The role of phylogeny and ecology in experimental host specificity: insights from a eugregarine-host system.

The degree to which parasites use hosts is fundamental to host-parasite coevolution studies, yet difficult to assess and interpret in an evolutionary manner. Previous assessments of parasitism in eugregarine-host systems suggest high degrees of host specificity to particular host stages and host species; however, rarely have the evolutionary constraints on host specificity been studied experimentally. A series of experimental infections were conducted to determine the extent of host stadium specificity (larval vs. adult stage) and host specificity among 6 tenebrionid host species and 5 eugregarine parasite species. Eugregarines from all host species infected both the larva and adult stages of the host, and each parasite taxa colonized several host species (Tribolium spp. and Palorus subdepressus). Parasite infection patterns were not congruent with host phylogeny, suggesting that host phylogeny is not a significant predictor of host-parasite interactions in this system. However, the 2 host stages produced significantly different numbers of parasite propagules, indicating that ecological factors may be important determinants of host specificity in this host-parasite system. While field infections reflect extant natural infection patterns of parasites, experimental infections can demonstrate potential host-parasite interactions, which aids in identifying factors that may be significant in shaping future host-parasite interactions.

Alternative life cycle strategies of Megalodiscus temperatus in tadpoles and metamorphosed anurans.

Megalodiscus temperatus (Stafford, 1905) is a common paramphistome trematode of North American amphibians with a two host life cycle and has been reported to infect frogs and rarely tadpoles. In this study we document the alternative life cycle strategy of M. temperatus in tadpoles and metamorphosed anurans. We show through field work and experimental infections that M. temperatus can establish in both anuran life stages and worms become gravid and release eggs in both tadpoles and metamorphosed frogs. However, worms exhibit differences in route of infection, development, egg production, and diet in tadpoles and metamorphosed anurans. These alternative life history strategies of M. temperatus suggest different selective pressures on the development and reproductive success of these worms in tadpoles and metamorphosed anurans, and we discuss the evolutionary avenues for and constraints on amphibian trematode life cycles presented by these two different anuran life stages.

New and emended descriptions of gregarines from flour beetles (Tribolium spp. and Palorus subdepressus: Coleoptera, Tenebrionidae).

The following new gregarine taxa are described from larvae of flour beetles (Coleoptera: Tenebrionidae): Awrygregarina billmani, n. gen., n. sp., from Tribolium brevicornis; Gregarina cloptoni, n. sp., from Tribolium freemani; Gregarina confusa, n. sp., from Tribolilum confusum; and Gregarina palori, n. sp., from Palorus subdepressus. In addition, the description of Gregarina minuta Ishii, 1914, from Tribolium castaneum, is emended. Scanning electron micrograph studies of these species' oocysts reveal differences in surface architecture. The Gregarina species have oocysts with longitudinal ridges, visible with SEM, whereas Awrygregarina billmani oocysts have fine circumferential striations; surface architecture is the main feature distinguishing the 2 gregarine genera. Although parasites from adult beetles are not included in the descriptions, adults of all host species can be infected experimentally using oocysts from the new taxa.

Gregarina niphandrodes (Eugregarinorida: Septatorina): oocyst surface architecture.

The surface architecture of oocysts produced by Gregarina niphandrodes (Eugregarinorida) from Tenebrio molitor adults (Coleoptera: Tenebrionidae) as revealed by scanning electron microscopy is reported. Gametocysts were allowed to dehisce on 15-mm, round cover glasses; the cover glasses with their oocysts chains were then mounted on stubs without further processing, and sputter-coated with 20-nm gold palladium. Scanning electron microscopy was performed at 10-15 kV with a Hitachi 3000N SEM. Oocysts retained their characteristic shapes as reported in the original species description but showed longitudinal ridges of relatively uniform height, width, and spacing, in separate fields on either side of a central equatorial bulge in the oocysts. There was no ultrastructural evidence of an enclosing external sheath holding the oocysts in a chain. Oocyst ends were flared slightly, and the chain itself was twisted, with adjacent oocysts offset slightly from one another. This article now provides an additional set of structural characters potentially useful in gregarine systematics.

Problems in the comparative physiology of some trypanosomatid flagellates.

There is considerable evidence that trypanosomatid species vary in their metabolic characteristics, that they vary in the mechanisms by which they control these characteristics, and that a single species may vary metabolically without varying structurally. Studies to date from a number of laboratories also indicate there is still reason to believe that metabolic characteristics of trypanosomatid flagellates, as manifest in culture, are at least correlated (in some cases) with the behavior of the flagellate species in the metazoan host. It appears that our major tasks for the next several years are 1. to discover the extent to which these correlations are manifestations of characteristics required for life in the metazoan host, 2. to discover the extent to which these correlations are manifestations of characteristics which determine infection site within the metazoan host, 3. to discover which if any metabolic control mechanisms contribute to the multiplicity of clinical infections often seen in this group of protozoa, and 4. to discover the critical links between energy metabolism, the control of that metabolism, and life inside a host or host cell.

Life cycle and paratenesis of American gordiids (Nematomorpha: Gordiida).

To complete their life cycle, gordiids must make a transition from aquatic to terrestrial environments. However, epibenthic aquatic larvae and their terrestrial definitive hosts do not overlap in habitat. This has led many investigators to suggest that infections are acquired through the ingestion of insects, which become infected as aquatic larvae with gordiid cysts and subsequently carry gordiids to land. This proposed life cycle was experimentally tested using 3 common American species of gordiids: Gordius robustus, Paragordius varius, and Chordodes morgani. Cysts of all 3 species survived the metamorphosis of Tanytarsus sp., a midge. Subsequent infection trials of definitive hosts with cysts from imagos show that cysts surviving the metamorphosis of insects remained viable and free of host internal defense reaction. Data from naturally infected mayflies, Callibaetus sp., show that encystment and survival of gordiids within aquatic insects occur in nature. Paratenesis between paratenic hosts was also shown to be possible in these 3 species. This latter finding appears to indicate that cysts formed in spurious paratenic hosts may not be lost but may eventually transfer to normal paratenic hosts.

