A comparison of within-plant karyological heterogeneity between agamospermous and sexualTaraxacum (Compositae) as assessed by the nucleolar organiser chromosome

Morphological variation for the NOR chromosome was studied for four half-siblings of a sexual outbreedingTaraxacum, for three siblings of the obligate agamospermT. pseudohamatum, and for two individuals of the agamospermT. brachyglossum. No rearrangement was detected for the 113 chromosomes of sexuals, or for 41 chromosomes of two agamospermous individuals. In the other three agamospermous individuals, 3/16, 5/50, and 5/20 chromosomes showed evidence of chromosomal rearrangement. The majority of rearrangement events (10/13) occurred to the satellite rather than to the body of the NOR-chromosome. It is considered that such high levels of somatic chromosomal rearrangement in agamospermousTaraxacum may be the result of activity by transposable genetic elements. This recombination may be of selective advantage to asexual plants which cannot generate genetic variability through the sexual process.     

Ecological and evolutionary consequences of the trophic polymorphism in Cichlasoma citrinellum (Pisces: Cichlidae)

The neotropical cichlid fish Cichlasoma citrinellum is polymorphic in the structure of its pharyngeal jaw apparatus and external morphology. The pharyngeal jaws are either gracile and bear slender, pointed teeth (papilliform) or robust with strong, rounded teeth (molariform). Molariform morphs have a ‘benthic’, and papilliform morphs a ‘limnetic’ body form. Furthermore, this species is also polychromatic, with yellow and black morphs. The molariform morphology of the pharyngeal jaw apparatus adapts the fish for cracking and feeding on snails. Based on analysis of stomach contents, 94% of the molariform morph ate snails whereas only 19%, of the papilliform morph did so. This result suggests that the morphs occupy different ecological niches. The morphology of the pharyngeal jaw apparatus does not correlate significantly with sex, but it does with body colouration (P<0.005). Cichlasoma citrinellum mate assortatively with their own colour; therefore a mating preference for colour may lead to genetic isolation of trophic morphs. The frequency of the molariform morph differs strikingly among populations of five Nicaraguan lakes and its abundance is correlated with the abundance of snails, the fishes' principal prey item. Among populations the frequency of molariform morphs decreases in the dry season. Morphology possibly changes reversibly within particular individuals between seasons. These results suggest that phenotypic plasticity and polymorphisms may be an adaptive characteristic of cichlid fishes. Patterns of intraspecific morphological variation match patterns of interspecific morphological diversification which suggests that universal developmental mechanisms canalize the possible expressions of morphology. The ability to respond morphologically to environmental shifts, in conjunction with genetically determined trophic polymorphisms and sexual selection via mate choice, could be the basis for speciation through intermediate stages of polymorphism of the impressive adaptive radiation of cichlid fishes.     

Haematological disturbances and osmotic shifts in rainbow trout,Oncorhynchus mykiss (Walbaum) under acid and aluminium exposure

Summary Elevated values of haematocrit were observed in rainbow trout (Oncorhynchus mykiss) during combined acid and aluminium exposure. Possible causes of this, i.e., decreased plasma volume, swelling of erythrocytes, and/or mobilization of erythrocytes into the blood were investigated. Slight haematocrit increases (10–20%) during mild acid stress (pH 5.0, 25 mol Ca²⁺·1^-1) were mostly caused by osmotic shifts. Both swelling of erythrocytes rocytes and a significant decrease of the plasma volume were demonstrated in fish at pH 5.0. These osmotic disturbined were greater during acid exposure (pH 5.0) combined with Al (60 g·1^-1; 200 g·1^-1). In addition, numbers of erythrocytes increased by 40% compared to acid exposure, which contributed to the severe haematocrit rise (35%) during Al exposure. A contraction of the spleen releasing erythrocytes into the blood is suggested to occur as an adrenergic response to hypoxia, which is observed in fish acutely exposed to Al.Abbreviations BV blood volume - ECFV extracellular fluid volume - ICFV intracellular fluid volume - ISFV interstitial fluid volume - MCHC mean cell haemoglobin concentration - MCV mean cell volume - PV plasma volume     

Electrooptical studies on proton-binding and -release of bacteriorhodopsin

Electric field induced pH changes of purple membrane suspensions were investigated in the pH range from 4.1 to 7.6 by measuring the absorbance change of pH indicators. In connection with the photocycle and proton pump ability, three different states of bacteriorhodopsin were used: (1) the native purple bacteriorhodopsin (magnesium and calcium ions are bound, the M intermediate exists in the photocycle and protons are pumped), (2) the cation-depleted blue bacteriorhodopsin (no M intermediate), and (3) the regenerated purple bacteriorhodopsin which is produced either by raising the pH or by adding magnesium ions (the M intermediate exists). In the native purple bacteriorhodopsin there are, at least, two types of proton binding sites: one releases protons and the other takes up protons in the presence of the electric field. On the other hand, blue bacteriorhodopsin and the regenerated purple bacteriorhodopsin (pH increase) show neither proton release nor proton uptake. When magnesium ions are added to the suspensions; the field-induced pH change is observed again. Thus, the stability of proton binding depends strongly on the state of bacteriorhodopsin and differences in proton binding are likely to be related to differences in proton pump activity. Furthermore, it is suggested that the appearance of the M intermediate and proton pumping are not necessarily related.     

