Early blooming's challenges: extended flowering season, diverse pollinator assemblage and the reproductive success of Gynodioecious Daphne laureola

BACKGROUND AND AIMS: The scarcity and unpredictability of active pollinators during late winter in temperate areas tends to favour extended flowering seasons and increased floral longevity in early blooming species, which are usually pollinated by diverse sets of insects. Daphne laureola is a gynodioecious woody perennial that flowers from January to April in southern Spain, a period characterized by cold temperatures, frequent rains and irregular snowfalls. METHODS: Pollinators were excluded at four different times during the flowering season in order to determine the effect of decreased exposure to pollinators on fruit set in female and hermaphrodite individuals. The role of nocturnal and diurnal pollination on reproductive success in each gender was simultaneously evaluated by selective exclusion. KEY RESULTS: A 50 % reduction in the flowering period decreased fruit set of females by 50 %, whereas the corresponding decrease in self-compatible hermaphrodites was only approx. 25 %. Day-active hymenopterans and lepidopterans were infrequent visitors, and nocturnal pollinators were inefficient, suggesting that pollen beetles, Meligethes elongatus, were the main pollinators of D. laureola in the study region. CONCLUSIONS: Beetles were less abundant in pollenless females, although discrimination did not apparently result in pollination limitation of female reproduction. A preference of beetles for sunny locations emphasized the relevance of abiotic conditions for pollination of this early blooming shrub.     

Guanylate cyclase activity in permeabilized Dictyostelium discoideum cells

Dictyostelium discoideum cells respond to chemoattractants by transient activation of guanylate cyclase. Cyclic GMP is a second messenger that transduces the chemotactic signal. We used an electropermeabilized cell system to investigate the regulation of guanylate cyclase. Enzyme activity in permeabilized cells was dependent on the presence of a nonhydrolysable GTP analogue (e.g., GTPγS), which could not be replaced by GTP, GDP, or GMP. After the initiation of the guanylate cyclase reaction in permeabilized cells only a short burst of activity is observed, because the enzyme is inactivated with a t_1.2 of about 15 s. We show that inactivation is not due to lack of substrate, resealing of the pores in the cell membrane, product inhibition by cGMP, or intrinsic instability of the enzyme. Physiological concentrations of Ca²⁺ ions inhibited the enzyme (half-maximal effect at 0.3 μM), whereas InsP₃ had no effect. Once inactivated, the enzyme could only be reactivated after homogenization of the permeabilized cells and removal of the soluble cell fraction. This suggests that a soluble factor is involved in an autonomous process that inactivates guanylate cyclase and is triggered only after the enzyme is activated. The initial rate of guanylate cyclase activity in permeabilized cells is similar to that in intact, chemotactically activated cells. Moreover, the rate of inactivation of the enzyme in permeabilized cells and that due to adaptation in vivo are about equal. This suggests that the activation and inactivation of guanylate cyclase observed in this permeabilized cell system is related to that of chemotactic activation and adaptation in intact cells. © 1996 Wiley-Liss, Inc.     

Exploring the origin of egg protein in an oviparous water snake (Natrix maura)

Reptiles are typical capital breeders that fuel reproduction by the use of lipids stored in fat bodies. However, little is known about the origin (exogenous or endogenous) of egg protein. We have examined the origin of egg protein by means of the analysis of protein content of liver and carcass (skeletal muscle) of the oviparous snake Natrix maura throughout an annual cycle. We have also measured monthly variation of the digestive-content mass and the ovarian mass. Results showed that protein in liver peaked during vitellogenesis according to the role of the liver in the synthesis of vitellogenin. Partial correlations showed the path of protein from the prey (digestive-content) to the liver, and finally to the ovaries, as well as an inverse relation between carcass protein and ovarian mass. Carcass muscle is the only body part that may act as a potential reserve for endogenous protein, although we did not find significant variation in carcass protein during female reproduction. As females with large follicles did not stop foraging activity, we assumed that egg protein was derived from the diet as partial correlations indicated. Our results suggest that N. maura is a capital breeder for lipids and tend to be income breeder for protein. This conclusion contrasts with that observed in capital breeders for which egg protein was derived from muscle. We discuss the idea that flexibility in the origin of egg protein could affect the body condition in post-reproductive females.     

