Evolutionary senescence in plants

Senescence is a decline in age-specific survival and reproduction with advancing age. Studies of evolutionary plant senescence are designed to explain this decline in life history components within the context of natural selection. A review of studies of plant demography reveals senescent declines in both annual and perennial plants, but also suggests that there are some plant species which may not be expected to show senescence. Thus, future comparative studies of closely related species, with and without senescence, should be possible. The assumptions of the major evolutionary theories of senescence are evaluated for their validity with respect to plants. Different plant species violate one or more of the assumptions of the theories, yet the consequences of violating these assumptions have never been investigated. Whereas, to date, evolutionary senescence has been studied only indirectly in plants, it is concluded that plants provide good experimental systems for clarifying our understanding of senescence in natural populations.     

The influence of lecithin on plasma choline concentrations in triathletes and adolescent runners during exercise

An investigation was carried out on the effect of lecithin (phosphatidylcholine, 90%) on the plasma choline concentrations during continuous strain in 10 top level triathletes (4 women and 6 men), trial I, and 13 excellent adolescent runners (3 girls and 10 boys), trial II. Venous blood, collected before and immediately after the race, was separated and plasma was assayed by an improved high performance liquid chromatography method for choline. Each study comprised three experiments. In trial I the triathletes performed two periods of bicycle exercise each lasting 2 h at an average speed of 35 km · h^–1, and in the second study (trial II) the subjects were subjected to severe physical stress on two occasions during cross-country races of durations between 30–60 min according to their ages. The participants received either a placebo or 0.2 g lecithin · kg body mass^–1, 1 h before each exercise. As a control the same dose of lecithin was administered without any exercise (both trials I and II). Bicycle exercise without lecithin supply decreased plasma choline concentrations in all the triathletes, on average by 16.9% (P0.01). When lecithin was given before exercise, average plasma choline concentrations remained at the same level as the initial values. The supply of lecithin without exercise led to a significant increase of the plasma choline concentrations, on average by 26.9% (P0.01). In trial II, when running without a supply of lecithin, the mean plasma choline concentrations in the adolescent runners remained stable which may have been due to the duration of the physical stress. When lecithin was given before exercise, plasma choline concentrations increased, on average by 18.9% (P0.01). The administration of lecithin without exercise led in these participants to an increase in plasma choline concentrations, on average by 54% (P0.001).It was found from the present study that a combination of both lecithin intake and hard physical stress prevented in most subjects a decrease in plasma choline and this could affect performance.     

Distribution of NADPH-diaphorase activity in rat paravertebral,prevertebral and pelvic sympathetic ganglia

Summary Paravertebral (superior cervical and stellate), prevertebral (coeliac-superior mesenteric, inferior mesenteric) and pelvic (hypogastric) sympathetic ganglia of the rat were investigated by enzyme histochemistry to ascertain the distribution of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) activity. In the paravertebral ganglia the majority of the sympathetic neuronal perikarya contained lightly and homogeneously distributed formazan reaction product but there was a range of staining intensities amongst the neuron population. In contrast, in the prevertebral ganglia, intense NADPH-diaphorase staining was present in certain neurons. Firstly, a population of neurons of the coeliac-superior mesenteric ganglion complex were surrounded by densely NADPH-diaphorase-positive baskets of fibres and other stained fibres were seen in interstitial nerve bundles and in nerve trunks connected to the ganglion complex. Secondly, in both the inferior mesenteric ganglion and hypogastric ganglion there were many very intensely NADPH-diaphorase positive neurons. Stained dendritic and axonal processes emerged from these cell bodies. In both ganglia this population of neurons was smaller in size than the lightly stained ganglionic neurons and commonly had only one long (presumably axonal) process. The similarity of these highly NADPH-diaphorase-positive neurons with previously described postganglionic parasympathetic neurons in the hypogastric ganglion is discussed.     

Numerical solution of structured population models

Numerical methods are presented for a general age-structured population model with demographic rates depending on age and the total population size. The accuracy of these methods is established by solving problems for which alternate solution techniques are available and are used for comparison. The methods reliably solve test problems with a variety of dynamic behavior. Simulations of a blowfly population exhibit cyclic fluctuations, whereas a simulated squirrel population reaches a stable age distribution and stable equilibrium population size. Life-history attributes are easily studied from the computed solutions, and are discussed for these examples. Recovery of a stressed population back to equilibrium is examined by computing the transition in age structure, and the transient behavior of other properties of the population such as the per capita growth rate, the average age, and the generation length.     

