ALBERT

Notes: In ciliates, development of the polyploid somatic macronucleus after sexual events involves extensive and reproducible rearrangements of the germ-line genome, including chromosome fragmentation and precise excision of numerous internal sequence elements. In Paramecium aurelia, alternative macronuclear versions of the same germ-line genome can be maternally inherited across sexual generations, showing that rearrangement patterns are not strictly determined by the germ-line sequence. Homology-dependent maternal effects can be evidenced by transformation of the vegetative macronucleus with cloned macronuclear sequences: new fragmentation patterns or internal deletions are specifically induced during differentiation of a new macronucleus, in sexual progeny of transformed clones. Furthermore, transformation of the maternal macronucleus with germ-line sequences containing internal eliminated sequences (short single-copy elements) can result in a specific inhibition of the excision of the same elements in the zygotic macronucleus. These experiments show that the processing of many germ-line sequences in the developing macronucleus is sensitive to the structure and copy number of homologous sequences in the maternal macronucleus. The generality and sequence specificity of this trans-nuclear, epigenetic regulation of rearrangements suggest that it is mediated by pairing interactions between germ-line sequences and sequences imported from the maternal macronucleus.

Notes: We have studied eddy current heating and shielding for conducting materials in a cylindrical geometry, and we have performed numerical calculations as a function of the important parameters. Results are presented in graphical form. Eddy current heating in OFHC copper at low temperatures and means of suppressing it have been studied in a Bitter magnet in a field of 15 T. Results show that it is possible to use conducting materials for experiments in high magnetic fields at temperatures below 1 K.

Notes: Recently, Luo et al. reported indirect evidence for the formation of dimers in a supersonic expansion of helium. They interpreted their data as confirmation of the existence of a bound state of 4He–4He with a binding energy of order 1 mK. We show that their data are actually consistent with the formation of helium trimers and not dimers. The two control experiments that they performed do not provide any evidence for discarding this alternative hypothesis.

Notes: . Paramecium tetraurelia, like some other ciliate species, uses an alternative nuclear genetic code where UAA and UAG are translated as glutamine and UGA is the only stop codon. It has been postulated that the use of stop codons as sense codons is dependent on the presence of specific tRNAs and on modification of eukaryotic release factor one (eRFl), a factor involved in stop codon recognition during translation termination. We describe here the isolation and characterisation of two genes, eRF1-a and eRF1-b, coding for eRFl in P. tetraurelia. The two genes are very similar, both in genomic organization and in sequence, and might result from a recent duplication event. The two coding sequences are 1,314 nucleotides long, and encode two putative proteins of 437 amino acids with 98.5% identity. Interestingly, when compared with the eRFl sequences either of ciliates having the same variant genetic code, or of other eukaryotes, the eRFl of P. tetraurelia exhibits significant differences in the N-terminal region, which is thought to interact with stop codons. We discuss here the consequences of these changes in the light of recent models proposed to explain the mechanism of stop codon recognition in eukaryotes. Besides, analysis of the expression of the two genes by Northern blotting and primer extension reveals that these genes exhibit a differential expression during vegetative growth and autogamy.

Notes: In this contribution I propose to examine an aspect of Sri Lankan agrarian history which is often alluded to but rarely studied in depth: the process of high land appropriation for the development of coffee, tea, rubber and coconut plantations. The development of a land market in the Indian subcontinent is becoming a promising field of research for the study of imperial impact as a process at work in specific contexts.The Sri Lankan case differs from the Indian one in that land appropriation was originally meant for and followed by large scale land alienation to outsiders–the planters. This process has attracted the interest of most historians writing on the history of the Raj in Ceylon, but usually the only aspect stressed has been the appropriation by the Colonial State of forest and chena(land devoted to slash-and-burn cultivation) for sale to British planters.

