Studies on the assembly of apo B-100-containing lipoproteins in HepG2 cells

The relationship between apoB-100 and the membrane of the endoplasmic reticulum (ER) has been studied by a combination of pulse-chase methodology and subcellular fractionation. HepG2 cells were pulse-labeled with [35S]methionine for 3 min and chased with cold methionine for periods between 0 and 20 min. ApoB-100 and albumin, present in the membrane as well as in the luminal content of the ER vesicles, were isolated after each chase period. The results indicated that apoB-100 was cotranslationally bound to the membrane of the ER, and from this membrane-bound form, was transferred to the lumen after a delay of 10-15 min. Albumin was, as could be expected for a typical secretory protein, cotranslationally sequestered in the lumen of the ER. Apo-B-100-containing lipoproteins present in the microsomal lumen were analyzed by ultracentrifugation in a sucrose gradient. ApoB-100 occurred on rounded particles in three density regions: (i) d 1.1065-1.170 g/ml (Fraction I), (ii) d 1.011-1.045 g/ml (Fraction II), and (iii) d less than 1.011 g/ml (Fraction III). Fraction I, isolated from cells cultured in the absence of oleic acid, contained a homogenous population of particles with a mean diameter of approximately 200 A. Fraction I isolated from cells cultured in the presence of oleic acid was slightly more heterogeneous and had a mean diameter of approximately 250 A. Fractions II and III had mean diameters of 300 and 500 A, respectively. Cholesterol esters and triacylglycerol were the quantitatively dominating lipid constituents of all three fractions. Pulse-chase experiments indicated that Fraction I contained the newly assembled lipoproteins. With increasing chase time, the apoB-100 radioactivity was redistributed from Fraction I to Fractions II and III, indicating that Fraction I is converted into Fractions II and III during the intracellular transfer. Particles corresponding to Fractions II and III were by far the most abundant lipoproteins found in the medium. The results presented support the possibility of a sequential assembly of apoB-100-containing lipoproteins.     

Diel and seasonal locomotor activity patterns in Arctic charr, Salvelinm alpinus (L.)

One-year-old Arctic charr, Sulvelinus alpinus (L.), of the Hornavan strain were tested from February 1985 to January 1986 in an attempt to get an increased understanding of the annual rheotactic behaviour as well as the die1 and seasonal locomotor activity pattern. An annular stream tank equipped with photocells was used to measure the direction of swimming movements as well as the number of passings. From February to late May the locomotor activity was low but increased in July and peaked in September. After November the locomotor activity was again at low winter levels. During the activity peak from July to November the majority ofall movements was directed against the current while no preference for direction was noted during the rest of the year. The high level of swimming movements directed against the current in late summer and autumn may be related to an innate habitat change. From February until June, the charr exhibited a bimodal diurnal activity pattern. In July activity was evenly spread over the whole 24- hour period and in August and September activity was again mainly diurnal with a bimodal pattern. In October and November the activity was mainly nocturnal and in December and January activity was concentrated in the short light period. Both annual and die1 activity are discussed in relation to earlier findings in general locomotor activity in Arctic charr and other salmonids.     

Unpredictable environments,nuptial gifts and the evolution of sexual size dimorphism in insects: an experiment

Many insects have a mating system where males transfer nutrients to females at mating, which are often referred to as ''nuptial gifts''. Among butterflies, some of the characteristic features of these species are polyandry (females mate multiple times), and relatively large male ejaculates. When males produce part of the resources used for offspring, the value of body size might then increase for males and decrease for females. The male/female size ratio is also observed to increase when the degree of polyandry and gift size increase. Butterfly species where gift-giving occurs are generally more variable in body size, suggesting that food quality/quantity fluctuates during juvenile stages. This will cause some males to have much to provide and some females to be in great need, and could be conducive to the evolution of a gift-giving mating system. In such a system, growing male and female juveniles should react differently to food shortage. Females should react by maturing at a smaller size since their own lack of reproductive resources can partly be compensated for by male contributions. Males have to pay the full cost of decreased reproduction if they mature at a small size, making it more important for males to keep on growing, even when growth is costly. An earlier experiment with the polyandrous and gift-giving butterfly, Pieris napi, supported this prediction. The pattern is expected to be absent or reversed for species with small nuptial gifts, where females do not benefit from mating repeatedly, and will thus be dependent on acquiring resources for reproduction on their own. To test this prediction, we report here on an experiment with the speckled wood butterfly, Pararge aegeria. We find that growth response correlates with mating system in the two above species, and we conclude that differences in environmental conditions between species may act as an important factor in the evolution of the mating system and sexual size dimorphism.     

Endogenous nitric oxide is present in the exhaled air of rabbits, guinea pigs and humans.

The presence of nitric oxide (NO) in the exhaled air of humans and of anaesthetized rabbits and guinea pigs was demonstrated by chemiluminescence, diazotization and mass spectrometry. This NO is endogenously produced in the lung by an NO synthase, since its generation in guinea pigs and rabbits was inhibited by N omega-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine, inhibitors of this enzyme. The effect of the inhibitors was reversed by the precursor of NO synthesis, L-arginine. Since NO is produced by normal vascular endothelium for the physiological regulation of blood flow and pressure and also by activated macrophages to contribute to non-specific immunity, our experiments suggest that NO may play both vascular regulatory and host defence roles in pulmonary physiology and pathophysiology.     

