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This collection of articles provides an account of six presentations delivered at the 19th International Conference of the World Association for the Advancement of Veterinary Parasitology(WAAVP) (held in New Orleans, Louisiana, USA, from 10 to 14 August 2003) in a symposium session on Molecular Systematics and Diagnosis, organised and chaired by R.B. Gasser and D.S. Zarlenga. The focus was on recent advances in molecular tools for specific and genotypic identification,diagnosis, systematics and population genetics, with special emphasis on investigations of parasitic nematodes and protists.

Relationships between the strongyloid nematodesRugopharynx delta, R. zeta, R. omega, R. longibursaris, R. mawsonae andR. sigma, all from macropodid marsupials, were investigated using allozyme data. The phylogenetic trees derived from the electrophoretic data set were congruent with those of the hosts and were consistent with the hypothesis that the species complex originated in pademelons of the genusThylogale and diversified in rock-wallabies (Petrogale spp.) and scrub wallabies of the subgenusNotamacropus. Host switching is evident only between closely related macropodid taxa.

The design and construction of a computer-controlled beam transport system and its application to slow positron implantation spectroscopy is described. The control features include the facility to vary the positron beam energy, observe visually the associated spatial movement of the beam profile, and change the beam position so as to hit the sample centrally at all energies selected. This procedure allows the automatic collection of annihilation gamma ray energy spectra and calculation of lineshape parameters as a function of incident positron energy. Examples of such measurements for metallic and semiconductor samples are used to illustrate the performance of the system.

An electrophoretic study was conducted on nematodes of the genus Physaloptera that occur in the stomachs of two species of Australian bandicoot, Perameles nasuta and P. gunnii. Each nematode was genetically characterised at 28 enzymes encoding a presumptive 30 loci. No fixed genetic differences were detected between the nematodes in P. gunnii from two localities. A comparison of nematodes from the two host species, however, revealed fixed genetic differences at 15 (50%) loci. This suggests that each host species is infected by a different species of Physaloptera.

Rugopharynx zeta (Johnston & Mawson) (Nematoda: Strongyloidea) is redescribed from the rock wallabies Petrogale penicillata penicillata, P. p. herberti, P. inornata and P. assimilis from Queensland and New South Wales, Australia. Specimens formerly assigned to this nematode taxon from the wallabies Macropus dorsalis and M. parma are treated as a new species, R. mawsonae. R. mawsonae n. sp. differs from R. zeta in the shape of the dorsal ray, length of spicules, morphology of spicule tip, length of female tail and position of deirid. The morphological differences are supported by electrophoretic data. R. zeta and R. mawsonae n. sp. had fixed genetic differences at 45.0% of the 21 enzyme loci examined, while each differed at 38.1% and 45.0% of loci respectively from the morphologically distinct R. delta (Johnston & Mawson). The known host and geographical distributions of R. zeta and R. mawsonae n. sp. are reviewed.

Amorphous SiO2 films grown by both thermal oxidation and plasma chemical vapor deposition (CVD) in a tetraethylorthosilicate and O2 atmosphere were studied using variable‐energy positron annihilation spectroscopy and infrared (IR) spectroscopy. For thermally grown SiO2 layers with growth temperatures of over 1000 °C it was found that the measured Doppler broadening parameter was identical for both wet and dry growth atmospheres. The measured Doppler broadening parameter in the case of SiO2 prepared by plasma CVD was found to be strongly influenced by the substrate temperature during deposition. For a substrate growth temperature of 600 °C, the data are essentially identical to those of the thermally grown oxide layers. IR analysis revealed that the concentration of Si–OH in the SiO2 layer is affected by the substrate temperature during growth. The level of the Doppler broadening parameter in the SiO2 film exhibited changes that can be correlated with this Si–OH concentration. We thus show that the concent...

An electrophoretic comparison of the nematodes Rugopharynx longibursaris and R. omega, both from Macropus rufogriseus in south-eastern Australia, revealed fixed genetic differences at 4.5% of the 23 enzyme loci examined. The electrophoretic data do not therefore reject the null hypothesis that the two taxa are conspecific. R. longibursaris was found in Tasmania and in the western mainland population of M. rufogriseus, while R. omega occurred only in the eastern mainland population. Implications for the taxonomic status of the western host population are considered. Specimens formerly assigned to R. omega, from Thylogale stigmatica from Queensland, were found to differ at 45.0% of enzyme loci from specimens from M. rufogriseus. Morphological examination revealed differences in the shape of the buccal capsule, the position of the deirid, the morphology of the spicule tip and the presence of a gubernaculum. A new species, R. sigma, is erected for specimens from T. stigmatica, T. thetis and T. calabyi.

