Songbird frequency selectivity and temporal resolution vary with sex and season

Many species of songbirds exhibit dramatic seasonal variation in song output. Recent evidence suggests that seasonal changes in auditory processing are coincident with seasonal variation in vocal output. Here, we show, for the first time, that frequency selectivity and temporal resolution of the songbird auditory periphery change seasonally and in a sex-specific manner. Male and female house sparrows (Passer domesticus) did not differ in their frequency sensitivity during the non-breeding season, nor did they differ in their temporal resolution. By contrast, female house sparrows showed enhanced frequency selectivity during the breeding season, which was matched by a concomitant reduction of temporal resolution. However, males failed to show seasonal plasticity in either of these auditory properties. We discuss potential mechanisms generating these seasonal patterns and the implications of sex-specific seasonal changes in auditory processing for vocal communication.     

Cross-talk between EGFR and T-cadherin: EGFR activation promotes T-cadherin localization to intercellular contacts

Reciprocal cross-talk between receptor tyrosine kinases (RTKs) and classical cadherins (e.g. EGFR/E-cadherin, VEGFR/VE-cadherin) has gained appreciation as a combinatorial molecular mechanism enabling diversification of the signalling environment and according differential cellular responses. Atypical glycosylphosphatidylinositol (GPI)-anchored T-cadherin (T-cad) was recently demonstrated to function as a negative auxiliary regulator of EGFR pathway activation in A431 squamous cell carcinoma (SCC) cells. Here we investigate the reciprocal impact of EGFR activation on T-cad. In resting A431 T-cad was distributed globally over the cell body. Following EGF stimulation T-cad was redistributed to the sites of cell–cell contact where it colocalized with phosphorylated EGFR^Tyr1068. T-cad redistribution was not affected by endomembrane protein trafficking inhibitor brefeldin A or de novo protein synthesis inhibitor cycloheximide, supporting mobilization of plasma membrane associated T-cad. EGF-induced relocalization of T-cad to cell–cell contacts could be abrogated by specific inhibitors of EGFR tyrosine kinase activity (gefitinib or lapatinib), lipid raft integrity (filipin), actin microfilament polymerization (cytochalasin D or cytochalasin B), p38MAPK (SB203580) or Rac1 (compound4). Erk1/2 inhibitor PD98059 increased phospho-EGFR^tyr1068 levels and not only amplified effects of EGF but also per se promoted some relocalization of T-cad to cell–cell contacts. Rac1 activation by EGF was inhibited by gefitinib, lapatinib or SB203580 but amplified by PD98059. Taken together our data suggest that T-cad translocation to cell–cell contacts is sensitive to the activity status of EGFR, requires lipid raft domain integrity and actin filament polymerization, and crucial intracellular signalling mediators include Rac1 and p38MAPK. The study has revealed a novel aspect of reciprocal cross-talk between EGFR and T-cad.     

Chemical,biochemical and microbiological properties of a brominated nitrovinylfuran with broad-spectrum antibacterial activity

A di-bromo substituted nitrovinylfuran with reported broad-spectrum antibacterial activity was found to be a potent inhibitor of MurA, a key enzyme in peptidoglycan biosynthesis. Further characterization of the compound was carried out to assess its reactivity towards thiol nucleophiles, its stability and degradation under aqueous conditions, inhibitory potential at other enzymes, and antibacterial and cytotoxic activity. Our results indicate that the nitrovinylfuran derivative is reactive towards cysteine residues in proteins, as demonstrated by the irreversible inhibition of MurA and bacterial methionine aminopeptidase. Experiments with proteins and model thiols indicate that the compound forms covalent adducts with SH groups and induces intermolecular disulfide bonds, with the intermediate formation of a monobromide derivative. The parent molecule as well as most of its breakdown products are potent antibiotics with MIC values below 4 μg/mL and are active against multiresistant strains such as methicillin-resistant Staphylococcus aureus (MRSA). Further development of the bromonitrovinyl scaffold towards antibiotics with clinical relevance, however, requires optimization of the antibiotic–cytotoxic selectivity profile.     