Notes: [Auszug] The G surface antigen is one of the set of surface antigens expressed by P. primaurelia in a mutually exclusive manner6'7. It is a highly abundant protein covering the whole surface of the cell, and comprises a unique polypeptide chain of relative molecular mass 250,000-300,000 (refs 8, 9). Its ...

Notes: Summary It has recently been claimed (Ribi 1979, 1980) that “bee ommatidia show no twist” and that our earlier finding of rhabdom twist in bees (Wehner et al. 1975; forMyrmecia ants see Menzel and Blakers 1975) had been due toartefactual twisting of the rhabdoms. In this paper we present new histological data substantiating our previous hypothesis that bee rhabdoms twist in vivo. In addition, we demonstrate that Ribi's longitudinal section (Ribi 1979; Fig. 1) through part of a bee's rhabdom does not provide compelling evidence against the hypothesis of rhabdomeric twist. 1. Rhabdomeric twist is found in preparations in which special care was taken not to cut the eye (glutar-aldehyde fixation) or even the whole head (acrolein fixation) during the processes of preparation and fixation. Thus, changes in intraocular pressure that might be one cause of artefactual twist have been reduced to a minimum in both the whole-eye and the whole-head preparations. On the other hand, even when the retinal tissue is mechanically manipulated, e.g. by compressing the compound eye or by excising the retina, the rhabdoms twist as in the usual preparations. The direction of twist (clockwise or counter-clockwise) is always correlated with the geometry of the rhabdom (X-type rhabdoms or Y-type rhabdoms). The twist rate is about the same in the whole-eye preparations, the manipulated-eye preparations, and the isolated-retina preparations. Besides the absence of structural pecularities like disrupted membranes or enlarged intercellular clefts, there is some further support for the hypothesis that artefactual twist does not occur: Slender processes of the secondary pigment cells invade the intercellular clefts between adjacent photoreceptor cells but never show any signs of being sheared off or distorted. Furthermore, membrane junctions can regularly be found between secondary pigment cells belonging to different retinulae. 2. Ribi's (1979) electron micrograph presenting a relatively short longitudinal section through a bee's rhabdom does not allow for the conclusion that the rhabdoms of bees are straight. This is mainly a consequence of the fact that the section of the rhabdom shown in his figure was cut at about right-angles to its microvilli instead of parallel to them. It follows from simple geometrical considerations that the cut microvilli can appear rather circular even in the presence of substantial amounts of twist.

Notes: Summary Anatomically and physiologically specialized ommatidia at the dorsal rim of insect compound eyes play a key role in polarization vision. In this eye region the polarization sensitivity (PS) of photoreceptors is significantly higher than elsewhere in the eye. We have compared the optical properties of the dorsal rim area and normal eye region of desert ants,Cataglyphis bicolor, and field crickets,Gryllus campestris. The objective was to find the cause of the enhanced PS in the dorsal rim area of eyes where the situation is not complicated by rhabdom twist. Three pieces of information were derived: 1. From qualitative optical experiments we conclude that rhabdom waveguide effects do not enhance or reduce PS. Quantitative measurements of depolarization in the retina demonstrate unambiguously that all parts of the eye retain the state of polarization very well through the full retinal depth. The factor limiting inherent PS of receptor cells must thus be the dichroic absorption of the rhabdomeres, which is determined by the dichroic ratio of microvilli and the degree of microvillar alignment. 2. A theoretical model of light propagating in a dichroic rhabdom reveals a strong influence of random microvillar misalignment on PS. Using measured values of misalignment, we predict differences in PS between dorsal rim and unspecialized retina that explain previous electrophysiological results. 3. Theoretical modelling also demonstrates the advantage of having a predominant microvillar direction in a rhabdom (as is the case in theCataglyphis dorsal rim).