Effect of Gossypol on Trypomastigotes and Amastigotes of Trypanosoma cruzi

Bloodstream Trypanosoma cruzi trypomastigotes isolated from infected mice undergo reduction of motility and structural damages after 5 to 45 min exposure to gossypol at concentrations ranging from 5 to 50 μM. When 1% serum albumin is added to the incubation medium, no alterations of parasites are observed, even with 100 μM gossypol. Intracellular T. cruzi amastigotes in infected Vero cell cultures exposed to 5 μM gossypol for 2 h do not show changes. Incubation with 5 μM gossypol for 48 h produces complete disruption of host cells; however, the amastigotes they contain show only mineor alterations. The observations indicate that, in protein-rich media, gossypol is complexed into associations which have no activity on the different forms of the T. cruzi biological cycle.     

Cytogenetic characteristics of a population of Chironomus riparius Meigen 1804 (Diptera, Chironomidae) from a polluted Po river station

A population of Chironomus riparius from a Po river station near Moncalieri (a trace-metal polluted station) was studied. In this population was established a great variability of band structure of polytene chromosomes as well as paracentric heterozygous inversions, deletions, deficiencies, partial breaks, diploid chromosome fragments, and changes in functional activity and appearance of heterochromatin. In arms A through F, some bands had an increased size compared to the standard chromosomic map. Some bands appeared in a heterozygous or normal homozygous state or were amplified. In all arms, many condensed stable bands appeared in the decondensed state when compared to the standard map. Asynaptic zones in arms E and G as well as heterozygous Balbiani rings and NORs were established. Very often the 4th chromosome was almost completely heteropycnotic and looded like a pompon chromosome. For the first time in this species, a high frequency of ectopic pairings of different arms was observed. Telomeric regions involved in ectopic pairings had a granular appearance, as did some centromeres. The hypothesis is advanced that such a high frequency of structural rearrangements could be correlated with genomic distribution of specific mobile elements.     

Zooplankton of Lake Peipsi-Pihkva in 1909–1987

164 taxa were identified in the net zooplankton of the pelagial of L. Peipsi-Pihkva in 1909–1987, including 3 species of protozoans, 74 species of rotifers, 58 species of cladocerans, 28 species of copepods and 1 mollusc. One rotifer species, Ploesoma peipsiense Mäemets et Kutikova, has been described as new for science here. The zooplankton of L. Peipsi-Pihkva is remarkably rich in species including rarities in Estonia: Limnosida frontosa, Drepanothrix dentata, Bythotrephes longimanus, B. cederstroemi etc. Due to its large surface area, L. Peipsi-Pihkva provides a large scale of biotopes of a diverse trophic state and humic content, which support species with different ecological requirements. Most of the aquatory of the lake has lately been mesotrophic, favouring the coexistence of indicators of oligo- and mesotrophic state and species preferring a higher trophic state. The occurrece of 10 species of the genus Bosmina including B. berolinensis, B. gibbera, B. lilljeborgi, B. thersites and B. crassicornis, sparse in Estonian lakes, is the most noteworthy feature of the zooplankton of L. Peipsi-Pihkva. The coexistence of B. coregoni and B. berolinensis, B. gibbera, B. lilljeborgi etc. which were earlier regarded as subspecies of B. c. coregoni proves that they are different species producing usually no hybrids. The species composition was subjected to certain changes during the years under consideration. Larvae of Dreissena were first found in zooplankton in 1962. The oligo-mesotrophic indicator Holopedium gibberum occurred in the lake in 1909–1964, but was lacking in later samples.     

Changes of the forest-savanna boundary in Brazilian Amazonia during the Holocene revealed by stable isotope ratios of soil organic carbon

The possibility of ecosystem boundary changes in northern Brazilian Amazonia during the Holocene period was investigated using soil organic carbon isotope ratios. Determination of past and present fluctuations of the forest-savanna boundary involved the measurement of natural ¹³C isotope abundance, expressed as ¹³C, in soil organic matter (SOM). SOM ¹³C analyses and radiocarbon dating of charcoal fragments were carried out on samples derived from soil profiles taken along transects perpendicular to the ecotonal boundary. SOM ¹³C values in the upper soil horizons appeared to be in equilibrium with the overlying vegetation types and did not point to a movement of the boundary during the last decades. However, ¹³C values obtained from deeper savanna and forest soil layers indicated that the vegetation type has changed in the past. In current savanna soil profiles, we observed the presence of mid-Holocene charcoals derived from forest species: fire frequency at that time was probably greater, and more extensive savanna may have resulted. Isotope data and the presence of these charcoals thus suggest that the forest-savanna boundary has shifted significantly in the recent Holocene period, forest being more extensive during the early Holocene than today. During the middle Holocene, the forest could have strongly regressed, and fires appeared, with a maximum development of the savanna vegetation. At the beginning of the late Holocene, the forest may have invaded a part of this savanna, and fires occurred again.     

Polypeptide folding with off-lattice Monte Carlo dynamics: the method

The MC dynamics of an off-lattice all-atom protein backbone model with rigid amide planes are studied. The only degrees of freedom are the dihedral angle pairs of the C-atoms. Conformational changes are generated by Monte Carlo (MC) moves. The MC moves considered are single rotations (simple moves, SM's) giving rise to global conformational changes or, alternatively, cooperative rotations in a window of amide planes (window moves, WM's) generating local conformational changes in the window. Outside the window the protein conformation is kept invariant by constraints. These constraints produce a bias in the distribution of dihedral angles. The WM's are corrected for this bias by suitable Jacobians. The energy function used is derived from the CHARMM force field. In a first application to polyalanine it is demonstrated that WM's sample the conformational space more efficiently than SM's.Abbreviations CPU Central Processing Unit - MC Monte Carlo - MCD Monte Carlo Dynamics - MD Molecular Dynamics - RMS Root-Mean-Square - RMSD Root-Mean-Square-Deviation - SM Simple Move - WM Window Move