Influence of the Molecular Structure of Steroids on Their Ability to Interrupt Gap Junctional Communication

17β-estradiol propionate was found to reduce the gap junctional communication in a concentration range similar to that of testosterone propionate, in primary cultures of rat Sertoli cells and cardiac myocytes. Uncoupling was reversible on washing out and occurred without concomitant rise in the intracellular calcium concentration. Esterification was a prerequisite for the activity of extracellularly applied steroid compounds (for example, testosterone was ineffective even at external concentrations up to 100 μm, whereas its intracellular application at 1 μm totally interrupted intercellular communication), but their uncoupling efficiency did not depend on the nature of the ester chain nor on its position on the steroid nucleus. The derivatives of two other androgen hormones (derivatives of the androstane nucleus) were also efficient as junctional uncouplers. Among five steroid molecules belonging to the pregnane family, only one (pregnanediol diacetate) interrupted the junctional communication. Neither cholic acid nor cholesteryl acetate or ouabain showed this effect. Altogether, no correlation with the presence or position of double bonds nor with the trans- or cis-fusion of the A and B rings could be recognized. These results suggest that this reversible, nondeleterious uncoupling effect of steroids is independent of the shape of the molecules and is more probably related to their size and liposolubility, that condition their insertion into the lipid bilayer. Their incorporation into the membrane could disturb the activity of the membrane proteins by a physical mechanism. Received: 10 April 1995/Revised: 27 October 1995     

Evolution of the Larval Cuticle Proteins Coded by the Secondary Sex Chromosome Pair: X2 and Neo-Y of Drosophila miranda: I. Comparison at the DNA Sequence Level

The larval cuticle protein genes (Lcps) represent a multigene family located at the right arm of the metacentric autosome 2 (2R) in Drosophila melanogaster. Due to a chromosome fusion the Lcp locus of Drosophila miranda is situated on a pair of secondary sex chromosomes, the X2 and neo-Y chromosome. Comparing the DNA sequences from D. miranda and D. melanogaster organization and the gene arrangement of Lcp1–Lcp4 are similar, although the intergene distances vary considerably. The greatest difference between Lcp1 and Lcp2 is due to the occurrence of a pseudogene in D. melanogaster which is not present in D. miranda. Thus the cluster of the four Lcp genes existed already before the separation of the melanogaster and obscura group. Intraspecific homogenizations of different cluster units must have occurred repeatedly between the Lcp1/Lcp2 and Lcp3/Lcp4 sequence types. The most obvious example is exon 2 of the Lcp3 gene in D. miranda, which has been substituted by the corresponding section of the Lcp4 gene rather recently. The homogenization must have occurred before the translocation which generated the neo-Y chromosome. Lcp3 of D. melanogaster has therefore no orthologous partner in D. miranda. Rearrangements in the promoter regions of the D. miranda Lcp genes have generated new, potentially functional CAAT-box motifs. Since three of the Lcp alleles on the neo-Y are not expressed and Lcp3 is expressed only at a reduced level, it is suggestive to speculate that the rearrangements might be involved as cis-regulatory elements in the up-regulation of the X2-chromosomal Lcp alleles, in Drosophila an essential process for dosage compensation. The Lcp genes on the neo-Y chromosome have accumulated more base substitutions than the corresponding alleles on the X2. Received: 27 December 1995 / Accepted: 30 April 1996     

Chromosomal heteromorphism linked to the mating type locus of the oomycetePhytophthora infestans

The mating type locus of the oomycete,Phytophthora infestans, is embedded in a region of DNA that displays distorted and non-Mendelian segregation. By using DNA probes linked to the mating type locus to genetically and physically characterize that region, a large zone of chromosomal heteromorphism was detected. LocusS1 was shown to represent a tandemly repeated array of DNA that was typically present in a hemizygous state in A1 isolates while being absent from A2 isolates. The analysis of the parents and progeny of seven crosses indicated that the tandem array was linked in cis to the A1-determining allele of the mating type locus. A worldwide survey of genotypically diverse field isolates ofP. infestans indicated thatS1 was present in each of 48 isolates of the A1 mating type that were tested, but was absent in 46 of 47 A2 strains. Physical analysis ofS1 indicated that the tandemly repeated DNA sequence spanned about 300 kb and had evolved from a 1.35-kb monomer. Internal deletions occurred withinS1 during sexual propagation. This and other mutations apparently contributed to a high degree of polymorphism within theS1 array.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Body size,litter size,timing of reproduction,and juvenile survival in the Unita ground squirrel,Spermophilus armatus