The measurement of implantation profiles and median implantation depths for keV positrons in solids is described. Details of the experimental technique, which is based on the difference between positron annihilation lineshape parameters in two materials that otherwise appear similar to the incident positrons, are presented, using the examples of aluminium-glass and gold-tungsten. Criteria for the suitability of the two elements of the binary sample are discussed, as are limiting factors such as post-implantation diffusion and trapping at interfaces.

The subsurface vacancy profile in Czochralski (Cz) Si(100) implanted to a dose of 2×1014 cm−2 with 120 keV fluorine ions has been determined using a slow positron beam. The unique sensitivity of the positron technique to vacancy concentrations of part per million (ppm) order is utilized. We demonstrate the presence of vacancies at ppm concentrations at depths up to 1 μm greater than the depth predicted by Monte Carlo modeling. The vacancies caused by the implantation process are suggested to consist of an immobile and a mobile component, the latter component being able to diffuse to such depths.

An allozyme study of four populations of Paramacropostrongylus typicus from Macropus fuliginosus and M. giganteus detected the presence of a distinct species, Paramacropostrongylus iugalis n. sp., which is described from the stomach of the eastern grey kangaroo Macropus giganteus. The description is based on samples possessing fixed allelic differences at 10 of 37 enzyme loci examined, minor morphological differences from its congener P. typicus in M. fuliginosus, and presumed limitation to a single host species, M. giganteus. Its differentiation from P. typicus is more clearly demonstrated by biochemical characters than by morphological ones. The two parasite species probably co-speciated with their hosts.

New measurements and Monte Carlo simulations for 1-30 keV positron implantation in Al are reported. The annihilation lineshape was measured as a function of positron energy E and the thickness of Al overlayers on a glass substrate. Median implantation depths vary as 117(6) E1.64(2) AA, with E in keV. The implantation profiles differ significantly from the widely accepted Gaussian derivative form. The excellent agreement between experiment and simulation confirms the power of Monte Carlo method in positron (and by implication electron) spectroscopies.

The Doppler broadening of radiation from the annihilation of positrons in p+-silicon epilayers 1 and 0.4 mu m thick epitaxially grown at different temperatures has been measured. The signal from positrons decaying within the epilayers is enhanced at the expense of any significant contribution from surface annihilation, enabling direct measurement of the Doppler lineshape parameter S characteristic of the epilayers. The epilayer S-value is observed to decrease for samples grown at temperatures below 700 degrees C, for which the expected concentration of oxygen defects increases. The correlation between S and oxygen concentration is interpreted in terms of (a) transition-limited trapping by atomic oxygen defects and (b) diffusion-limited trapping by SiOx precipitates. The former is consistent with secondary-ion mass spectrometry measurements if the specific trapping rate is reduced to at least half of the value quoted by earlier researchers; the latter implies that the results are consistent with the formation of SiO1.75 precipitates of mean radius 3.2+or-0.7 nm.

Using variable‐energy positron annihilation spectroscopy, we have investigated the defects induced by ArF excimer laser (6.4 eV) irradiation of SiO2 films (500 nm thick) on Si(100). Films of both thermally grown oxide, and plasma enhanced chemical vapor deposited oxide were studied. It is proposed that E’ centers (≡Si⋅), nonbridging oxygen hole centers (NBOHC, ≡Si‐O⋅) and their charge transfer states may trap positrons and affect the positronium annihilation modes. For the thermally grown oxide, the content of E’ centers, and hence original structural defects such as oxygen vacancies (≡Si‐Si≡) at the SiO2/Si interface, is much higher than that in the SiO2 film itself. For SiO2 films deposited on Si by plasma‐enhanced chemical vapor deposition, the impurity of ≡Si‐OH (1.2 wt %) throughout the oxide layer acts as a precursor to the radiation‐induced defects. The NBOHC formed strongly trap positrons and decrease the positronium formation fraction. The annealing behavior of the radiation‐induced defects in bo...