A surplus no more? Variation in krill availability impacts reproductive rates of Antarctic baleen whales

The krill surplus hypothesis of unlimited prey resources available for Antarctic predators due to commercial whaling in the 20th century has remained largely untested since the 1970s. Rapid warming of the Western Antarctic Peninsula (WAP) over the past 50 years has resulted in decreased seasonal ice cover and a reduction of krill. The latter is being exacerbated by a commercial krill fishery in the region. Despite this, humpback whale populations have increased but may be at a threshold for growth based on these human-induced changes. Understanding how climate-mediated variation in prey availability influences humpback whale population dynamics is critical for focused management and conservation actions. Using an 8-year dataset (2013–2020), we show that inter-annual humpback whale pregnancy rates, as determined from skin-blubber biopsy samples (n = 616), are positively correlated with krill availability and fluctuations in ice cover in the previous year. Pregnancy rates showed significant inter-annual variability, between 29% and 86%. Our results indicate that krill availability is in fact limiting and affecting reproductive rates, in contrast to the krill surplus hypothesis. This suggests that this population of humpback whales may be at a threshold for population growth due to prey limitations. As a result, continued warming and increased fishing along the WAP, which continue to reduce krill stocks, will likely impact this humpback whale population and other krill predators in the region. Humpback whales are sentinel species of ecosystem health, and changes in pregnancy rates can provide quantifiable signals of the impact of environmental change at the population level. Our findings must be considered paramount in developing new and more restrictive conservation and management plans for the Antarctic marine ecosystem and minimizing the negative impacts of human activities in the region.     

Evidence for nuclear internalization of exogenous DNA into mammalian sperm cells

Mature sperm cells have the spontaneous capacity to take up exogenous DNA. Such DNA specifically interacts with the subacrosomal segment of the sperm head corresponding to the nuclear area. Part of the sperm-bound foreign DNA is further internalized into nuclei. Using end-labelled plasmid DNA we have found that 15–22% of the total sperm bound DNA is associated with nuclei as determined on isolated nuclei. On the basis of autoradiographic analysis, nuclear permeability to exogenous DNA seems to be a wide phenomenon involving the majority of the sperm nuclei. In fact, the foreign DNA, incubated with sperm cells for different lengths of time, is found in 45% (10 min) to 65% (2 hr) of the sperm nuclei. Ultrastructural autoradiography on thin sections of mammalian spermatozoa, preincubated with end-labelled plasmid DNA, shows that the exogenous DNA is internalized into the nucleus. This conclusion is further supported by ultrastructural autoradiographic analysis on thin sections of nuclei isolated from spermatozoa preincubated with end-labelled DNA. © 1993 Wiley-Liss, Inc.     

Collective behavior in relation with changing environments: Dynamics,modularity, and agency

Collective behavior operates without central control, using local interactions among participants to adjust to changing conditions. Many natural systems operate collectively, and by specifying what objectives are met by the system, the idea of agency helps to describe how collective behavior is embedded in the conditions it deals with. Ant colonies function collectively, and the enormous diversity of more than 15K species of ants, in different habitats, provides opportunities to look for general ecological patterns in how collective behavior operates. The foraging behavior of harvester ants in the desert regulates activity to manage water loss, while the trail networks of turtle ants in the canopy tropical forest respond to rapidly changing resources and vegetation. These examples illustrate some broad correspondences in natural systems between the dynamics of collective behavior and the dynamics of the surroundings. To outline how interactions among participants, acting in relation with changing surroundings, achieve collective outcomes, I focus on three aspects of collective behavior: the rate at which interactions adjust to conditions, the feedback regime that stimulates and inhibits activity, and the modularity of the network of interactions. To characterize the dynamics of the surroundings, I consider gradients in stability, energy flow, and the distribution of resources and demands. I then propose some hypotheses that link how collective behavior operates with changing environments.     

Novel thieno[2,3-b]- and [3,4-b]pyrans as potassium channel openers. Thiophene systems—XVII

The syntheses and antihypertensive activity of the thieno[3,4-b]pyran and thieno[2,3-b]pyran isosteres of the potassium channel opener (PCO) RWJ 26629 (± 2a) are reported. While the unsubstituted thiophene derivatives were active at 20 mg/kg, introduction of a strong electron withdrawing group in the 2-position of the thieno[3,2-b] series increased potency. Similar substitution on the thieno[3,4-b] series significantly lowered potency. Compounds 26 and 30 are approximately 5-fold more potent than the prototypic PCO cromakalim (± 1).