The timing of reproduction affected litter size, offspring mass, and offspring survival in the Uinta ground squirrel, Spermophilus armatus, in Grand Teton National Park, Wyoming. Survival of juvenile females to yearling age varied negatively with date of weaning and positively with individual offspring mass. At the same time, juveniles weaned early in the season were lighter, and juveniles weaned later in the season were heavier. The coefficient of variation for juvenile body mass, originally measured at weaning, significantly decreased by the time juveniles entered hibernation, indicating that individuals weaned early and light caught up in body mass to individuals weaned later and heavier. From the perspective of the mother's investment in the litter, litter size (corrected for mother's mass) decreased with later wcaning dates, while the relationship of weaning date to litter mass (corrected for mother's mass) was significant in only one year. Maternal allocation of resources in litters changed over the season so that mothers produced many, small offspring early in the season, and fewer, large offspring late in the season.     

The relationship between the relative growth rate and nitrogen economy of alpine and lowland Poa species

This study investigates the nitrogen economy of six altitudinally contrasting Poa species which differ in their relative growth rate (R). Two alpine (Poa fawcettiae and P. costiniana), one sub-alpine (P. alpina)and three temperate lowland species (P. pratensis, P. campressa and P. trivialis) were grown hydroponically under identical conditions in a growth room. The low R exhibited by the alpine species was associated with lower plant organic nitrogen concentration (n_p) and lower nitrogen productivity (Π_p, amount of biomass accumulation per mol organic nitrogen and time). The differences in Π_p between the alpine and lowland species did not appear to be due to differences in the carbon concentration or the proportion of total plant organic nitrogen allocated to the leaves, stems or roots. Variations in Π_P were also not due to variations in photosynthetic nitrogen use efficiency (Ψ_N, the rate of photosynthesis per unit organic leaf nitrogen) or shoot or root respiration rates per unit organic nitrogen (Λ_SH and Λ_R, respectively) per se. Rather, the lower Λ_p in the alpine species was probably due to a combination of small variations in several of the parameters (e.g. slightly lower Ψ_N, slightly higher Λ_SH and Λ_R, and slightly higher proportions of total plant organic nitrogen allocated to the roots). The alpine species exhibited lower organic acid and mineral concentrations. However, no differences in whole-plant construction costs (grams of glucose needed to synthesize one gram of biomass) were observed between She alpine and lowland Poa species. The lack of sub-stantial differences in Ψ_N between the alpine and lowland species contrasts with the large differences in Ψ_N between slow- and fast-growing lowland species that have been reported in the literature. The reasons for the unusually high Ψ_N values exhibited by the alpine Poa species are discussed.     

Effects of various environmental conditions on growth and reproduction of the sea hareAplysia oculifera (Adams and Reeve, 1850)

We studied the influences of food type, food quantity, water currents, starvation and light on growth and reproduction of the sea hareaplysia oculifera (Adams and Reeve, 1850) under laboratory conditions. Out of five species of algae served as food,Enteromorpha intestinalis promoted the fastest growth ofA. oculifera, Ulva spp. slower growth,Cladophora sp. allowed maintenance spp. slower growth,Cladophora sp. allowed maintenance of steady body mass, and the brown algaeColpomenia sp. andPadina pavonia were rejected by the sea hares. When sea hares were exposed to four levels of water currents, growth rates decreased as water currents increased. Sea hares fed on 50% ration grew slower than those fed on 100% ration (ad libitum). During 10 days of starvation sea hares lost weight, but when subsequently fed 100% ration they recovered and grew at a rate similar to those fed continuously with 100% ration. Under shade and under natural sunlight sea hares grew at the same rates. Whenever growth rates decreased, sea hares began to spawn at a smaller body size.A. oculifera demonstrated physiological plasticity that adapted them to varied and unpredictable environmental conditions. At different conditions of food availability they applied different tactics of resource allocation between growth and reproduction.     