Positron behavior in thin native oxide layers grown on an initially hydrogen‐terminated Si(100) surface was investigated and correlated with the chemical structure of the layers determined using Fourier‐transform infrared absorption attenuated total reflection spectroscopy, and x‐ray photoelectron spectroscopy. Hydrogen termination of the Si surface by 4 vol % HF treatment gave rise to a narrower Doppler‐broadened positron‐electron annihilation line than that of bulk Si. By a process of oxidation in pure water very thin (up to 7.8 A) layers were grown on the H‐terminated Si. The Doppler broadening of annihilations from the Si surface was seen to increase monotonically with thickness away from the value seen for amorphous bulk SiO2. The positronium fraction was monitored throughout but was found to be independent of the oxidation duration. It was found that the chemical structure of the surface, in particular the oxidized Si‐OH bond, was correlated with the positron annihilation mode and to the level of ob...

Boron ion implantation‐induced defects in SiO2 were investigated using slow positron annihilation spectroscopy and electron spin resonance (ESR). The defects caused by ion implantation are manifest as a particularly low S parameter in the region of the SiO2 layer in which B implantation damage occurs. The annealing behavior of the defect responsible for positron trapping was studied. The defect to which the positron is sensitive is found to be unobservable in ESR measurements. The defect is suggested to be dissolved O2 or a charged Frenkel defect, such as the negative nonbridging‐oxygen hole center (≡Si—O−).

Species-specific identification of ascaridoid nematodes at any developmental stage is a prerequisite for detailed investigation of the life cycles, systematics and epidemiology of this important group, and is also crucial for the diagnosis of associated infections. The morphological identification of some species and/or their larval stages can, however, present considerable difficulty. Recently, PCR-based methods, using genetic markers in the internal transcribed spacers (ITS) of ribosomal DNA, have been shown to provide reliable alternatives to more traditional methods for the specific identification of nematodes. This article provides an account of recent research on the development of PCR-based methods (utilizing ITS sequences) for the specific identification of ascaridoid nematodes of zoonotic potential, for the diagnosis of infections, and for the analysis of genetic variation within and among individual nematodes and their populations. Prospects for using these diagnostic and analytical tools to investigate epidemiological and population genetic questions relating to ascaridoid parasites are also discussed.

Single-strand conformation polymorphism (SSCP) analysis was employed to screen for sequence heterogeneity in the second internal transcribed spacer (ITS-2) of ribosomal (r) DNA of Labiostrongylus longispicularis, a parasitic strongylid nematode occuring in some species of kangaroo in different geographical regions of Australia. The results showed that most of the nematodes screened had different SSCP profiles, which were subsequently shown to correspond to polymorphisms and/or an indel in the ITS-2 sequence. These variable sites related mainly to unpaired regions of the predicted secondary structure of the precursor rRNA molecule. SSCP profiles could be used to distinguish L. longispicularis in Macropus robustus robustus (New South Wales) from L. longispicularis in Macropus robustus erubescens and Macropus rufus (South Australia). This difference corresponded to a transversional change in the ITS-2 sequence at alignment position 82. The study demonstrated clearly the effectiveness of SSCP analysis for future large-scale population genetic studies of L. longispicularis in order to test the hypothesis that L. longispicularis from different geographical regions represents multiple sibling species.

In order to maximise the positional homology in the primary sequence alignment of the second internal transcribed spacer for 30 species of equine strongyloid nematodes, the secondary structures of the precursor ribosomal RNA were predicted using an approach combining an energy minimisation method and comparative sequence analysis. The results indicated that a common secondary structure model of the second internal transcribed spacer of these nematodes was maintained despite significant interspecific differences (2-56%) in primary sequences. The secondary structure model was then used to refine the primary second internal transcribed spacer sequence alignment. The 'manual' and 'structure' alignments were both subjected to phylogenetic analysis to compare the effect of using different sequence alignments on phylogenetic inference. The topologies of the phylogenetic trees inferred from the manual second internal transcribed spacer alignment were usually different to those derived from the structure second internal transcribed spacer alignment. The results suggested that the positional homology in the second internal transcribed spacer primary sequence alignment was maximised when the secondary structure model was taken into consideration.

Metastrongylus species are important parasites of free-range pigs and wild boar, but little is known about the genetic make-up of natural populations. This study was undertaken to examine sequence variation in the internal transcribed spacer 2 of ribosomal DNA within and among three species of Metastrongylus using PCR-linked restriction fragment length polymorphism analysis. In contrast to many other species of bursate nematodes, significant intraspecific variation was detected in restriction fragment length polymorphism profiles among individual worms. In spite of this, it was possible to identify the three species by their distinctive restriction profiles. The findings suggest that the internal transcribed spacer 2 region should be useful for analysing population variation within Metastrongylus species.

