Dry matter partitioning in a tomato plant: one common assimilate pool?

The influence of the distance (transport resistance) betweensource and sink on dry matter distribution between fruits andvegetative parts in tomato was studied. In two glasshouse experiments,a control treatment (single-shoot plants, no truss removal)was conducted, together with two double-shoot treatments: double-shootplants with no trusses removed from one shoot and all trussesremoved at anthesis from the other shoot (100–0) and double-shootplants with every second truss removed from both shoots (50–50).Plant growth and dry matter distribution was recorded by periodicaldestructive harvests, during a period of about 100 d after anthesisof the first truss. In experiment 2, plants were probably sink-limited.At the end of both experiments, 58–60% of dry matter wasin the fruits for control plants, whereas for both double-shoottreatments this was 43% (Experiment 1) or 38% (Experiment 2).Until 60–65 d after first flowering, vegetative growthof the individual shoots in both double-shoot treatments wasthe same. Results supported the assumption of one common assimilatepool and showed no significant influence of distance (transportresistance) between source and sink on dry matter partitioning. Key words: Allocation, distance, dry matter distribution, one assimilate pool, partitioning, transport resistance, tomato     

Caveolae meet endosomes: a stable relationship?

The endocytic trafficking of caveolae has been the subject of some controversy for many years. A new study (Pelkmans et al.) shows that budded caveolae can interact with both the caveosome and the early endosome and that caveolin-enriched regions of the early endosomal membrane form unusually stable domains that can incorporate cargo in a regulated manner.     

Foraging in surfperches: resource partitioning or individualistic responses?

Synopsis On Naples Reef off southern California, five sympatric species of surfperch (Embiotocidae) eat small animal prey associated with algal turf and other benthos. Although they tend to select different foods in different microhabitats, their different foraging traits are probably individualistic responses (non-interactive or autecological species responses to resources) because the species' pairwise resource overlaps are neither uniformly small nor generally complementary. Mean overlap in both food and foraging microhabitat was significantly greater than random as most species converged in resource use. Only two exploiters of extremes in the available spectra of prey types and microhabitats had significantly narrow overlap. Thus, resource overlaps may simply reflect similarities and differences in the species' independently evolved foraging traits, which were rendered more or less inflexible as different morphological specializations for picking, crunching, or winnowing prey. Therefore, any one species may occur about the reef to its best advantage independently of the others. Its distribution and abundance may simply reflect its tolerance of the local environment. The evidence indicates that the surfperches are not concertedly partitioning resources on Naples Reef, i.e., that the five species had not all coevolved to avoid interspecific competition in the past nor have they all mutually shifted their niches to minimize it in the present.     

Six authors in search of a citation: villains or victims of the Vancouver convention?

OBJECTIVES--To analyse trends in the number of authors per article over the past 10 years. DESIGN--Analysis of articles from random volumes of eight biomedical journals. SUBJECTS--Cell, Nature, Proceedings of the National Academy of Sciences USA (PNAS), Journal of Clinical Investigation (JCI), Biochemical and Biophysical Research Communications (BBRC), Journal of Clinical Oncology (JCO), New England Journal of Medicine (NEJM), Lancet. MAIN OUTCOME MEASURES--Median and modal numbers of authors. RESULTS--All journals except Cell and Nature showed a trend towards increasing authorship numbers over the study period. The trend was most noticeable in journals such as JCO which feature clinical research. General medical journals (Lancet, NEJM) with a median of six to seven authors per article published far fewer seven author than six author studies, which suggests that author number may be influenced by the Vancouver convention which precludes citation of more than six authors. CONCLUSIONS--The phenomenon of expanding authorship in biomedical journal articles is not explained by the hypothesis that newer research technologies have necessitated more extensive collaboration. Rather, the data suggest that conferral of authorship may sometimes have a volitional component which contributes to rising author numbers. It is proposed that replacement of the Vancouver convention with a "first author, last author" citation system may help stem this rise in author numbers.     