The assembly and secretion of apoB 100 containing lipoproteins in Hep G2 cells. Evidence for different sites for protein synthesis and lipoprotein assembly

Pulse-chase studies combined with subcellular fractionation indicated that LpB 100 (i.e. the apoprotein B (apoB) 100 containing lipoproteins) was released to the lumen of the secretory pathway in a subcellular fraction enriched in smooth vesicles, and referred to as SMF (the smooth membrane fraction). The migration of SMF during gradient ultracentrifugation as well as kinetic studies indicated that the fraction was derived from a pre-Golgi compartment, probably the smooth endoplasmic reticulum (ER). Only small amounts of LpB 100 could be detected during these pulse-chase experiments in the subcellular fractions derived from the rough endoplasmatic reticulum (RER). SMF contained the major amount of the diacylglycerol acyltransferase activity present in the ER, while the major amount of membrane bound apoB 100 was present in the RER. Pulse-chase studies of the intracellular transfer of apoB 100 demonstrated the formation of a large membrane-bound preassembly pool in the ER, while no significant amount of apoB 100 radioactivity was present in the membrane of the Golgi apparatus. The maximal radioactivity of LpB 100, recovered from the ER or the Golgi lumen, was small compared with the radioactivity recovered from the ER membrane, indicating that the assembled LpB 100 rapidly leaves the cells. This in turn indicates that the rate-limiting step in the secretion of apoB 100 was the transfer of the protein from the ER membrane to the LpB 100 in the lumen. A portion of the intracellular pool of apoB 100 was not secreted but underwent posttranslational degradation.     

Clumping versus spacing out: Experiments on nest predation in fieldfares (Turdus pilaris)

Fieldfares (Turdus pilaris), which nest solitarily as well as in colonies, offer an opportunity to assess within one species the relative advantages of clumping versus spacing out. An experiment with artificial, egg-baited nests showed that predation in the absence of fieldfares was higher on clumped than on scattered nests. In another experiment, we tested whether artificial nests run a higher risk of predation near solitary fieldfare pairs than near fieldfare colonies. Nest predation was higher near solitary fieldfare pairs than outside and inside fieldfare colonies. The risk probably was higher outside than inside colonies. Communal antipredator attacks is the likely cause of reduced predation near colonies. Even solitary fieldfare pairs confer some degree of protection, because predation was lower at artificial nests near solitary fieldfare pairs than at nests without neighbouring fieldfares.     

Innate preference hierarchies coupled with adult experience,rather than larval imprinting or transgenerational acclimation,determine host plant use in Pieris rapae

The evolution of host range drives diversification in phytophagous insects, and understanding the female oviposition choices is pivotal for understanding host specialization. One controversial mechanism for female host choice is Hopkins’ host selection principle, where females are predicted to increase their preference for the host species they were feeding upon as larvae. A recent hypothesis posits that such larval imprinting is especially adaptive in combination with anticipatory transgenerational acclimation, so that females both allocate and adapt their offspring to their future host. We study the butterfly Pieris rapae, for which previous evidence suggests that females prefer to oviposit on host individuals of similar nitrogen content as the plant they were feeding upon as larvae, and where the offspring show higher performance on the mother's host type. We test the hypothesis that larval experience and anticipatory transgenerational effects influence female host plant acceptance (no‐choice) and preference (choice) of two host plant species (Barbarea vulgaris and Berteroa incana) of varying nitrogen content. We then test the offspring performance on these hosts. We found no evidence of larval imprinting affecting female decision‐making during oviposition, but that an adult female experience of egg laying in no‐choice trials on the less‐preferred host Be. incana slightly increased the P. rapae propensity to oviposit on Be. incana in subsequent choice trials. We found no transgenerational effects on female host acceptance or preference, but negative transgenerational effects on larval performance, because the offspring of P. rapae females that had developed on Be. incana as larvae grew slower on both hosts, and especially on Be. incana. Our results suggest that among host species, preferences are guided by hard‐wired preference hierarchies linked to species‐specific host traits and less affected by larval experience or transgenerational effects, which may be more important for females evaluating different host individuals of the same species.     

Heterospecific courtship,minority effects and niche separation between cryptic butterfly species

Species interacting in varied ecological conditions often evolve in different directions in different local populations. The butterflies of the cryptic Leptidea complex are sympatrically distributed in different combinations across their Eurasian range. Interestingly, the same species is a habitat generalist in some regions and a habitat specialist in others, where a sibling species has the habitat generalist role. Previous studies suggest that this geographically variable niche divergence is generated by local processes in different contact zones. By varying the absolute and relative densities of Leptidea sinapis and Leptidea juvernica in large outdoor cages, we show that female mating success is unaffected by conspecific density, but strongly negatively affected by the density of the other species. Whereas 80% of the females mated when a conspecific couple was alone in a cage, less than 10% mated when the single couple shared the cage with five pairs of the other species. The heterospecific courtships can thus affect the population fitness, and for the species in the local minority, the suitability of a habitat is likely to depend on the presence or absence of the locally interacting species. If the local relative abundance of the different species depends on the colonization order, priority effects might determine the ecological roles of interacting species in this system.