The Size of Macronuclear DNA and Its Relationship to Models for Maintaining Genic Balance*†

Synopsis.
A considerable amount of evidence is now available to indicate that the DNA in the ciliates Oxytricha and Stylonychia undergoes fragmentation when the micronucleus forms a macronucleus. Some evidence suggests that fragmentation may also occur in Tetrahymena and possibly in Paramecium . It is shown that some regulatory or nonrandom segregational mechanism must operate during cell divisions to maintain genic balance in Tetrahymena . Both the hypothesis of macronuclear subunits and also a new hypothesis based on replicative control of DNA are capable of explaining the currently know biochemical, cytological, and genetic facts.     

Effects of experience on parasitoid movement in odour plumes

Abstract. Insects commonly improve the effectiveness with which they locate biotic resources through learning, but the mechanism by which experience exerts its effects has rarely been studied in detail. The effect of oviposition experience on upwind movement of the eucoilid parasitoid, Leptopilina heterotoma (Thomson) (Hym.: Eucoilidae), in odour plumes of host microhabitats, was quantified with the use of a Kramer-type locomotion compensator. A 2h exposure to host Drosophila melanogaster larvae in either fermenting apple-yeast or decaying mushroom substrate (known to affect their preference for these odours in glasshouse and field choice experiments) had a number of effects on movement in plumes of each substrate. Females experienced with a particular substrate walked faster and straighter, made narrower turns and spent more time in upwind movement (i.e. toward the source) in a plume of odour from that substrate than in odour from an alternative substrate. Inexperienced females, by contrast, generally showed little or no significant difference in responses to alternative odours. In addition to affecting the mean values of movement parameters, experience also affected variability around those means. When walking speed or path straightness in an odour plume was increased by experience, variability among individuals was correspondingly decreased. The consequences of odour learning for microhabitat choice is discussed briefly.     

Behaviour of male and female parasitoids in the field: influence of patch size,host density,and habitat complexity

1. Two field experiments were carried out to examine the role of patch size, host density, and complexity of the surrounding habitat, on the foraging behaviour of the parasitoid wasp Cotesia glomerata in the field. 2. First, released parasitoids were recaptured on patches of one or four Brassica nigra plants, each containing 10 hosts that were placed in a mown grassland area. Recaptures of females were higher than males, and males and females aggregated at patches with four plants. 3. In experiment 2, plants containing 0, 5 or 10 hosts were placed in unmown grassland plots that differed in plant species composition, on bare soil, and on mown grassland. Very low numbers of parasitoids were recaptured in the vegetated plots, while high numbers of parasitoids were recaptured on plants placed on bare soil or in mown grassland. Recaptures were higher on plants on bare soil than on mown grassland, and highest on plants containing 10 hosts. The host density effect was significantly more apparent in mown grassland than on bare soil. 4. Cotesia glomerata responds in an aggregative way to host density in the field. However, host location success is determined mostly by habitat characteristics, and stronger host or host‐plant cues are required when habitat complexity increases.     

LATE HETTANGIAN (EARLY JURASSIC) AMMONITES FROM TASEKO LAKES, BRITISH COLUMBIA, CANADA

Abstract: The Taseko Lakes area yields the most diverse and well‐preserved late Hettangian ammonite assemblage in British Columbia. In total, 27 ammonite taxa are described, of which Fergusonites hendersonae, Eolytoceras constrictum and Pseudaetomoceras victoriense are new. Based on this study, Phylloceras cf. asperaense Hillebrandt, Togaticeras togatum (Neumayr), T. aff. goisernense Rakús, Eolytoceras aff. guexi Taylor, Discamphiceras aff. silberlingi Guex, Schlotheimia cf. cuevitensis Hillebrandt and Franziceras aff. graylockense Taylor are newly recognized in the North American Mineralense Zone whereas Nevadaphyllites psilomorphus (Neumayr), N. aff. compressus Guex, Fergusonites hendersonae sp. nov., Eolytoceras chongi Hillebrandt, E. constrictum sp. nov., Paracaloceras spp. A and B, P.? spp., Metophioceras? sp. and Schreinbachites? aff. laqueoides (Hyatt) are newly recognized in the Rursicostatum Zone. Discamphiceras silberlingi, D. sp. nov., Alsatites aff. bipartitus (Hillebrandt), Alsatites sp., Pseudaetomoceras victoriense sp. nov. and Paracaloceras aff. varaense Hillebrandt are newly recognized in both the Mineralense and Rursicostatum zones. The Taseko Lakes fauna allows clear correlations with contemporaneous strata in other areas of North America as well as with the Canadensis Zone and probably the Arcuatum horizon of the South American succession. Broad correlations are also possible with Hettangian or lowest Sinemurian strata in New Zealand and the eastern Tethys. Furthermore, the Taseko Lakes fauna permits correlation of the Mineralense and Rursicostatum zones with the circum‐Mediterranean Marmoreum Zone. Finally, this fauna suggests that the Mineralense Zone and the lower to middle portion of the Rursicostatum Zone in Taseko Lakes are equivalent to the Complanata Subzone whereas the upper portion of the Rursicostatum Zone correlates with the Depressa Subzone of the north‐west European Angulata Zone.     

