Small hosts infrequently disrupt laying by Brown-headed Cowbirds and Bronzed Cowbirds

ABSTRACT Brood parasites often must overcome host defenses that may include behaviors that serve other functions, such as deterrence of predators and nest attendance during laying and incubation. Host use by brood parasites may also be influenced by competitors in areas where more than one parasitic species occurs. We identified the degree to which behavior of potential hosts and potential competitors affected laying by Brown‐headed Cowbirds (Molothrus ater) and Bronzed Cowbirds (M. aeneus) at a site in south Texas where they co‐occur. We watched potential host nests during the presunrise period to record cowbird laying and document nest visitation, laying, cowbird‐host encounters, and nest attentiveness by hosts. Hosts were frequently at their nests when cowbirds laid eggs (83% of 121 watches among nests of five host species) and cowbirds regularly encountered hosts (43–74% and 40–77% of watches per species of host for Brown‐headed and Bronzed cowbirds, respectively). Host nest defense infrequently interfered with cowbird laying and cowbirds rarely interacted with one another during laying. Overall, 12% of the 42 cowbird laying attempts that elicited host nest defense failed, resulting in cowbird eggs either laid atop hosts as they sat in nests or laid outside the nest cup. We clearly documented that relatively small hosts can thwart parasitism by cowbirds. Thus, the potential for successful defense of nests should be considered when assessing the evolution of behaviors to deter the removal of host eggs by cowbirds and mechanisms leading to nest abandonment. Regarding the latter, the presence of a cowbird at a nest would be a poor indicator for parasitism as some laying attempts were thwarted and unparasitized broods were reared at those nests. Despite the potential for nest defense to affect host use by cowbirds, we did not detect an effect of nest defense. Because most host defense was ineffective, we examined hypotheses for the timing of cowbird laying and host nest attendance. Our analysis of time of day of laying by Brown‐headed Cowbirds at our site and data compiled from the literature suggests that laying time is best predicted by the time of civil twilight (first light) rather than sunrise.     

Intermolecular recE4-independent recombination in Bacillus subtilis: formation of plasmid pKBT1

Summary The plasmid pKBT1 was derived by in vivo recE4-independent recombinational event(s) yielding a structure containing regions of plasmid and chromosomal origin. BamHI digests of plasmid pUB110 (Kan^r/Neo^r) and Bg/II digests of pTL12 (Tmp^r, leuA) were mixed, ligated and used to transform competent cells of a recE4 strain of Bacillus subtilis. Kanamycin-resistant transformants were electrophoretically screened for hybrid plasmids. Plasmid pKBT1 (8.0 kb) was smaller than pTL12 (10.4 kb) but larger than monomeric pUB110 (4.5 kb). Plasmid PKBT1 was stably maintained in recE4 strains of B. subtilis and conferred kanamycin resistance but did not specify trimethoprim resistance or leucine prototrophy. At least 86% of the pUB110 monomer length was present in pKBT1 and was completely contained within a single 5.58 kb HindIII fragment. The other segment of pKBT1 was of chromosomal origin as evidenced by lack of homology to pTL12 and strong hybridization to B. subtilis chromosomal DNA. At least one of the in vivo recE4-independent event(s) which produced pKBT1 must have involved intermolecular recombination between transforming and chromosomal DNA. This finding differs from previous reports in which recE4-independent recombination involving pUB110 sequences was a strictly intramolecular event.     

The influence of diet on the growth of Stenonema vicarium (Walker) (Ephemeroptera: Heptageniidae)

Laboratory studies compared the growth rate of Stenonema vicarium (Walker) nymphs on diets of detritus and natural stream periphyton. In three consecutive runs of the experiment, growth rates were consistently higher on periphyton (mean growth rate = 2.1% wet wt. d⁻¹) than detritus (mean = 1.8% wet wt. d⁻¹). The starting date of each run also significantly influenced growth rates. In each treatment growth rates generally decreased over the course of the 3 runs, and ca. one-half of the nymphs in the last run did not molt or grow. It appeared that growth of S. vicarium may be partially controlled by seasonal factors.     