Are Bradyrhizobium japonicum stable during a long stay in soil?

Two strains of Bradyrhizobium japonicum, recognizable by their intrinsic resistance to high levels of antibiotics and their serological features were introduced into three calcareous soils under field conditions. These strains were re-isolated 16 or 20 years later and compared with the parental strains kept lyophilized. In the Dijon location, the survival was high although soybean was never grown in the field. But the B. japonicum completely disappeared in the Montpellier field after 10 years under vineyard. In the Toulouse field after the two initially introduced strains, inoculation of subsequent soybean crops with a new strain enabled this strain to occupy 70–80% of the nodules; these results suggest that under such conditions the problem of competition can be solved by repeated inoculation. In this field, the number of introduced B. japonicum remained high during 4 years without soybeans, but a new inoculation would be necessary after 5 years. In the two fields where the survival was high, the two strains remained at about the same relative level as at introduction, there was no detectable exchange of characters between them. With regards to agronomic characteristics, there were no important changes in the competitivity of the strains. Among the eight field isolates tested in a greenhouse for efficiency by comparison with eight lyophilized isolates, seven showed no significant difference for the total weight of soybean or seed yield but one field isolate showed a loss of efficiency corresponding to 27% less seed weight. This long-term experiment allowed us to conclude that the B. japonicumstrains used were stable for many characters, but variations in efficiency may rarely occur.     

Clustering versus random segregation of plasmids lacking a partitioning function: a plasmid paradox?

Plasmids lacking a functional partition system are randomly distributed to the daughter cells; plasmid-free daughter cells are formed with a frequency of (1/2)2n per cell and cell generation where 2n is the (average) copy number at cell division. Hence, the unit of segregation is one plasmid copy. However, plasmids form clusters in the cells. A putative solution to this potential paradox is presented: one plasmid copy at a time is recruited from the plasmid clusters to the replication factories that are located in the cell centres. Hence, replication offers the means of declustering that is necessary in a growing host population. The daughter copies diffuse freely and each copy may with equal probability end up in either of the two cell halves. In this way, the random segregation of the plasmids is coupled to replication and occurs continuously during the cell cycle, and is not linked to cell division. The unit of segregation is the plasmid copy and not the plasmid clusters. In contrast, the two daughters of a Par+ plasmid are directed in opposite directions by the plasmid-encoded partition system, thereby assuring that each daughter cell receives the plasmid.     

Stress hormones in relation to breeding status and territory location in colonial king penguin: a role for social density?

Because glucocorticoid (stress) hormones fundamentally affect various aspects of the behaviour, life history and fitness of free-living vertebrates, there is a need to understand the environmental factors shaping their variation in natural populations. Here, we examined whether spatial heterogeneity in breeding territory quality affected the stress of colonial king penguin (Aptenodytes patagonicus). We assessed the effects of local climate (wind, sun and ambient temperature) and social conditions (number of neighbours, distance to neighbours) on the baseline levels of plasma total corticosterone (CORT) in 77 incubating and 42 chick-brooding birds, breeding on territories of central or peripheral colony location. We also assessed the oxidative stress status of a sub-sample of central vs. peripheral chick-brooders to determine whether chronic stress arose from breeding on specific territories. On average, we found that brooders had 55 % higher CORT levels than incubators. Regardless of breeding status, central birds experienced greater social density (higher number of neighbours, shorter distance between territories) and had higher CORT levels than peripheral birds. Increasing social density positively explained 40 % of the variation in CORT levels of both incubators and brooders, but the effect was more pronounced in brooders. In contrast, climate was similar among breeding territories and did not significantly affect the CORT levels of breeding birds. In brooders, oxidative stress status was not affected by local density or weather conditions. These results highlight that local heterogeneity in breeding (including social) conditions may strongly affect the stress levels of breeding seabirds. The fitness consequences of such variation remain to be investigated.     