Acetylcholine turnover rates in rat brain regions during cocaine self-administration

The involvement of cholinergic neurons in the brain processes underlying reinforcement has been recently demonstrated. This experiment assessed the potential role of cholinergic neurons in cocaine reinforcement by measuring the turnover rates of acetylcholine in brain regions of rats self-administering cocaine and in yoked cocaine and yoked vehicle-infused controls. The activity of cholinergic innervations of and/or interneurons in the olfactory tubercle, caudate putamen, diagonal band-pre-optic region, ventral pallidum, lateral and medial hypothalamus, hippocampus, ventral tegmental area and visual cortices reflected by the turnover rates of acetylcholine were significantly altered in rats self-administering cocaine compared to yoked cocaine infused controls. These changes implicate the involvement of cholinergic neurons with cell bodies in the diagonal band-pre-optic region, the medial septum and several brainstem nuclei and interneurons in the caudate-putamen and ventral pallidum in the processes underlying cocaine self-administration. The identified cholinergic neuronal systems may have a broader role in the brain processes for natural reinforcers (i.e. food, water, etc.) since drugs of abuse are believed to produce reinforcing effects through these systems.     

Life-cycle and genome of OtV5, a large DNA virus of the pelagic marine unicellular green alga Ostreococcus tauri

Large DNA viruses are ubiquitous, infecting diverse organisms ranging from algae to man, and have probably evolved from an ancient common ancestor. In aquatic environments, such algal viruses control blooms and shape the evolution of biodiversity in phytoplankton, but little is known about their biological functions. We show that Ostreococcus tauri, the smallest known marine photosynthetic eukaryote, whose genome is completely characterized, is a host for large DNA viruses, and present an analysis of the life-cycle and 186,234 bp long linear genome of OtV5. OtV5 is a lytic phycodnavirus which unexpectedly does not degrade its host chromosomes before the host cell bursts. Analysis of its complete genome sequence confirmed that it lacks expected site-specific endonucleases, and revealed the presence of 16 genes whose predicted functions are novel to this group of viruses. OtV5 carries at least one predicted gene whose protein closely resembles its host counterpart and several other host-like sequences, suggesting that horizontal gene transfers between host and viral genomes may occur frequently on an evolutionary scale. Fifty seven percent of the 268 predicted proteins present no similarities with any known protein in Genbank, underlining the wealth of undiscovered biological diversity present in oceanic viruses, which are estimated to harbour 200Mt of carbon.     

Variability in the behavioural responses of three generalist herbivores to the most abundant coumarin in Daphne laureola leaves

We evaluated the responses of three generalist herbivores (Lepidoptera: Noctuidae) to the most abundant coumarin in young leaves of spurge-laurel, Daphne laureola L. (Thymelaeaceae), a perennial shrub consistently fed upon by several noctuid species. Pseudenargia ulicis Staud and Noctua janthe Borkhausen are natural herbivores of the species in southeastern Spain, whereas Spodoptera littoralis Boisduval is a model species frequently used to address the antifeedant role of secondary compounds. Discrimination between control and coumarin-coated D. laureola leaves was investigated by short-term choice experiments. We found species-specific behavioural responses to the treatment: P. ulicis preferred the control leaves, whereas S. littoralis did not discriminate between leaf treatments, although it clearly avoided coumarin when incorporated into an artificial diet, and N. janthe preferred the coumarin-coated leaves. Furthermore, N. janthe larvae reduced consumption when the proportion of treated leaves ingested increased and consumption of S. littoralis larvae was also reduced in coumarin-containing diet under no-choice conditions. Our results highlight that different herbivore species feeding simultaneously on the same host plant respond differently to a single chemical defence compound, likely constraining a directional response of the plant to selection.