Heuristic optimization of the general life history problem: A novel approach

Summary The general life history problem concerns the optimal allocation of resources to growth, survival and reproduction. We analysed this problem for a perennial model organism that decides once each year to switch from growth to reproduction. As a fitness measure we used the Malthusian parameterr, which we calculated from the Euler-Lotka equation. Trade-offs were incorporated by assuming that fecundity is size dependent, so that increased fecundity could only be gained by devoting more time to growth and less time to reproduction. To calculate numerically the optimalr for different growth dynamics and mortality regimes, we used a simplified version of the simulated annealing method. The major differences among optimal life histories resulted from different accumulation patterns of intrinsic mortalities resulting from reproductive costs. If these mortalities were accumulated throughout life, i.e. if they were senescent, a bangbang strategy was optimal, in which there was a single switch from growth to reproduction: after the age at maturity all resources were allocated to reproduction. If reproductive costs did not carry over from year to year, i.e. if they were not senescent, the optimal resource allocation resulted in a graded switch strategy and growth became indeterminate. Our numerical approach brings two major advantages for solving optimization problems in life history theory. First, its implementation is very simple, even for complex models that are analytically intractable. Such intractability emerged in our model when we introduced reproductive costs representing an intrinsic mortality. Second, it is not a backward algorithm. This means that lifespan does not have to be fixed at the begining of the computation. Instead, lifespan itself is a trait that can evolve. We suggest that heuristic algorithms are good tools for solving complex optimality problems in life history theory, in particular questions concerning the evolution of lifespan and senescence.     

Relative growth rate, biomass allocation pattern and water use efficiency of three wheat cultivars during early ontogeny as dependent on water availability

We have investigated the water use efficiency of whole plants and selected leaves and allocation patterns of three wheat cultivars (Mexipak, Nesser and Katya) to explore how variation in these traits can contribute to the ability to grow in dry environments. The cultivars exhibited considerable differences in biomass allocation and water use efficiency. Cultivars with higher growth rates of roots and higher proportions of biomass in roots (Nesser and Katya) also had higher leaf growth rates, higher proportions of their biomass as leaves and higher leaf area ratios. These same cultivars had lower rates of transpiration per unit leaf area or unit root weight and higher biomass production per unit water use. They also had higher ratios of photosynthesis to transpiration, and lower ratios of intercellular to external CO₂ partial pressure. The latter resulted from large differences in stomatal conductance associated with relatively small differences in rates of photosynthesis. There was little variation between cultivars in response to drought, and differences in allocation pattern and plant water use efficiency between cultivars as found under well-watered conditions persisted under dry conditions. At the end of the non-watered treatment, relative growth rates and transpiration rates decreased to similar values for all cultivars. High ratios of photosynthesis to transpiration, and accordingly high biomass production per unit of transpiration, is regarded as a favourable trait for dry environments, since more efficient use of water postpones the decrease in plant water status.     

The influence of plant density on the responses of Sinapis alba to CO2 and windspeed

Plants in nature live in populations of variable density, a characteristic which may influence individual plant responses to the environment. We investigated how the responses of Sinapis alba plants to different wind speeds and CO₂ concentrations could be modified by plant density. In our wind-density experiment the expectation that mechanical and physiological effects of wind will be ameliorated by growing in high density, as a result of positive plant interactions, was realised. Although individual plants were smaller at higher densities, the effect of increasing windspeed was much less than at lower plant densities. A similar reduced sensitivity of individual plant growth under high densities was also observed under CO₂ enrichment. When measured as a population or stand response, there was no effect of density on the CO₂ responses, with all stands showing very similar increases in total biomass with CO₂ enrichment. In the wind speed experiment, total biomass per stand increased significantly with density, although there was no effect of density on the wind speed response. Specific leaf area decreased with increasing wind speed and this response was significantly affected by the density at which the plants grew.