The two ribosomal DNA internal transcribed spacers (1 and 2) of the hookworms Ancylostoma caninum, A. tubaeforme, A. ceylanicum and A. duodenale were sequenced. The sequence lengths were similar among the four species, except that A. ceylanicum had slightly longer (by 5-7 bp) internal transcribed spacer 1 and 2 sequences. The predicted secondary structure of the internal transcribed spacer 2 precursor rRNA was similar for all species, despite interspecific differences in primary sequence ranging from 0.9% to 13.2%. Interspecific differences in internal transcribed spacer 1 sequence ranged from 0.9% to 7.5%. A cladistic analysis of the sequence data, using the human hookworm Necator americanus as the outgroup, provided little resolution of the phylogenetic relationships, except that A. ceylanicum occurred on a branch external to the other three species. Nonetheless, internal transcribed spacers 1 and 2 may provide useful phylogenetic information at higher taxonomic levels within the superfamily Ancylostomatoidea.

In this study, molecular data sets were used to address the controversies relating to the systematics of strongyloid nematodes of equids utilising morphological data sets. DNA sequences of the first and second internal transcribed spacers (ITS-1 and ITS-2) of ribosomal DNA were determined for 30 species of equine strongyles and the systematic relationships reconstructed using phenetic and phylogenetic tree-building methods. The molecular data provided support for the hypothesis that the genera with large subglobular buccal capsules are ancestral to those with small cylindrical buccal capsules, but did not provide support for the current division of the subfamilies Strongylinae and Cyathostominae or for some taxonomic groupings (i.e. generic designations of species) within the Cyathostominea based on morphological data. Although not entirely concordant, the current molecular data provide a systematic framework for future studies of equine strongyles, which could be exploited in combination with new, phylogenetically informative morphological data sets.

New experiments and Monte Carlo simulations of positron implantation in gold are presented which, when compared with earlier work on aluminum, clearly demonstrate that the material dependence of positron implantation profiles is not adequately described by the simple mass density scaling factor in the widely used expression for median implantation depths. There is excellent agreement between the experimental results and the simulations which use the Penn dielectric formalism to describe inelastic scattering.

We report on new observations of positron stopping in a series of Al foils mounted on glass backplates for incident energies E in the range 10–50 keV. The measured median penetration depths for E≳30 keV are found to differ significantly from values derived from the empirical power law model of A. P. Mills, Jr. and R. J. Wilson [Phys. Rev. A 26, 490 (1982)]. The associated disparity between the measured implantation profiles and their analytic representations, and the consequent implications for defect profiling analysis, are discussed.

The sequences of the nuclear ribosomal DNA region spanning the first internal transcribed spacer, the 5.8S rRNA gene and the second internal transcribed spacer were determined for Ascaris samples from pigs and humans from different geographical regions. The sequences of the 5.8S gene and the second internal transcribed spacer were the same for all samples examined, whereas all Ascaris samples from humans had six (1.3%) nucleotide differences in the first internal transcribed spacer compared with those from pigs. These differences provided some support for the existence of separate species of Ascaris or population variation within this genus. Using a nucleotide difference within a site for the restriction enzyme HaeIII, a PCR-linked restriction fragment length polymorphism method was established which allowed the delineation of the Ascaris samples from pigs and humans used herein. Exploiting the sequence differences in the first internal transcribed spacer, a PCR-based single-strand conformation polymorphism method was established for future analysis of the genetic structure of pig and human Ascaris populations in sympatric and allopatric zones.

The nucleotide sequences of the first and second internal transcribed spacers of nuclear ribosomal DNA were determined for adults of Cylicostephanus minutus from different geographical origins. The lengths of first and second internal transcribed spacer sequences ranged from 370 to 372bp and 215 to 216bp, respectively. Pairwise sequence comparisons revealed that some individuals of C. minutus had identical first and second internal transcribed spacer sequences, whereas others differed by 3.0% and 7.4% in their first and second transcribed spacers, respectively. Some individuals with sequence differences originated from the same host. The levels of difference within C. minutus were higher than that between the morphologically distinct species, Cylicostephanus goldi and Cylicostephanus longibursatus (0.8% for the first internal transcribed spacer and 3.8% for the second internal transcribed spacer). The data provide support for the proposal that C. minutus represents a complex of at least two species. In order to study the population genetic structure of C. minutus, a PCR-linked single-strand conformation polymorphism technique was also established.