Notes: Summary 1) Comparable to the bee, but in contrast to the majority of ants, the desert antCataglyphis bicolor has been shown to exhibit a highly developed repertoire of visually guided behavioural responses. This paper deals with the anatomy and fine structure of the peripheral visual pathway of this ant. In the first visual neuropile, the lamina, first and second order neurons are classified by applying Golgi methods adapted for electron microscopy. Synaptic connections within the lamina are described and discussed. The results are summarised in developing a three-dimensional model of the ant's lamina (Fig. 17). 2) Eachretinula within the central eye region is composed of four large (nos. 2, 4, 6 and 8) and four small retinular cells (nos. 1, 3, 5 and 7) and a basal ninth cell. Visual cells nos. 2, 3, 4, 6, 7, and 8 form short unbranched axons (Rs), which terminate within the lamina. The visual cells nos. 1 and 5 (Rl), as well as the basal cell no. 9, show arborisations in the lamina, but terminate in the second visual neuropile, the medulla. Within the lamina all nine retinular cell axons, originating from one retinula, form a cartridge within which they interact with the second order neurons, the monopolar cells. Collaterals of second order neurons and side branches of retinular cell axons form local neuronal circuits. 3) Five types ofmonopolar cells have been classified by means of their dendritic fields within the lamina and the medulla (L1a, b, c, L2 and L4). They relay the retinular cells with higher order neurons within the medulla. In the distal layer of the lamina (stratum A) the spreads of the monopolar cells are restricted to a single cartridge, whereas in the proximal stratum C their collateral processes extend laterally through more than one cartridge. The collaterals of the L4-type of monopolar cells are exclusively confined to stratum C. There they are arranged bilaterally along the dorsoventral axis of the eye. Within stratum A, where all neurons are organised in well defined columns (cartridges), the axons of the short visual cells seem to be distributed over any cross section of a cartridge at random. In this layer, tangential fibres are the only candidates for inter-cartridge cross talk. In stratum C, the columnar organisation of the neuropile becomes less obvious because of the wide spread ramifications of the second order neurons. For instance, the collaterals of the L1a-type of monopolar cell extend over up to 18 neighbouring cartridges. 4) Three types ofcentrifugal fibres running from the medulla to the lamina are observed (T-fibres). Some of them form wide field arborisations either in stratum A (type T2) or in stratum C (type T3). In linear scale, their collaterals may extend over more than 40% of the large (dorsoventral) axis of the lamina. 5) Receptor terminals, especially Rs-fibres, are densely packed with elongated synaptic vesicles, whereas in second order neurons round vesicles are arranged around the presynaptic elements. Especially in Rs-fibres analyses of serial sections reveal T-shaped synaptic ribbons, which are the presynaptic sites as regards four postsynaptic elements. In case of rod-like presynaptic elements diadic and triadic arrangements of postsynaptic fibres can also be observed. Four main types of synaptic configurations are discriminated: (1) Receptor terminals synapse on second order neurons. (2) Second order neurons synapse on receptor cell axons as well as on other second order profiles. These synapses are sometimes observed in feedback configurations. (3) Synapses occuring between receptor axon terminals. (4) A small, probably efferent neurosecretory nerve fibre synapses on second order neurons. Neurosecretory fibres of larger diameters (to 1.5 Μm are frequently found in stratum C.

Notes: Summary Structurally specialized ommatidia at the dorsal rim of the compound eyes of honey bees have been shown to be indispensable for polarized skylight navigation. In this study numerous other hymenopteran genera belonging to various superfamilies are shown to exhibit similar specializations in this part of the eye: (1) The cornea is penetrated by pore canals, which affect the optics of the ommatidia by scattering the light falling into the eye. In Andrena and Ammophila the cornea contains extensive cavities. (2) Each retinula contains 9 long receptor cells as opposed to 8 long ones in the adjacent dorsal area, and the rhabdom area is increased by a factor of up to 2. In all ant species examined there are no corneal but only retinal specializations at the dorsal rim of the eye. They include a specially shaped rhabdom as in Cataglyphis, in which polarization vision has also been demonstrated.