Mitochondrial DNA evolution in lagomorphs: Origin of systematic heteroplasmy and organization of diversity in European rabbits

Summary A characterization was conducted on mitochondrial DNA (mtDNA) molecules extracted separately from 107 European rabbits (Oryctolagus cuniculus) both wild and domestic, 13 European hares (Lepus capensis), and 1 eastern cottontail (Sylvilagus floridanus). Experimentally this study took into account restriction site polymorphism, overall length variation of the noncoding region, and numbers of repeated sequences. Nucleotide divergences indicate that the mtDNAs from the three species derived from a common ancestor some 6–8 million years (Myr) ago. Every animal appeared heteroplasmic for a set of molecules with various lengths of the noncoding region and variable numbers of repeated sequences that contribute to them. This systematic heteroplasmy, most probably generated by a rate of localized mtDNA rearrangements high enough to counterbalance the cellular segregation of rearranged molecules, is a shared derived character of leporids.The geographic distribution of mtDNA polymorphism among wild rabbit populations over the western European basin shows that two molecular lineages are represented, one in southern Spain, the second over northern Spain, France, and Tunisia. These two lineages derived from a common ancestor some 2 Myr ago. Their present geographical distribution may be correlated to the separation of rabbits into two stocks at the time of Mindel glaciation.Finally the distribution of mtDNA diversity exhibits a mosaic pattern both at inter- and intrapopulation levels.     

Active site conformation in myoglobin as determined by X-ray absorption spectroscopy

X-ray absorption fine structure experiments were performed to study structural and dynamic aspects of the active site of various forms of myoglobin. The structures determined for deoxyMb, MbCO, and MbO2 are consistent with the structure established by X-ray absorption fine structure experiment and X-ray crystallography. The first shell of ferrous MbNO determined contains 5 nitrogens located at 2.02 A and a short NO bond length of 1.76 A. This study focuses on the change of the XAFS Debye-Waller factor with temperature, which is a measure of thermal and static disorder. It was found that the changes of Debye-Waller factor with temperature for the Mb proteins, except deoxyMb, are consistent with a simple Einstein model, in which a single frequency was assumed for the bond stretching modes. In contrast, the temperature dependence of deoxyMb cannot be fitted to the Einstein model and a large disorder was found at low temperatures, which indicates the existence of conformational substates of the active site.     

The photoperceptive sites and the function of tissue light-piping in photomorphogenesis of etiolated oat seedlings*

Abstract. Responses to red light irradiation of discrete areas along the intact, etiolated oat seedling indicate that illumination of the region around the coleoptilar node results in maximal coleoptile growth stimulation and mesocotyl growth suppression. Quantitation of the fibre optic properties of these etiolated tissues shows that the amount of axially transmitted light is log linear as a function of distance for both the mesocotyl and coleoptile (plus primary leaf). Using the fibre optic properties of the tissues to predict the response of the etiolated seedling to defined illumination fields allows one to localize two sites of photoperception: although the mesocotyl response pattern can be explained by the action of a single site found near the top of the mesocotyl itself, the coleoptile response depends on irradiation of both the mesocotyl site and an additional site located just above the node. The very low- and the low-fluence responses of etiolated oats independently predict similar regions of the seedling as sites of photo-perception. The fibre optic properties of the seedling could allow the seedling to increase the effective light signal received by the photosensitive area significantly.     

Site Variation in Littoral Periphyton Populations: Correlation and Regression with Environmental Factors

Over a six month period, littoral periphyton communities on vertically oriented glass slides were quantitatively sampled from four stations in Elk Lake, British Columbia. Correlation and multiple regression analyses were employed to assess the temporal interrelationships between 15 environmental variables, concurrently measured, and the periphyton populations, expressed as total cell counts and species diversity, for the complete data set of each station. At all stations there was a statistically significant increase in total cell populations with increasing length of slide exposure; positive correlations also occurred with pH, temperature, nitrite, nitrate and copper, while negative ones occurred with oxygen, orthophosphate and hardness (magnesium and total). At all but one station there was a significant decrease in species diversity with increasing slide exposure duration, a decrease attributed to the simultaneous increase in standing crop resulting in intensified competition for diminishing substrate area suitable for cell colonization. Multiple stepwise regression analyses demonstrated that greater than 80% of the variation in periphyton total cells at all four stations could be reliably predicted or described in terms of length of slide exposure, phosphate (ortho and total), and hardness (calcium and total). On the other hand, exposure duration and total cell populations were the most important variables associated with changes in species diversity at three of the four stations. In general, correlations and regressions between standing crop and environmental variables were more statistically reliable and consistent between stations than comparable analyses with species diversity. Use of multiple regression and correlation disclosed the importance of small, non statistically significant variation in ecologically relevant variables and permitted objective comparison of stations.     

RELATIONSHIP BETWEEN CELL CYCLE AND LIGHT‐LIMITED GROWTH RATE IN OCEANIC PROCHLOROCOCCUS (MIT9312) AND SYNECHOCOCCUS (WH8103) (CYANOBACTERIA)1

The relationships between growth rate, cell‐cycle parameters, and cell size were examined in two unicellular cyanobacteria representative of open‐ocean environments: Prochlorococcus (strain MIT9312) and Synechococcus (strain WH8103). Chromosome replication time, C, was constrained to a fairly narrow range of values (～4–6 h) in both species and did not appear to vary with growth rate. In contrast, the pre‐ and post‐DNA replication periods, B and D, respectively, decreased with increasing growth rate from maxima of ～30 and 10–20 h to minima of ～4–6 and 2–3 h, respectively. The combined duration of the chromosome replication and postreplication periods (C+D), a quantity often used in the estimation of Prochlorococcus in situ growth rates, varied ～2.4‐fold over the range of growth rates examined. This finding suggests that assumptions of invariant C+D may adversely influence Prochlorococcus growth rate estimates. In both strains, cell mass was the greatest in slowly growing cells and decreased 2‐ to 3‐fold over the range of growth rates examined here. Estimated cell mass at the start of replication appeared to decrease with increasing growth rate, indicating that the initiation of chromosome replication in Prochlorococcus and Synechococcus is not a simple function of cell biomass, as suggested previously. Taken together, our results reflect a notable degree of similarity between oceanic Synechococcus and Prochlorococcus strains with respect to their growth‐rate‐specific cell‐cycle characteristics.     

Signs of stabilisation and stable coexistence

Many empirical studies motivated by an interest in stable coexistence have quantified negative density dependence, negative frequency dependence, or negative plant–soil feedback, but the links between these empirical results and ecological theory are not straightforward. Here, we relate these analyses to theoretical conditions for stabilisation and stable coexistence in classical competition models. By stabilisation, we mean an excess of intraspecific competition relative to interspecific competition that inherently slows or even prevents competitive exclusion. We show that most, though not all, tests demonstrating negative density dependence, negative frequency dependence, and negative plant–soil feedback constitute sufficient conditions for stabilisation of two‐species interactions if applied to data for per capita population growth rates of pairs of species, but none are necessary or sufficient conditions for stable coexistence of two species. Potential inferences are even more limited when communities involve more than two species, and when performance is measured at a single life stage or vital rate. We then discuss two approaches that enable stronger tests for stable coexistence‐invasibility experiments and model parameterisation. The model parameterisation approach can be applied to typical density‐dependence, frequency‐dependence, and plant–soil feedback data sets, and generally enables better links with mechanisms and greater insights, as demonstrated by recent studies.