The stable tyrosyl radical in photosystem II: why D?

Two redox-active tyrosines are present in Photosysytem II, the water-oxidizing enzyme. While the tyrosine that is kinetically competent in electron transfer, TyrZ, may also have a role in the enzyme mechanism, the second tyrosine, TyrD, has a stable radical and is not directly involved in the redox chemistry associated with enzyme function. Nevertheless, reasonable mechanistic roles for TyrD have been postulated that satisfy desires to rationalise the presence of this cofactor, or, in English, we think we know what it does. First, the TyrD radical acts an oxidant of the Mn cluster in the lowest state of the redox accumulation cycle (i.e., S(0)), providing potential benefits in maintaining the cluster in the more stable higher valence states. This redox role may also be important during Mn assembly and indeed overreduced forms of the Mn cluster appear to be oxidised by TyrD(*). Second, the proton generated by the TyrD radical is thought to remain in its vicinity having an electrostatic influence on the location and potential of the chlorophyll cation, P(+). This effect may be important for the kinetics of TyrZ oxidation and may provide a significant thermodynamic boost to the enzyme. In addition, through its electrostatic influence, TyrD(*)(H(+)) may confine the highly oxidising cation P(+) to the chlorophyll nearest to TyrZ, thereby accelerating TyrZ oxidation and restricting the potentially damaging redox chemistry to one side of the reaction centre: the disposable D1 side. This second role, evidence for which is beginning to emerge, constitutes a new role for a redox-active tyrosine in biology: as a positive charge generator in a hydrophobic environment. In this short review, we focus on work relevant to these two roles.     

The partitioning of water uptake between growth forms in a Neotropical savanna: do herbs exploit a third water source niche?

In addition to trees and grasses, the savannas of central Brazil are characterised by a diverse herbaceous dicot flora. Here we tested whether the coexistence of a highly diversified assemblage of species resulted in stratification or strong overlap in the use of soil water resources. We measured oxygen and hydrogen isotope ratios of stem water from herbs, grasses and trees growing side by side, as well as the isotopic composition of water in soil profile, groundwater and rainfall, and predawn (Ψ_pd) and midday (Ψ_md) leaf water potentials. We used a stable isotope mixing model to estimate vertical partitioning of soil water by the three growth forms. Grasses relied on shallow soil water (5–50 cm) and were strongly anisohydric. Ψ_pd and Ψ_md decreased significantly from the wet to the dry season. Trees extracted water from deeper regions of the soil profile (60–120 cm) and were isohydric. Ψ_pd and Ψ_md did not change from the wet to the dry season. Herbs overlapped with grasses in patterns of water extraction in the dry season (between 10 and 40 cm), but they took up water at soil depths intermediate (70–100 cm) to those of trees and grasses during the wet season. They showed seasonal changes in Ψ_pd but not in Ψ_md. We conclude that vertical partitioning of soil water may have contributed to coexistence of these three growth forms and resulted in a more complex pattern of soil water extraction than the two‐compartment model of soil water uptake currently used to explain the structure and function of tropical savanna ecosystems.     

Biodegradation of a phenolic mixture in a solid–liquid two-phase partitioning bioreactor

