The dipteran sperm tail: ultrastructural characteristics and phylogenetic considerations

The sperm tail from representatives of several families of Diptera has been examined by high resolution electron microscopy and a computer analysis that improved the visualization of recorded patterns. A considerable variability in sperm tail structure is found within Diptera, and is actually greater than that of any other insect order. The 'generalized insect sperm axoneme'. which is characterized as a 9+9+2 axoneme and by the accessory microtubules having 16 protofilaments, was found only in some dipterans; these are members of Mycetophilidae. From this fact we conclude that Mycetophilidae is likely to be the most primitive extant dipteran group. Another mycetophilid, Boletina , was seen to have accessory tubules with 15 protofilaments as have members of families Dixidae, Chironomidae, Culicidae, and Bibionidae. The last two families have spermatozoa of a type designated as 9+9+'1' there is a central rod rather than two microtubules. We regard this 9+9+'1'pattern with 15 protofilaments to represent a synapomorphic feature. Representatives of the neatoceran families Tipulidae and Trichoceridae have accessory tubules with 13 protofilaments as do examined members of several brachyceran families. Brachycera is hence likely to be derived from the vicinity of the tipulid family. The intertubular material is small in Mycetophilidae and most nematoceran groups, whereas in Tipulidae and Brachycera it is enlarged; here it bridges the space between the accessory tubules and contains various inclusions.     

APPLIED ISSUES Replacement of the native crayfish Astacus astacus by the introduced species Pacifastacus leniusculus in a Swedish lake: possible causes and mechanisms

1. The introduced North American crayfish Pacifastacits leniusculus is currently replacing the native crayfish Astacus astacus in a Swedish lake. Using field data from co-occurring populations on life-history traits and population size structure of the two species, this study evaluated possible mechanisms behind the replacement. 2. Pacifastacus showed a higher capacity for population increase; the species had a higher individual growth rate, reached sexual maturity at a smaller size and lower age, and had a higher per capita egg production than Astacus. 3. A higher frequency of non-lethal injuries in Astacus suggested the predominance of Pacifastacus in interference interactions. These results support a competitive exclusion hypothesis. 4. A dramatic change in relative abundance of the two species in recent years was the result of an almost complete cessation in recruitment of young-of-the-year (YOY) in Astacus. Poor recruitment in Astacus was probably caused by the combined effects of interspecific competition and predation, resulting in increased mortality among YOY Astacus, and by reproductive interference, suppressing the less common species. 5. The results support the hypothesis that the observed replacement of Astacus by Pacifastacus is governed by a combination of several interacting mechanisms, of which     

Translocation of 32P between interacting mycelia of a wood-decomposing fungus and ectomycorrhizal fungi in microcosm systems

Interactions between saprotrophic and ectomycorrhizal fungi have been largely ignored, although their mycelia often share the same microsites. The mycelial systems show general similarity to each other and, although the enzymatic potential of the saprotrophic fungi is generally considered to be higher, the importance of organic nutrient sources to ectomycorrhizal fungi is now widely accepted. In the experiments described here, nutritional interactions involving transfer of elements from one mycelium to the other have been monitored dynamically using radioactive tracers and a non-destructive electronic autoradiography system. Microcosms were used in which mycelial systems of the ectomycorrhizal fungi Suillus variegatus and Paxillus involutus , extending from Pinus sylvestris host plants, were confronted with mycelia of the saprotroph Hypholoma fasciculare extending from wood blocks. The fungi showed a clear morphological confrontation response. The mycorrhizal mycelium often formed dense patches over the Hypholoma mycelia. Up to 25% of the ³²P present in the Hypholoma mycelium was captured by the mycorrhizal fungi and translocated to the plant host within 30 d. The transfer of ³²P to the saprotroph from labelled mycorrhizal mycelium was one to two orders of magnitude lower. The significance of this transfer as a 'short cut' in nutrient cycling is discussed.     

Demographic and genetic effects on reproduction as related to population size in a rare, perennial herb, Senecio integrifolius (Asteraceae)

Seed-set of the rare and threatened plant Senecio integrifolius increased significantly with population size. Experimental studies as well as field observations showed this to be due to density-dependent seed-set (Allee effect). Hand-pollination revealed lower seed-set, and a lower germination rate of inbred progeny than of outbred progeny, with great differences among populations. Contrary to general predictions in models of minimum viable population sizes, the present study indicates little negative effects of inbreeding in small populations. A genetic load model was invoked to explain the results, hypothesizing that purging of deleterious alleles in small populations has reduced inbreeding depression. However, no clear correlation between population size and genetic load was found. The results in this paper suggest that demographic and environmental factors are of greater immediate importance than population genetics in determining extinction probabilities of small plant populations.     