Characterization and Localization of a Phenoloxidase in Mung Bean Hypocotyl Cell Walls

The occurrence of proteins able to oxidize polyphenols even in the absence of H2O2 was recently reported in mung bean (Vigna radiata L.) hypocotyl cell wall extracts (R. Goldberg, A. Chabanet, A.M. Catesson [1993] In K.G. Welinder, S.K. Rasmussen, C. Penel, H. Greppin, eds, Plant Peroxidases: Biochemistry and Physiology, pp. 296-300). Therefore, the possible presence of a laccase in the extracts was investigated using immunocytological and biochemical approaches. An enzyme catalyzing phenol oxidation in the presence of molecular O2 was extracted and purified from the cell walls. This 38-kD cationic protein, like o-diphenoloxidases, was unable to oxidize p-diphenols or p-diamines. However, it crossreacted with an anti-laccase antiserum and, like laccases, its activity was inhibited by N-cetyl-N,N,N-trimethylammonium bromide but not by ferulic acid salts. Immunolabeling data showed that the 38-kD oxidase was absent from all cellulosic cell walls. It was localized only in lignifying and lignified cell walls. This restricted localization suggests that this laccase-like phenoloxidase could participate in the lignification process but not in the primary wall stiffening, which develops in the epidermal and cortical tissues along the mung bean hypocotyl.     

Alcohol dehydrogenase polymorphism in barrel cactus populations of Drosophila mojavensis

Starch gel electrophoresis revealed that the alcohol dehydrogenase (ADH-2) locus was polymorphic in two populations (from Agua Caliente, California and the Grand Canyon, Arizona) of cactophilic Drosophila mojavensis that utilize barrel cactus (Ferocactus acanthodes) as a host plant. Electromorphs representing products of a slow (S) and a fast (F) allele were found in adult flies. The frequency of the slow allele was 0.448 in flies from Agua Caliente and 0.659 in flies from the Grand Canyon. These frequencies were intermediate to those of the low (Baja California peninsula, Mexico) and high (Sonora, Mexico and southern Arizona) frequency Adh-2^S populations of D. mojavensis that utilize different species of host cacti.     

The role of phytoplankton in the removal of arsenic by sedimentation from surface waters

The role of phytoplankton in the removal of arsenic (As) by particle adsorption and sedimentation was investigated in Moira Lake, Canada. Sampling water and suspended particles over one year illustrated significant variation in As partitioning between particulate and aqueous phases, but failed to establish a correlation between the partition coefficient, K _d, and indicators of phytoplankton biomass. A highly significant inverse logarithmic relationship was noted between K _d and the concentration of suspended particles (log K _d = 5.1 – 1.4 log SS; p = 0.0001) in an apparent demonstration of the particle concentration effect (O' Connor & Connolly, 1980).Particle deposition, measured by means of sediment traps, appeared to include a substantial component of resuspended surficial sediment making sediment trap results unreliable for quantifying the removal of substances from the water column. The As concentration of particles from deep traps deployed during late summer and early fall exceeded the As concentrations of suspended particles and surficial sediment, and may indicate that a highly contaminated nepheloid layer acts as a temporary sink for As.     

Intragroup competition predicts individual foraging specialisation in a group‐living mammal

Individual foraging specialisation has important ecological implications, but its causes in group‐living species are unclear. One of the major consequences of group living is increased intragroup competition for resources. Foraging theory predicts that with increased competition, individuals should add new prey items to their diet, widening their foraging niche (‘optimal foraging hypothesis’). However, classic competition theory suggests the opposite: that increased competition leads to niche partitioning and greater individual foraging specialisation (‘niche partitioning hypothesis’). We tested these opposing predictions in wild, group‐living banded mongooses (Mungos mungo), using stable isotope analysis of banded mongoose whiskers to quantify individual and group foraging niche. Individual foraging niche size declined with increasing group size, despite all groups having a similar overall niche size. Our findings support the prediction that competition promotes niche partitioning within social groups and suggest that individual foraging specialisation may play an important role in the formation of stable social groupings.     

Dispersal in cactophilic Drosophila

Three species of Drosophila each breed in necrotic tissue of specific columnar cacti endemic to the Sonoran Desert. Drosophila pachea breeds in senita ( Lophocereus schottii) , D. nigrospiracula breeds in saguaro ( Carnegiea gigantea ) or cardón ( Pachycereus pringlei ), and D. mojavensis uses organ pipe ( Stenocereus thurberi ) in Sonora, Mexico and southern Arizona. Patches of these three host cacti have very different spatial distributions, with those of senita being quite frequent and close together, while those of the other hosts are much father apart. Testing all three species simultaneously, we used capture-mark-release-recapture methods to ask if dispersal differed in these species and if differences were those predicted by the spatial availability of the host patches. D. pachea dispersed the shortest distance in all experiments. Furthermore, D. pachea was the only species showing sex-biased dispersal, with male flies exhibiting the greater propensity to disperse. The observations suggest that across similar spatial scales, D. pachea should show greater population genetic structure than the other two species, and that mitochondrial DNA, because of its maternal inheritance, might show greater evidence of structure than nuclear markers.