Larvae of three species of anisakid nematode from fish, Anisakis simplex, Hysterothylacium aduncum and Contracaecum osculatum, were characterised genetically using a molecular approach. The nuclear ribosomal DNA region spanning the first internal transcribed spacer, the 5.8S gene and the second internal transcribed spacer was amplified and sequenced. The lengths of the first and second internal transcribed spacer sequences of the three species ranged from 392 to 449 bp and 262 to 347 bp, respectively, whereas the 5.8S sequence was 157 bp. For the three species, the G+C contents for the three regions of ribosomal DNA ranged from 42.4 to 52.2%. While no intraspecific variation was detected in the second internal transcribed spacer or 5.8S sequence of any species examined, one polymorphic nucleotide position was detected in the first internal transcribed spacer sequence for A. simplex and H. aduncum. The extent of sequence differences in the first (approximately 34-45%) and second (approximately 50-53%) internal transcribed spacers among the species was greater than in the 5.8S gene (approximately 3-5%). Based on the sequence differences, PCR-based restriction fragment length polymorphism and single-strand conformation polymorphism methods were established for the unequivocal delineation of the three species. These methods should provide valuable tools for studying the life-cycle, transmission pattern(s) and population structure of each of the three anisakid nematodes examined herein, and for the diagnosis of anisakiasis in humans and animals.

The present study characterised seven species of the Chabertiidae (Nematoda: Strongyloidea) belonging to either the subfamily Oesophagostominae (Oesophagostomum radiatum, Oesophagostomum venulosum, Oesophagostomum dentatum, Oesophagostomum quadrispinulatum, Oesophagostomum columbianum, Oesophagostomum bifurcum) or to the subfamily Chabertiinae (Chabertia ovina) by their second internal transcribed spacer rDNA sequence, assessed the extent of intraspecific variation and interspecific differences in the sequence, and inferred the phylogenetic relationship of C. ovina with respect to members of the Oesophagostominae. In both the phenetic and cladistic analyses of the sequence data, Chabertia was nested within Oesophagostomum, suggesting either that the species examined represent members of the same genus, or alternatively, that Oesophagostomum may represent more than one genus.

Sequences of the second internal transcribed spacer ribosomal DNA for the parasitic trichostrongylid nematodes Trichostrongylus probolurus, Trichostrongylus rugatus and Camelostrongylus mentulatus were compared with previously published sequences for five other species within the genus Trichostrongylus. The secondary structures of the second internal transcribed spacer pre-rRNA for these nematodes were predicted using an energy minimisation method. The results indicate that a common secondary structure of the second internal transcribed spacer of these nematodes is maintained despite distinct differences in primary sequence between species. Sequence differences among Trichostrongylus species ranged from 1.3 to 7.6%, but each species differed by 22-26% in sequence when compared with C. mentulatus which belongs to a different subfamily.

The first internal transcribed spacer (ITS-1) of the ribosomal DNA of seven species of Trichostrongylus was sequenced. The length of ITS-1 in the different species varied from 387 to 390 bases. The G + C content of the ITS-1 sequences were approximately 42%. Little or no intraspecific variation was detected in the three species. Trichostrongylus axei, Trichostrongylus colubriformis and Trichostrongylus vitrinus, for which multiple isolates from different geographical regions were sequenced. In contrast, the level of ITS-1 sequence differences between species ranged from 1.3% to 5.7%. The greatest sequence differences were detected between T. tenuis, the parasite species which infects birds and the six species found in mammals. Some of the nucleotide differences occurred at sites corresponding to recognition sites for restriction endonucleases. These results are compared with previous data obtained for the second internal transcribed spacer (ITS-2). The ITS-1 data indicate that this region of rDNA may also be useful for systematic studies in trichostrongylid nematodes.

Esophagostomiasis in humans due to infection with Oesophagostomum bifurcum (nodular worm) is of major human health significance in northern Togo and Ghana, where Necator americanus (human hookworm) also exists at high prevalence. Yet, very little is known about the transmission patterns of O. bifurcum, which is in part due to the difficulties in diagnosis and in differentiating some life-cycle stages of O. bifurcum from N. americanus using morphological features. As a first step toward developing a molecular-diagnostic assay, it was evaluated whether ribosomal (r)DNA could provide genetic markers for the identification of O. bifurcum and N. americanus to species. Internal transcribed spacer rDNA (plus flanking and intervening sequences) was analyzed by polymerase chain reaction-linked restriction fragment length polymorphism (PCR-RFLP) using several restriction endonucleases. The analysis showed that there was no detectable intraspecific difference in the size of the PCR products among multiple samples, that there was a consistent size difference in the products (of 110 bp or 350 bp, depending on region amplified) between the species, and that there was no significant variation in restriction patterns within each species. These results indicate that the rDNA spanning the internal transcribed spacers provides useful genetic markers for the identification of O. bifurcum and N. americanus to species, which has important implications for developing PCR-based tools to study the epidemiology and population biology of O. bifurcum.