Ceratophyllum demersum hampers phytoplankton development in some small Norwegian lakes over a wide range of phosphorus concentrations and geographical latitude

1. Data on submerged and floating-leafed macrophytes, phytoplankton, nutrients (N, P) and calcium were collected from twenty-four small lakes ( 1 km²) over a wide range of latitudes in Norway. The majority of the investigated lakes were mesotrophic or eutrophic, and most of the lakes were markedly affected by diffuse and point-source runoff from agriculture. According to their macrophyte species composition, the majority of the lakes can be classified as Potamogeton lakes or Chara lakes, or a combination of these.
2. This study is consistent with the 'two alternative stable states' hypothesis. We observed clearwater lakes with dense macrophyte cover over a wider range of total P concentration than has been reported previously: from 30 to more than 700 mg P m^–3. The clearwater state was only observed in lakes with mean depths of less than 1.9 m.
3. Most clear lakes with high cover of submerged vegetation showed indications of N limitation.
4. In this study nearly all the macrophyte-dominated lakes with P concentrations above 30 mg m^–3 had dense stands of Ceratophyllum demersum (L.). This indicates that Ceratophyllum may also play an important role in stabilizing and maintaining a clearwater state at high P concentrations.     

Co-expression tools for plant biology: opportunities for hypothesis generation and caveats

Gene co‐expression analysis has emerged in the past 5 years as a powerful tool for gene function prediction. In essence, co‐expression analysis asks the question ‘what are the genes that are co‐expressed, that is, those that show similar expression profiles across many experiments, with my gene of interest?’. Genes that are highly co‐expressed may be involved in the biological process or processes of the query gene. This review describes the tools that are available for performing such analyses, how each of these perform, and also discusses statistical issues including how normalization of gene expression data can influence co‐expression results, calculation of co‐expression scores and P values, and the influence of data sets used for co‐expression analysis. Finally, examples from the literature will be presented, wherein co‐expression has been used to corroborate and discover various aspects of plant biology.     

Simple Methods for the Calibration of Light Measuring Equipment

Simple methods are described for

• a) absolute calibration of thermal radiation detectors (thermopiles, bolometers);
• b) absolute calibration at various wavelengths of photoelectric devices;
• c) obtaining the relative spectral sensitivity of the detector systems of spectrofluorimeters.

The procedures require no special standard lamp or other exotic equipment. The methods depend on the computation of the radiation characteristics of an ordinary tungsten lamp at various measured voltages and currents.     

Lost in translation – a history of systematic confusion and comments on the type species of Squalodon and Patriocetus (Cetacea,Odontoceti)

Abstract: The two odontocete taxa Squalodon grateloupii and Patriocetus ehrlichii, both the type species of their respective genera, have been at the centre of a great deal of taxonomic confusion. Originally regarded to be conspecific, these two taxa have been the subject of a bewildering taxonomic debate lasting for more than a century, which recently led to the suggestion to abandon these widely used names and replace S. grateloupii with the similar, yet independently and later proposed name S. gratelupi as the type species of Squalodon. Here, we attempt to summarise the events leading to the current confused situation in the hope of resolving this issue once and for all and argue that the name Squalodon grateloupii, as originally proposed, should be reinstated.     

The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations

The epidermis of Xenoturbella bocki Westblad was studied by scanning and transmission electron microscopy. Two cell types predominate in the epidermis: multiciliated epidermal cells and non-ciliated or monociliated gland cells. A conspicuous feature is the dense ciliary coverage and the numerous gland cell openings. Xenoturbella has a characteristic pattern of axonemal filament termination in the distal tips of their cilia. Each epidermal cilium has the typical 9 + 2 patten through the major part of its shaft. Near the tip there is a shelf at which doublets 4–7 terminate. Doublets 1, 2, 3, 8 and 9 continue into the thinner distal part of the cilium. A similar shelf in cilia is known only from the turbellarian orders Nemertodermatida and Acoela, and hence may be an apomorphic feature which indicates a close relationship between Xenoturbellida, Nemertoder-matida and Acoela. The basal body is provided with a so-called basal foot which has a cross-striated appearance and an expanded distal plate that seems to act as a microtubule organizing center. Approximately 15–25 microtubuli radiate from the endplate of the basal foot to the basal bodies caudally. The arrangement of basal foot and ciliary rootlets in Xenoturbella differs from that of Acoela and related orders in that there are two striated rootlets only (an anterior and a posterior one), rather than one main rootlet and two lateral rootlets.