Sexual selection effects on the evolution of senescence

Although the basic theories concerning evolution of senescence have been generally accepted for a half-century, interpretation of this paradigm has been constrained by an over-reliance on mortality as both the cause and the measure of senescence. Consideration of both survival and fecundity as components of reproductive value, and integration of sexual selection theory with senescence theory allows reconciliation of long-standing, as well as recent, discrepancies between data and theory. This approach demonstrates that sexual selection on males in polygynous mating systems can have significant effects on the evolution of senescence that could overshadow the selection effects of mortality rates among such animals.     

Plant Growth Regulators and Induction of Leaf Senescence in Nitrogen-Deprived Wheat Plants

The sequence of events and the signals that regulate the remobilization of nitrogen (N) reserves during senescence induced by N starvation were studied in leaf 3, the last fully expanded leaf, in 17-day-old wheat (Triticum aestivum L.) plants. The first event observed was a rapid decrease in the isopentenyl adenosine (iPA) concentration during the first 24 h of N starvation. No differences in t-zeatin riboside and dihydrozeatin riboside concentrations were observed until the end of the assay. During the following 6 days, a decrease in soluble amino acids, chlorophyll, and protein, as well as an increase in soluble sugar concentration and endoproteolytic activity, could be observed. At day 3 of the experiment, the abscisic acid (ABA) concentration in the leaves of N-deprived plants started to increase. After 6 days of N deprivation there was a rise in oxidative stress, as indicated by the increase in malondialdehyde concentration, as well as a decrease in the activities of antioxidant enzymes catalase and ascorbate peroxidase. To analyze interactions with leaf development, the first, second, third, and fourth leaves were studied. iPA concentration decreased in all the leaf stages, including leaf 4, which was not fully expanded. A linear correlation between iPA and protein concentration was determined. These results suggest that the sharp fall in iPA could be the earliest event that induces protein degradation during the development of senescence induced by N deficiency, and that only later is ABA accumulated and oxidative stress developed.     

Long-term molecular and cellular stability of human neural stem cell lines

Human Neural Stem Cells (hNSCs) are excellent candidates for in vitro and in vivo molecular, cellular, and developmental research, and also for ex-vivo gene transfer and cell therapy in the nervous system. However, hNSCs are mortal somatic cells, and thus invariably enter an irreversible growth arrest after a finite number of cell divisions in culture. It has been proposed that this is due to telomere shortening. Here, we show that long-term cultured (up to 4 years) v-myc perpetuated hNSC lines do preserve short but stable and homogeneous telomeres (TRF and Q-FISH determinations). hNSC lines (but not strains) express high levels of telomerase activity, which is activated by v-myc, as demonstrated here. Telomerase activity is not constitutive, becoming non-detectable after differentiation (in parallel to v-myc down-regulation). hNSC lines also maintain a stable cell cycle length, mitotic potential, differentiation and neuron generation capacity, and do not express senescence-associated beta-galactosidase over years, as studied here. These data, collectively, help to explain the immortal nature of v-myc-perpetuated hNSC lines, and to establish them as excellent research tools for basic and applied neurobiological and translational studies.     

Explaining the optimality of U-shaped age-specific mortality

Mortality is U-shaped with age for many species, declining from birth to sexual maturity, then rising in adulthood, sometimes with postreproductive survival. We show analytically why the optimal life history of a species with determinate growth is likely to have this shape. An organism allocates energy among somatic growth, fertility and maintenance/survival at each age. Adults may transfer energy to juveniles, who can then use more energy than they produce. Optimal juvenile mortality declines from birth to maturity, either to protect the increasingly valuable cumulative investments by adults in juveniles or to exploit the compounding effects of early investment in somatic growth, since early growth raises subsequent energy production, which in turn supports further growth. Optimal adult mortality rises after maturity as expected future reproduction declines as in Hamilton, but intergenerational transfers lead to postreproductive survival as in Lee. Here the Hamilton and transfer effects are divided by probabilities of survival in contrast to the fitness impact measures, which are relevant for mutation-selection balance. If energetic efficiency rises strongly with adult experience, then adult mortality could initially be flat or declining.     