There remains uncertainty about the species status of Teladorsagia circumcincta, T. davtiani and T. trifurcata. In this study, the second internal transcribed spacer (ITS-2) sequences of these taxa and 2 other members within the same subfamily (Ostertagiinae) were examined. Although some sequence variation was detected between and within single worms of each taxon of Teladorsagia, no unequivocal base differences were detected among their consensus ITS-2 sequence. In comparison, there was 9% sequence difference between O. ostertagi and O. leptospicularis and 13-15% difference between the genera Teladorsagia and Ostertagia. These findings indicate that T. circumcincta, T. davtiani and T. trifurcata represent a single species, T. circumcincta.

An electrophoretic analysis using 17 enzyme loci was carried out on specimens of the gastric nematode of macropodid marsupials, Rugopharynx australis (Monnig, 1926), collected from Macropus eugenii (Desmarest), M. fuliginosus (Desmarest), M. giganteus Shaw, M. robustus Gould, M. rufogriseus (Desmarest), M. rufus (Desmarest), Thylogale billardierii (Desmarest) and Wallabia bicolor (Desmarest) from south-eastern Australia. The extent of fixed genetic differences between nematodes from different host species ranged from 0–53%. The two distinct morphological forms of the parasite found in M. rufogriseus differed at 50% of loci. Specimens present in M. fuliginosus and M. giganteus were indistinguishable genetically, as were nematodes from M. rufus and M. robustus. Of the two morphologically distinct congeners included in the analysis as controls, Rugopharynx epsilon (Johnston & Mawson, 1939) was genetically distinct (46–69% fixed genetic differences) from all specimens of the R. australis complex while R. rufogrisea Magzoub, 1964 was closely related to one of the two species occuring in M. rufogriseus. It was concluded that R. australis is a species complex, with a genetically distinct species present in M. eugenii, M. fuliginosus/M. giganteus, M. robustus/M. rufus, W. bicolor and T. billardierii, and two species in M. rufogriseus.

Variable energy positron defect profiling has revealed the sub‐surface vacancy profile in Cz Si(100) implanted to a dose of 2×1014 cm−2 with 120 keV fluorine ions. We demonstrate the presence of vacancies at ppm concentrations at depths exceeding 1.5 μm, many times the depth at which implantation damage is expected to occur. The vacancy profile deduced was verified by removing layers of known thickness from the surface of the sample by chemical etching and repeating the defect profiling process. In the fitting procedure, the use of a Monte Carlo deduced positron implantation profile is required. The annealing behavior of the defects shows that the vacancies caused by the implantation process consist of two components. The deep vacancies are found to be relatively easily removed by a moderate temperature anneal, in contrast to those vacancies nearer to the surface. The former component is suggested to be the more mobile and thus may diffuse to the depths at which it is observed.

Boron ion implantation‐induced defects in SiO2 were investigated using slow positron annihilation spectroscopy and electron spin resonance (ESR). The defects caused by ion implantation are manifest as a particularly low S‐parameter in the region of the SiO2 layer in which B implantation damage occurs. The annealing behavior of the defect responsible for positron trapping was studied. The defect to which the positron is sensitive is found to be unobservable in ESR measurements. The defect is suggested to be dissolved O2 or a charged Frenkel defect, such as the negative non bridging‐oxygen hole center (≡S−O−).

Macropostrongyloides baylisi from four different species or subspecies of host were analysed electrophoretically at 27 enzyme loci. The results revealed the existence of two species, one in Macropus giganteus and the other in M. robustus robustus, M.r. erubescens and M.r. parryi, that had fixed genetic differences at 33% of loci. Populations of nematodes from two subspecies of M. robustus, M.r. robustus from Queensland and M.r. erubescens from South Australia, had fixed genetic differences at two (7.4%) of 27 loci and were considered to belong to the same species. No fixed genetic differences were detected between nematodes from M. parryi and M.r. robustus. A discriminant function analysis of morphological data assigned 96% of specimens to groups defined on the basis of the host species or subspecies from which they were obtained. This separation of Ma. baylisi into host-specific groups did not, however, totally correlate with the electrophoretic data. The species of M. baylisi in M. giganteus was genetically more distinct from the sibling species in M. robustus/M. parryi than to a related but morphologically dissimilar nematode, Ma. yamagutii from M. fuliginosus. This suggests an evolutionary parallel between host and parasite at the genetic level which is not reflected by morphological differences.