Fitness of two sibling species of Asobara (Braconidae:Alysiinae), larval parasitoids of Drosophilidae in different microhabitats

Abstract. 1. Two sibling species of larval endoparasitoids of Drosophilidae: Asobara tabida (Nees) and A.rufescens (Foerster) occur in the same macrohabitat, but inhabit different microhabitats. Each species is most attracted by odours of its own microhabitat.
2. In order to assess the adaptive value of the microhabitat preference we studied the survival of both parasitoids in the major host species that occur in these microhabitats.
3. Survival in the major host in the preferred microhabitat was shown to be 40% higher for A.tabida and 30% higher for A.rufescens when compared to survival in the major host in the non-preferred microhabitat.
4. Measurements of developmental rates, specific mortalities and dry weights of the parasitoids suggest that the differential survival is due to differences in synchronization with the hosts.
5. The possible evolutionary consequences of some biological characteristics in Asobara are discussed. Microhabitat selection, differential survival, development and mating behaviour are attributes likely to have played a role in the reduction of gene flow between populations of the ancestral species, either in primary or in secondary sympatry.     

The Sensitivity of Light Modulation of Enzyme Activity to Arsenite and Sulphite and of Photosynthetic Induction toArsenite is Determined by a Cytoplasmic Gene

Muschinek, G., Alscher, R. and Anderson, L. E. 1987. The sensitivityof light modulation of enzyme activity to arsenite and sulphiteand of photosynthetic induction to arsenite is determined bya cytoplasmic gene—J. exp. Bot. 38: 1069–1075. The membrane component of the light modulation system was moresensitive to arsenite and to sulphite in the Pisum cultivar‘Nugget’ than in the cultivar ‘Progress No.9’. Likewise, the induction phase of CO₂ fixation wasmore arsenite sensitive in chloroplasts isolated from ‘Nugget’plants. Sensitivity was controlled by a cytoplasmic gene. Key words: Induction, light modulation, arsenite sensitivity     

An architectural analysis of Archaeopteris9 a fossil tree with pseudomonopodial and opportunistic adventitious growth

Currently recognized hierarchies of lateral organs in Archaeopteris are presented and phenomena that mediate organography and organotaxis in the taxon are discussed. It is demonstrated that Archaeopteris is totally lacking extant architectural analogues, and the methodology employed in the architectural analysis of such fossil plants is introduced. Proposed growth categories or phases for the Archaeopteris plant are presented and tested against evidence from a large silicified log of Callixylon, the trunk of Archaeopteris. An architectural model is put forward that encompasses such growth phaseS. Finally, the occurrence of reiterative, opportunistic growth in Archaeopteris is discussed.     

Micronuclear DNA from Paramecium tetraurelia: Serotype 51 A Gene Has Internally Eliminated Sequences

A method for the isolation of micronuclear DNA from Paramecium tetraurelia has been developed. After cell lysis, a low speed centrifugation at 1,000 g is used to remove all of the unbroken cells and macronuclei and approximately two thirds of the macronuclear fragments. Next a higher speed centrifugation of 9,000 g sediments the micronuclei and frees them from small particulates and soluble constituents. Advantage is then taken of the fact that micronuclei have a lower density than do macronuclear fragments in 45%–60% Percoll. Micronuclei float to the top during centrifugation at 24,000 g , while macronuclear fragments sediment. After several cycles of centrifugation in Percoll, the micronuclei, although heavily contaminated with cytoplasmic components, are essentially free of macronuclei and macronuclear fragments. Micronuclear DNA can then be extracted from the suspension. The whole procedure is very rapid and in about an hour micronuclear and macronuclear DNA can be separated. About 2 μ g of micronuclear DNA can be obtained from 6 times 10⁷ paramecia. We find that there are internal sequences in the micronuclear A gene DNA in wild type cells which are eliminated when the micronuclei develop into macronuclei. They yield unique restriction fragments for micronuclei and macronuclei. Therefore the purity of the preparations is easily monitored by probing Southern blots of restriction enzyme-digested DNA with the cloned A gene. No differences have been found between the micronuclear A gene in wild type and the d48 mutant.     

Results of Intracecal Inoculation of Germfree and Conventional Guinea Pigs and Germfree Rats with Axenically Cultivated Entamoeba histolytica

SYNOPSIS. Germfree and conventional guinea pigs and germfree rats were inoculated with large numbers of axenically cultivated Entamoeba histolytica. Amebic lesions were not found in any of the animals, and there was no indication that the ameba had established lumen infections in any instance. The axenic amebae appeared to have lost their pathogenicity. This loss was believed to depend, in part at least, upon the fact that they did not encyst and thus did not complete their life cycle in axenic culture.