Nucleic acid-like structures III. Oligomerization of 3′-deoxyadenosine 2′,5′-diphosphoimidazolide

Summary The activated dimonophosphate of 3-deoxyadenosine (cordycepin) undergoes oligomerization to produce a new family of pyrophosphate-linked oligomers in which the average repeating unit involves a nine-atom structural group. The presence of a poly(U) template increase the relative yields of higher oligomers, although the template-free reaction is itself extremely efficient.For the previous paper in this series see Schwartz et al. (1987)     

Effects of α2-Adrenergic Agonists and Antagonists on Photoreceptor Membrane Currents

Type B photoreceptors of the nudibranch mollusc Hermissenda crassicornis receive excitatory synaptic potentials (EPSPs) whose frequency is controlled by potential changes of a neighboring cell known as the S optic ganglion cell which is thought to be electrically coupled to the presynaptic source of these EPSPs, the E optic ganglion cell. The frequency of the EPSPs increases when a conditioned stimulus (light) is paired with an unconditioned stimulus (rotation) during acquisition of a Pavlovian conditioned response. The results of the present study are consistent with an adrenergic origin for these EPSPs. Noradrenergic agonists (greater than 100 microM), norepinephrine and clonidine, only slightly depolarize the type B cell but clearly prolong its depolarizing response to light. Serotonin, by contrast, causes hyperpolarization of the type B cell's resting potential as well as after a light step. Clonidine reduces voltage-dependent outward K+ currents (IA, an early current, ICa2+-K+, a late Ca2+-dependent current) that control the type B cell's excitability (and thus its light response and membrane potential). These effects of clonidine are reduced or blocked by the alpha 2-receptor antagonist, yohimbine (0.5 microM), but not the alpha 1-blocker, prazosin. The same yohimbine concentration also blocked depolarizing synaptic excitation of the type B cell in response to depolarization of a simultaneously impaled S optic ganglion cell. Histochemical techniques (both the glyoxylic acid method of de la Torre and Surgeon and the formaldehyde-induced fluorescence or Falck-Hillarp method) demonstrated the presence of a biogenic amine(s) within a single neuron in each optic ganglion as well as three or four cells within the vicinity of previously identified visual interneurons. No serotonergic neurons were found within the optic ganglion or in proximity to visual interneurons. A clonidine-like synaptic effect on type B cells, therefore, could amplify conditioning-specific changes of membrane currents by increasing type B depolarization and possibly, as well, by elevating intracellular second messengers.     

Effect of differential gastric evacuation and multispecies prey items on estimates of daily energy intake in juvenile chinook salmon

Synopsis The caloric density of stomach contents in juvenile chinook salmon,Oncorhynchus tshawytscha, was not affected by gastric evacuation, suggesting a constant caloric density of stomach contents during evacuation. Differences in the caloric density of prey consumed did affect caloric density of stomach contents over a 24-h period. Consumption of the amphipodCorophium sp. was associated with reduced caloric densities of stomach contents. During periods whenCorophium contributed more than 4% of the stomach contents, average caloric density declined from 5.56 to 5.33 kcal g^–1. Despite this difference, estimates of daily energy intake of juvenile chinook salmon were only 3%, greater when developed from the mean caloric density of stomach contents excludingCorophium.     

Freeze-Thawing of Aquaspirillum magnetotacticum Cells Selectively Releases Periplasmic Proteins

Cells of the gram-negative bacterium Aquaspirillum magnetotacticum, when suspended in buffer and freeze-thawed, produced pinkish orange supernatant fluid. The fluid contained ≤2.0% of total extractable outer membrane component 2-keto-3-deoxyoctonate or of the cytoplasmic membrane marker succinic dehydrogenase. Electrophoretic banding patterns and difference spectra of proteins and hemoproteins released by freeze-thawing cells were distinct from those of membrane-associated substances and similar to those of periplasmic substances obtained by applying conventional fractionation methods to this organism.     

Monotonic Change of the Mean Phenotype in Two-Locus Models

It is shown that the mean phenotype monotonically approaches the optimum in a class of symmetric, two-locus, two-allele models with stabilizing selection. In this model, mean fitness does not change monotonically. Thus, Fisher's fundamental theorem does not hold, even though another quantity of evolutionary interest, the mean phenotype, can be shown to change monotonically. Using this quantity, it is proven that global stability results for this model.     

Isolation and sequencing of cDNA clones encoding alpha and beta subunits of Drosophila melanogaster casein kinase II.

Cloned cDNAs encoding both subunits of Drosophila melanogaster casein kinase II have been isolated by immunological screening of lambda gt11 expression libraries, and the complete amino acid sequence of both polypeptides has been deduced by DNA sequencing. The alpha cDNA contained an open reading frame of 336 amino acid residues, yielding a predicted molecular weight for the alpha polypeptide of 39,833. The alpha sequence contained the expected semi-invariant residues present in the catalytic domain of previously sequenced protein kinases, confirming that it is the catalytic subunit of the enzyme. Pairwise homology comparisons between the alpha sequence and the sequences of a variety of vertebrate protein kinase suggested that casein kinase II is a distantly related member of the protein kinase family. The beta subunit was derived from an open reading frame of 215 amino acid residues and was predicted to have a molecular weight of 24,700. The beta subunit exhibited no extensive homology to other proteins whose sequences are currently known.