Defects in crystalline and amorphous SiO 2 were studied by variable-energy positron annihilation spectroscopy in an attempt to clarify what defects in SiO 2 are observable in positron annihilation studies. The level of the S parameter in amorphous SiO 2 films grown by plasma chemical vapor deposition (CVD) in a tetraethylorthosilicate (TEOS) and O 2 atmosphere was found to be correlated to the concentration of Si-OH in the silicon dioxide layer. The difference in S parameter between crystalline and amorphous SiO 2 is assumed to be due to the effects of para-Ps self-annihilation in free volumes in the amorphous sample. The S parameters in crystalline and themally grown silicon dioxide were both found to decrease after C ion implantation (1×10 14 cm -2 , 140 keV) or alternatively, ArF excimer laser (6.4eV) irradiation. Both methods are expected to produce numerous types of Frenkel defects of which, it is most likely that negatively charged species such as ≡Si-O - are the trapping sites. In the case of ion implantation into amorphous SiO 2 a greatly lowered S parameter is observed, this is explained as a combination of defect trapping and reduction in para Ps production after ion implantation

SUMMARYThis study examined whether allelic changes observed when clinical isolates ofGiardia intestinalismade in suckling mice were adapted toin vitrogrowth occurred as a result of gene switching (alternate isoenzymes) or through selection of organisms with different genotypes from mixed infections. Samples were compared electrophoretically at 20 enzyme loci. Marked allelic differences were detected between the uncloned clinical isolates grown in mice and the axenic cultures established from them. Furthermore, the allelic profiles of the uncloned isolates changed during the course ofin vivoorin vitrogrowth. In contrast, all clones produced from each isolate retained identical allelic profiles, regardless of whether they were grownin vivoorin vitro. These findings argue against gene switching as an explanation for the observed allozyme changes and support preferential selection of organisms with specific genotypes by growth conditions. The data indicate the presence of at least 2 and possibly up to 4 distinct genotypes within each clinical isolate. The genetic differences detected between clinical isolates in suckling mice were of similar magnitude to those that separate different axenic isolates ofG. intestinalisinto cryptic species. Conversely, the genetic differences between the isolates were limited when sampled after establishmentin vitro. These findings have significant implications for research onGiardiaand other medically important parasites and raise the possibility that culture may exert a similar selective bias on the genotypes isolated from infections with other parasitic protozoa.

An electrophoretic study of Pelecitus roemeri from Macropus robustus, M. giganteus and Wallubia bicolor revealed no genetic differences at 23 enzyme loci. The genetic data support the existing morphological evidence that P. roemeri from these three hosts represents a single species. The data show no genetic variation between nematodes from the same or different host species collected in northern and southern Australia. This result is discussed briefly in relation to Price's model of parasite speciation.

Allozyme electrophoresis of 98 Hypodontus macropi from eight different species of hosts using 24 enzymes revealed a complex of at least six sibling species, with 15–50% fixed genetic differences between taxa. Except for the taxon parasitizing Macropus rufus/M. robustus , pairs of parasite taxa were, in each case, sympatric at each locality examined, thus supporting the conclusion that they represent valid species. The existence of a series of host-specific nematode taxa explains many of the inconsistencies noted previously in the host distribution of H. macropi . Comparison of parasite allozyme phenograms with host phylogeny suggests that four of the speciation events could be attributable to cospeciation and two to host switching. A clear case of host switching between M. rufus/M. robustus and M. fuliginosus was found.

The implantation profile P(x) for 7 and 10 keV positrons in aluminium, and the median penetration depths for positrons in the energy range 5–10 keV, have been measured by monitoring the mean S parameter for a 0–800 nm thick wedge of aluminium deposited on a glass substrate. Because of the short positron diffusion lengths in both the aluminium and glass, and the uniform density of the sample, P(x) is essentially equal to the annihilation profile A(x), and is obtained by differentiating the S vs wedge thickness data. The results are distinctly non‐Makhovian, and are close in shape to the recent Monte Carlo results of Jensen and Walker. The measured median penetration depths vary as (42.3±3.6)En A (n=2.07±0.04; E=positron energy in keV), in marked contrast with the widely used form. The effect on these last measurements of positron diffusion to the Al‐glass interface is discussed, as are the consequences for the analyses of prositronium fraction ad S parameter profiling data.

