Stopover strategies in passerine bird migration: a simulation study

Long-distance bird migration consists of a series of stopovers (for refuelling) and flights, with flights taking little time compared to stopovers. Therefore, it has been hypothesized that birds minimize the total time taken for migration through efficient stopover behaviour. Current optimality models for stopover include (1) the fixed expectation rule and (2) the global update rule. These rules maximize the speed of migration by determining the optimal departure fuel load for a given fuel deposition rate. We were interested in simple behavioural rules approaching the stopover behaviour of real birds and how these rules compare to the time minimizing models above with respect to the total time taken for migration. The simple strategies were to stay at a site (1) until a fixed fuel load was reached or (2) for a constant number of days. We simulated migration of small nocturnal passerine birds across an environment of continuously distributed but variable fuel deposition rates, and investigated the influence of different stopover strategies on the duration of migration. Staying for a constant number of days at each stopover site, irrespective of the fuel deposition rate, resulted in only slightly longer than minimum values for migration duration. Additionally, the constant stopover duration, e.g. 10 days, may change by a day or two (per stopover) without having a large effect on total migration duration. There is therefore a possibility that real birds may be close to optimal migration speed without the need for very complex behaviour. When assessing the sensitivity of migration duration to factors other than stopover duration, we found that flight costs, search and settling time, mean fuel deposition rate and the accuracy in the choice of flight direction were the factors with the largest influence. Our results suggest that migrating birds can approximate optimal stopover duration relatively easy with a simple rule, and that other factors, e.g. those above, are more relevant for travel time.     

It pays to be sweet: sugars in mutualistic interactions

It is common knowledge that many flowering plants produce nectar to reward pollinators. Not so well known is the fact that plants, fungi and insects also employ sugar-rich solutions to obtain transport or defensive services. However, not all of these interactions are honest. In many cases, cheaters use tricks instead of treats.     

Two new scorpion toxins that target voltage-gated Ca2+ and Na+ channels

This report describes the isolation, primary structure determination, and functional characterization of two similar toxins from the scorpion Parabuthus granulatus named kurtoxin-like I and II (KLI and KLII, respectively). KLII from P. granulatus is identical to kurtoxin from Parabuthus transvaalicus (a 63 amino-acid long toxin) whereas KLI is a new peptide containing 62 amino acid residues closely packed by four disulfide bridges with a molecular mass of 7244. Functional assays showed that both toxins, KLI and kurtoxin from P. granulatus, potently inhibit native voltage-gated T-type Ca(2+) channel activity in mouse male germ cells. In addition, KLI was shown to significantly affect the gating mechanisms of recombinant Na(+) channels and weakly block alpha(1)3.3Ca(V) channels expressed in Xenopus oocytes. KLI and kurtoxin from P. granulatus represent new probes to study the role of ion channels in germ cells, as well as in cardiac and neural tissue.     

Maleimide-oligo(ethylene glycol) derivatives of camptothecin as albumin-binding prodrugs: synthesis and antitumor efficacy

In situ binding of thiol-reactive prodrugs to the cysteine-34 position of circulating albumin is a new approach in drug delivery. Therefore, five maleimide-bearing derivatives of the anticancer drug camptothecin (CPT) were developed as albumin-binding prodrugs. These compounds were synthesized by reacting heterobifunctional cross-linkers based on oligo(ethylene glycols) [3-6 (O-CH(2)-CH(2)) units] bearing a maleimide group on one end and a carboxylic acid group on the other with camptothecin 20-O-glycinate. Incorporating oligo(ethylene glycol) chains into the prodrugs enhanced their water-solubility when compared to the parent compound (up to 27-fold). HPLC studies showed that the prodrugs react almost quantitatively with the cysteine-34 position of endogenous albumin within a few minutes after incubation of the CPT derivatives with human blood plasma. The therapeutic potential of two of the prodrugs was assessed in nude mice bearing a colon xenograft (HT-29). Both albumin-binding derivatives of camptothecin were well-tolerated and showed enhanced antitumor efficacy when compared to CPT.     

Aluminum resistance in two cultivars of Zea mays L.: Root exudation of organic acids and influence of phosphorus nutrition

Root exudation of organic acids as Al-chelating compounds and P nutrition have been suggested to play a major role in Al-resistance in higher plants. Effects of Al exposure on maize plant growth, and organic acid root content and root exudation under various levels of P nutrition were examined. Sikuani, a Colombian maize cultivar tolerant to acid soils with high Al saturation, and Corso, a Swiss cultivar, were grown in sterile hydroponic conditions for 21 days. Al-caused inhibition of root growth was lower in Sikuani than in Corso. Al effect on plant growth was decreased with increasing P content in roots. Al content in roots increased with increasing P content and was higher in Sikuani than in Corso. When exposed to Al, the contents in root apices as well as the root exudation of citric and malic acids in Corso and citric, malic and succinic acids in Sikuani increased, and were higher in Sikuani than in Corso. Increased PEP carboxylase (PEPC) activity in root apices after Al exposure partially explained the variations of organic acid content in the roots. These Al-induced changes in PEPC activity, organic acid content and exudation were reduced in plants supplied with higher P concentrations during the 21 days prior to treatment. Increased secretion of organic acids after exposure to Al appeared to be specific to Al and was not totally explained by increased root content in organic acids.