Fetal mesenchymal stem cells derived from human umbilical cord sustain primitive characteristics during extensive expansion

Stem cells of fetal origin lie between embryonic and adult stem cells in terms of potentiality. Because of the ethical controversy surrounding embryonic stem cells and the relatively inferior quality of adult stem cells, the use of fetal stem cells would be an attractive option in future therapeutic applications. Here, we have investigated primitive characteristics of human umbilical-cord-derived fetal mesenchymal stem cells (UC fMSCs) during extensive expansion. We have successfully isolated and cultured UC fMSCs from all UC samples, but with two early fungal contaminations. UC fMSCs proliferated without significant evidence of morphological changes, and the average cumulative population-doubling level was over 25 for about 3 months. UC fMSCs showed the positive expression of several CD markers, known to be related to MSCs, including CD73 (SH-3, 4), CD90 (Thy-1), CD105 (SH-2), CD117 (c-kit), and CD166 (ALCAM). They demonstrated primitive properties throughout the expansion period: multilineage differentiation potentials examined by functional assays, a variety of pluripotent stem cell markers including Nanog, Oct-4, Sox-2, Rex-1, SSEA-3, SSEA-4, Tra-1–60, and Tra-1–81, minimal evidence of senescence as shown by β-galactosidase staining, and the consistent expression of telomerase activity. These results suggest that UC fMSCs have more primitive properties than adult MSCs, which might make them a useful source of MSCs for clinical applications. This work was supported by the Seoul R&BD Program (10548).     

Electron Microscopic Observation on the Infected Cells During Pea (Pisum sativum L.) Nodule Senescence

Ultrastructural changes of the infected cells have been observed by transmission electron microscopy during pea root nodule senescence. The infected cells and bacteroids of pea nodules ultimately senesce, their senescence has certain laws and features. Firstly, peribacteroid membrane were loosened, leaving a large electron-empty space with fibrillar and vesicular material. Then bacter0id cytoplasm lost features and aggregated into some clustered electron- dense material. At next stage bacteroids were structurally emtpy and appeared like “ghost” cells. Companying bacteroid senescence, host cytoplasm changed from dark to light in electron density and cell organelles gradually decreased. After the host cell tonoplasts and plasmalemma broke down, the infected cells showed a chaotic state of bacteroids and host cell debrises. Finally, infected cells disintegrated completely. Sometimes some young bacteria were seen in the intercellular spaces surrounded by degenerating cells, in the degenerating cytoplasm. A few infection threads were also found among the disintegrated bacteroids, even some of them were releasing the bacteria into the degenerating host cytoplasm.     

Structural changes in flowers ofIpomoea tricolor during flower opening and closing

Summary Examination of rib tissue of developing flower buds and flowers ofIpomoea tricolor utilizing the light and electron microscopes resulted in the identification of a group of inner epidermal cells which undergo dynamic structural changes during the two days prior to flowering, during flowering and during flower fading. Over a three-day period these cells undergo enlargement, modification of shape, reduction in wall thickness, extensive vacuolation and autophagic activity, transition from a ribosome-rich cytoplasm with stacked RER and dictyosome complexes to one with only a few isolated organelles and a limited number of ribosomes, and modification of the tonoplast membrane. In contrast, other cells of the rib vacuolate prior to flowering and exhibit no further changes. The structural changes in the inner epidermal cells suggest that they affect the turgor status of the cells and initiate the opening and eventual closing of the flower.     

Leaf senescence in a non-yellowing mutant of Festuca pratensis: Photosynthesis and photosynthetic electron transport

The photosynthetic capacity of detached leaves of a non-yellowing mutant of Festuca pratensis Huds. declined during senescence at a similar rate to that in a normal cultivar. Respiratory oxygen uptake in the dark continued at similar rates in both genotypes during several days of senescence. In chloroplasts isolated from leaves at intervals after excision, the rate of photosystem I (PS I)-mediated methyl viologen reduction using reduced N,N,N,N-tetramethyl-p-phenylene diamine as electron donor also declined in both genotypes, possibly due to loss of integrity of the photosynthetic apparatus in the cytochrome f-plastocyanin region. There was a similar fall in PS II electron transport using water as electron donor and measured at the rate of reduction of 2,6-dichlorophenolindophenol. Partial restoration of this activity by the addition of diphenyl carbazide was evidence for lability of the oxygen-evolving complex during senescence. An accentuated difference between mutant and normal material in this case indicated that the mutant retains a greater number of functional PS II centres. Changes in the light-saturation characteristics of the two photosystems have been discussed in relation to the organization of the photosynthetic membranes during senescence.Abbreviations and symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP 2,6-dichlorophenolindophenol - DMSO dimethyl sulphoxide - DPC diphenyl carbazide - MV methyl viologen - PS I, PS II photosystem I, II - TMPD N,N,N,N-tetramethyl-p-phenylene diamine