A solid–liquid two-phase partitioning bioreactor (TPPB) in which the non-aqueous phase consisted of polymer (HYTREL) beads was used to degrade a model mixture of phenols [phenol, o-cresol, and 4-chlorophenol (4CP)] by a microbial consortium. In one set of experiments, high concentrations (850 mg l⁻¹ of each of the three substrates) were reduced to sub-inhibitory levels within 45 min by the addition of the polymer beads, followed by inoculation and rapid (8 h) consumption of the total phenolics loading. In a second set of experiments, the beneficial effect of using polymer beads to launch a fermentation inhibited by high substrate concentrations was demonstrated by adding 1,300 and 2,000 mg l⁻¹ total substrates (equal concentrations of each phenolic) to a pre-inoculated bioreactor. At these levels, no cell growth and no degradation were observed; however, after adding polymer beads to the systems, the ensuing reduced substrate concentrations permitted complete destruction of the target molecules, demonstrating the essential role played by the polymer sequestering phase when applied to systems facing inhibitory substrate concentrations. In addition to establishing alternative modes of TPPB operation, the present work has demonstrated the differential partitioning of phenols in a mixture between the aqueous and polymeric phases. The polymeric phase was also observed to absorb a degradation intermediate (arising from the incomplete biodegradation of 4CP), which opens the possibility of using solid–liquid TPPBs during biosynthetic transformation to sequester metabolic byproducts.     

Habitat partitioning among neotropical pioneers: a consequence of differential susceptibility to browsing herbivores?

Four species of fast-growing pioneer tree species in the genus Cecropia exist in the forests in central Panama. Cecropia insignis is dominant in old-growth forests but absent from nearby secondary forests; C. obtusifolia, and C. peltata are abundant in secondary forests but rare in old-growth forest, and C. longipes is uncommon in both. To determine whether Cecropia habitat associations are a consequence of local dispersal or differences in recruitment success, we grew seedlings of these species in common gardens in large treefall gaps in secondary and old-growth forest. In contrast to the observed adult distribution, only C. insignis grew significantly over 16 months in secondary forests; remaining species were heavily browsed by herbivores. C. insignis also grew and survived best in old-growth forest. Differences in susceptibility to herbivory did not result from an ant defence mutualism; none of the plants were colonised by ants during the experiment. To test whether C. insignis, the species least susceptible to herbivory, trades off investment in growth in favour of defence, we also grew the four Cecropia species in a screened growing house under light conditions comparable to large forest gaps. Contrary to expectation, species growth rates were similar; only C. peltata grew significantly faster than C. insignis. These results suggest that (1) conditions in ~40-year-old secondary forests no longer support the recruitment of Cecropia species, which are canopy dominants there; and (2) among congeners, differences in plant traits with little apparent cost to growth can have large impacts on recruitment by affecting palatability to herbivores.     

Split sex ratios in perennial social Hymenoptera: a mixed evolutionary stable strategy from the queens' perspective?

In social Hymenoptera, relatedness asymmetries due to haplodiploidy often generate conflicts of genetic interest between queens and workers. Split sex ratios are common in ant populations and may result from such conflicts, with workers favoring the production of males in some colonies and of gynes in others. Such intercolonial differences may result from variations in relatedness asymmetries among colony members, but several examples are now known in which this hypothesis does not hold. We develop here a simple model assuming monogynous, monoandrous, worker-sterile, perennial colonies without dispersal restrictions. Workers may eliminate eggs of either sex and determine the caste of the female brood, but the queen controls the number of eggs of each sex she lays. In such conditions, we demonstrate that split sex ratios can result from queens adopting a mixed evolutionary stable strategy (ESS), with one option being to put a strict limit to the number of diploid eggs available and the alternative one to provide diploid eggs ad lib. In the former situation, workers should raise all diploid eggs as workers and release only male sexuals. In the latter, workers should adjust the caste ratio so as to reach the maximum sexual productivity for the colony, which is entirely invested into gynes. For a particular relative investment in gynes at the population level, between 0.5 (ESS under full queen control) and 0.75 (ESS under full worker control), an equilibrium is reached at which both strategies yield an equal genetic payoff to the queen. Male-specialized colonies are predicted to be equally abundant but less populous and less productive than gyne-specialized ones. Available data on the monogyne form of the fire ant, Solenopsis invicta, suggest that this model may apply in this case, although more specific studies are required to test these predictions.     

