue SADIE method, used as a reference. The two methods were also compared with respect to counts of codling moth larvae in orchards. Thanks to its better theoretical properties than SADIE, MAPCOMP was efficient in detecting spatial inhomogeneity when clusters were located on square or elongated spatial domains and more or less close to the edges, even for small sample sizes. It also appeared not very sensitive to edge effects. Another advantage of MAPCOMP is a bandwidth parameter that allows assessing the spatial extent of heterogeneity, if any.     

Seasonal cycles and persistence in an acarine predator-prey system on cassava in Africa

We applied time series analysis and a mechanistic predator-prey model to long-term data of monthly population counts of the herbivorous pest mite Mononychellus tanajoa and its introduced phytoseiid predator Typhlodromalus aripo from a cassava field in Benin, West Africa. In this approach, we determined the extent to which the main features of the observed predator-prey fluctuations in cassava fields can be explained from biotic traits inherent to the biology of predator and prey, and the extent of the significance of abiotic factors in determining population levels. The time series analyses with cross-correlation showed that the period of predator-prey fluctuations coincided with the annual pattern of intense rainfall and onset of dry season. A pronounced M. tanajoa peak followed after a short lag (2 weeks) by a T. aripo peak coincided with a trough in rainfall intensity. Both the prey and predator had local and lower peaks that coincided with high rainfall intensity, but with a considerably longer lag (ca. 3 months) compared with the high peaks occurring at the onset of the dry season. Regression of log-transformed data series (over a 7-year period) showed that—except for the first year after predator release—M. tanajoa fluctuated around an almost time-invariant mean population density, while T. aripo densities showed a consistent decline over the full observation period. To explain observed trends and periodic components in the data-series of predator and prey densities, we review hypotheses that are based on (1) the annual patterns and trends in abiotic factors, (2) mechanisms endogenous to the predator-prey system and (3) a combination of exogenous and endogenous factors.     

Les environnements sédimentaires des gisements pré-acheuléens et acheuléens des wadis Akerdil et Bamouéré (Guelb er-Richât, Adrar, Mauritanie), une première approche

In the Mauritanian Adrar, the Guelb er-Richât and, in particular, the depression of its external ring are the location of rather exceptional Acheulian industry accumulations discovered by Théodore Monod. These sites may have originated from one or more non dated humid phases where seasonal water presence induced faunal and thus predator presence. Piedmont relief, foot slope rock flow and deltaic spreads attest, until the last humid Holocene, the permanence of particularly favourable sectors like upstream of Akerdil and Bamouéré wadis. Outcrops or chaos of Chinguetti quartzite sandstone have supplied the necessary raw material for tool confection; the second factor favouring the presence of these Acheulian sites. The rather difficult aim of the authors is first, to locate lithics in stratigraphic position and second, to establish a chronostratigraphy. Some first examples are presented.     

Inhibition of ribosome recruitment induces stress granule formation independently of eukaryotic initiation factor 2alpha phosphorylation

Cytoplasmic aggregates known as stress granules (SGs) arise as a consequence of cellular stress and contain stalled translation preinitiation complexes. These foci are thought to serve as sites of mRNA storage or triage during the cell stress response. SG formation has been shown to require induction of eukaryotic initiation factor (eIF)2α phosphorylation. Herein, we investigate the potential role of other initiation factors in this process and demonstrate that interfering with eIF4A activity, an RNA helicase required for the ribosome recruitment phase of translation initiation, induces SG formation and that this event is not dependent on eIF2α phosphorylation. We also show that inhibition of eIF4A activity does not impair the ability of eIF2α to be phosphorylated under stress conditions. Furthermore, we observed SG assembly upon inhibition of cap-dependent translation after poliovirus infection. We propose that SG modeling can occur via both eIF2α phosphorylation-dependent and -independent pathways that target translation initiation.