The trapping of positrons in p+‐Si epilayers grown at different temperatures on n+(001)Si has been studied by Doppler broadening spectroscopy. The signal from positrons decaying in the epilayers is enhanced through the influence of the subsurface electric field present in p+‐Si; this enables direct measurement of the lineshape parameter S characteristic of the epilayers. The correlation between S for the epilayers and oxygen concentration is discussed, and application of the diffusion‐limited trapping model suggests that positrons are being trapped by SiO1⋅75 precipitates of mean radius 3.2±0.7 nm.

Evidence is presented based on morphological, electrophoretic and behavioural data, and on geographical distribution, that Aponomma tachyglossi Roberts is an independent species parasitic primarily on echidnas. Females of A. tachyglossi are distinguishable from the closely related species A. hydrosauri (Denny), parasitic on reptiles, in the shape of the porose areas, and males by the extent of the punctate areas between the scutum and the festoons. Nymphs and larvae of the two species could not be distinguished. In electrophoretic studies, A. tachyglossi was found to differ from A. hydrosauri at 16% of the 18 loci examined. Larvae of A. tachyglossi failed to attach and engorge on lizards, the usual host of A. hydrosauri. Futhermore, A. tachyglossi was found to have a limited geographical range in coastal areas of central Queensland. These data support the hypothesis that A. tachyglossi is a distinct species.

The occurrence of species flocks within the nematode genus Cloacina was examined using the criteria of host specificity, co-occurrence and monophyly. Species of Cloacina generally exhibited a high degree of host specificity, with most species occurring either in a single host species or in two closely related host species. The frequency distribution of numbers of component species of Cloacina per host species indicated that most host species harboured 2–4 species of nematodes, with an approximately exponential decline in the number of species of parasites to a maximum of 20 species of nematode per host species. Host species harbouring eight or more species of Cloacina were found within a single recent macropodid clade, but there was no correlation between evolutionary age of the host and the number of parasite species harboured. Sampling effort was significantly correlated with the number of nematode species found and, in partial regression analysis, subsumed the effects of host body size and geographic range, which were found to be significant correlates with the number of nematode species present in preliminary analyses. Analysis of co-occurrences of nematode species indicated significant variation between host species, with some hosts (e.g. Macropus agilis) most commonly harbouring a single species of Cloacina, while closely related host species (e.g. M. dorsalis) most commonly harboured numerous species. Parsimony analysis of species of Cloacina based on morphological data suggested that while small series of related nematode species could be identified within a single host species, the species flock in each host species is polyphyletic in origin. Species flocks contributed significantly to community richness in some host species.

The diffusion of positrons in the near-surface regions of n − , n + , p − and p + -Cz-grown silicon has been studied by implanting positrons with energies controllable in the range 1–32 keV and measuring the mean Doppler broadening of the annihilation line. While the first three samples exhibit normal positron diffusion to the surface, this diffusion appears to be much diminished in the p + sample. This result is discussed in terms of (a) shallow trapping of positrons at B − centres, and (b) the effect of an electric field in the subsurface region.

The occurrence of gastrointestinal helminth parasites in 40 Macropus agilis, 12 M. antilopinus, 39 M. dorsalis, 28 M. giganteus, 29 M. parryi, 30 M. robustus and 26 Wallabia bicolor from north and central Queensland was examined. A total of 124 morphologically defined species of helminth was encountered, comprising 103 species of strongyloid nematodes, 6 species of trichostrongyloid nematodes, 2 species of spiruroid nematodes, 4 species of oxyuroid nematodes, 7 species of anoplocephalid cestodes and 2 species of digenetic trematodes. Helminth communities in each macropodid host species exhibited a high level of diversity, and were dominated numerically by strongyloid nematodes. A high proportion of the helminth species was restricted to a single host species and there was a low level of similarity between helminth communities in different host species. Similarities that did occur were not apparently related to the phylogenetic relationships between hosts and are best explained by host switching between hosts sharing overlapping habitats and feeding preferences. There was poor separation of the helminth species into ‘core’, ‘secondary’ and ‘satellite’ members of communities.