O' mother where wert thou? Maternal strategies in the southern elephant seal: a stable isotope investigation

Maternal effects are widespread in ecology and can alter the dynamics of a population. We investigated the impact of maternal foraging strategies on offspring weaning mass—a proxy of maternal foraging success and of offspring survival—in southern elephant seals on îles Kerguelen. Using 4 years of data, we modelled pup weaning mass as a two-component mixture and used blood stable isotope values to discriminate between maternal foraging strategies previously identified from bio-logging studies. Carbon isotope ratio was a strong predictor of weaning mass, but the relationship was non-monotonic in contrast to a priori expectations. Females foraging in the interfrontal zone weaned pups with a smaller mass compared with females foraging in Antarctic waters. Pup mass was positively correlated with a proxy of global primary production in the interfrontal zone for small weanlings. Maternal effects, via a poor foraging efficiency in the 1970s, may help explain the large population decrease observed at that time on îles Kerguelen because of an overall decrease in pup weaning mass, survival and subsequent recruitment.     

Sensory-based niche partitioning in a multiple predator–multiple prey community

Many predators and parasites eavesdrop on the communication signals of their prey. Eavesdropping is typically studied as dyadic predator–prey species interactions; yet in nature, most predators target multiple prey species and most prey must evade multiple predator species. The impact of predator communities on prey signal evolution is not well understood. Predators could converge in their preferences for conspicuous signal properties, generating competition among predators and natural selection on particular prey signal features. Alternatively, predator species could vary in their preferences for prey signal properties, resulting in sensory-based niche partitioning of prey resources. In the Neotropics, many substrate-gleaning bats use the mate-attraction songs of male katydids to locate them as prey. We studied mechanisms of niche partitioning in four substrate-gleaning bat species and found they are similar in morphology, echolocation signal design and prey-handling ability, but each species preferred different acoustic features of male song in 12 sympatric katydid species. This divergence in predator preference probably contributes to the coexistence of many substrate-gleaning bat species in the Neotropics, and the substantial diversity in the mate-attraction signals of katydids. Our results provide insight into how multiple eavesdropping predator species might influence prey signal evolution through sensory-based niche partitioning.     

IAPs: what's in a name?

Originally described in insect viruses, cellular proteins with Baculoviral IAP repeat (BIR) motifs have been thought to function primarily as inhibitors of apoptosis. The subsequent finding that a subset of IAPs that contain a RING domain have ubiquitin protein ligase (E3) activity implied the presence of other functions. It is now known that IAPs are involved in mitotic chromosome segregation, cellular morphogenesis, copper homeostasis, and intracellular signaling. Here, we review the current understanding of the roles of IAPs in apoptotic and nonapoptotic processes and explore the notion that the latter represents the primary physiologic activities of IAPs.     

Carbapenemases: a problem in waiting?

Carbapenems are stable to most prevalent beta-lactamases, and chromosomal carbapenemases are restricted to Stenotrophomonas maltophilia, to a few Bacteroides fragilis, and to rare pathogens. Nevertheless, an acquired metallo-beta-lactamase called IMP-1 is beginning to emerge in Pseudomonas aeruginosa and Enterobacteriaceae isolates in Japan, and has also been found in isolates from Singapore. Furthermore, IMP-producing Acinetobacter spp. have been identified in Italy and Hong Kong. Recently a second group of acquired metallo-carbapenemases, the VIM types, has been recorded from P. aeruginosa isolates in five Eurasian countries. Weak carbapenemases belonging to molecular class D are emerging in A. baumannii world-wide, with two sub-groups apparent. A few acquired carbapenemases belonging to molecular class A also have been reported. Finally it has also been shown that enzymes with feeble carbapenemase activity (e.g. AmpC types and some SHV enzymes) may confer resistance in exceptionally impermeable strains; counterwise, even potent carbapenemases, such as IMP-1, may only give a small reduction in susceptibility in Enterobacteriaceae that lack permeability lesions. Is the emergence of carbapenemase a problem waiting to happen?