The life cycle of a horsehair worm, Gordius robustus (Nematomorpha: Gordioidea).

Aspects of the life cycle of the nematomorph Gordius robustus were investigated. Gordius robustus larvae fed to Tenebrio molitor (Coleoptera: Tenebrionidae) readily penetrated and subsequently encysted in the posterior portion of the midgut wall. Parasite development was then arrested in each infected beetle. Upon feeding these cysts to laboratory-reared Gryllus firmus, worm larvae developed to adults in the cricket hemocoel. In an additional experiment, G. robustus larvae fed to G. firmus did not develop to adults. These findings are in contrast to other studies that found direct infection to be an alternative mode of infection. This experiment is the first employing a laboratory-maintained stock of hosts to rear nematomorph worms.

Ultrastructure of interlamellar Henneguya exilis in the channel catfish.

Ultrastructural aspects of interlamellar Henneguya exilis infections in channel catfish are reported. The plasmodium wall of this form differs from that of other species in that it is composed of two outer unit membranes which give rise to a zone of numerous pinocytic canals. Single-membraned canals appeared to be a stable feature of the wall while double-membraned canals are interpreted as those actively carrying out pinocytosis. Evidence suggests that host cellular cytoplasm as well as interstitial material is taken in by plasmodia. Plasmodium wall integrity, aggregation of parasite ectoplasmic components, numbers of pinocytic canals, and number of mitochondria proximal to the wall vary among different plasmodium profiles and may be related to plasmodium maturity. The parasite causes extensive hyperplasia of basal cells, which in turn replaces most other cell types found in noninfected gill filaments. Cytoarchitectural differences between basal cells of noninfected filaments and basal cells adjacent to plasmodia include significantly shorter microfilament bundles in the latter.

Diversity of the parasite assemblage of Fundulus zebrinus in the Platte River of Nebraska.

Changes in the values of the Shannon H' diversity index as determined for individual hosts (infraassemblage diversity), host samples (sample assemblage diversity), and for species density are reported for an assemblage of 7 parasites in Fundulus zebrinus in the Platte River in Nebraska for a 5-yr period. The parasites were: Myxosoma funduli (gill), Trichodina sp. (gill), Gyrodactylus bulbacanthus (gill), Salsuginus sp. (gill), Gyrodactylus stableri (body surface), and Neascus sp. (= Posthodiplostomum; eyes and body cavity). In addition, relative abundance and equitability are given for each of the study years. Mean infraassemblage diversity, sample assemblage diversity, species density, and equitability were all significantly negatively correlated with river streamflow (measured in cubic feet per second) of the year prior to the sample, but were independent of the concurrent year's streamflow. Over the long term, M. funduli and Trichodina sp. were the most, and G. bulbacanthus was the least, abundant. Species pair prevalence and relative density correlations showed few long-term patterns of co-occurrence or microallopatry. The strongest association was between M. funduli and the Neascus sp. and was attributed to similarities in ecological requirements of intermediate hosts.

Host specificity of Rhabdochona canadensis (Nematoda: Rhabdochonidae) in Nebraska.

Intermediate and definitive host specificity of Rhabdochona canadensis in Nebraska were investigated. Mayfly nymphs Trichorythodes sp. and Caenis sp. were found to serve as experimental intermediate hosts. Development inside the nymphs required approximately 10 days, with the worms passing through 2 molts and then becoming encapsulated in the hemocoel as infective third-stage juveniles. Survey data revealed that only the red shiner Cyprinella lutrensis serves as definitive host for R. canadensis in nature. Laboratory infections of Notropis dorsalis, N. stramineus, and Fundulus zebrinus, all of which were uninfected in nature, were attempted to determine if observed specificity was due to physiological or ecological factors. Two individuals of N. dorsalis became infected with R. canadensis, but no development was observed. Both N. stramineus and F. zebrinus were incapable of becoming infected. Thus, definitive host specificity in this system seems to be mediated by both physiological and host ecological factors.

Second intermediate host-specificity of Haematoloechus complexus and Haematoloechus medioplexus (Digenea: Haematoloechidae).

Second intermediate host-specificity was examined for 2 species of the frog lung fluke genus Haematolechus. Nine species of freshwater arthropods were exposed to cercariae of H. complexus and H. medioplexus. Metacercariae of H. complexus developed in all arthropod species used. Metacercariae of H. medioplexus developed only in anisopteran odonate naiads. This difference in host utilization may have epizootiological implications. The potential development of H. complexus in a greater number of arthropods than H. medioplexus may increase the chances of ingestion of H. complexus by an anuran host. The range of arthropods parasitized by H. complexus indicates that host-specificity of the metacercarial stage of this fluke is not restricted by the phylogeny of aquatic arthropods.

Epimerite-host epithelium relationships among eugregarines parasitizing the damselflies Enallagma civile and Ischnura verticalis.

The host-parasite interface between 2 species of damselflies and 4 species of eugregarines was examined at the ultrastructural level. Nubenocephalus nebraskensis organisms attached to the host midgut epithelium by means of a sucker-like protomerite; the space between the epicytic folds and host epithelium was filled with electron-dense material interpreted to be adhesive in nature. Actinocephalus carrilynnae organisms attached by means of the epimerite, which had no epicytic folds, and by the fluted stalk with characteristic epicytic folds: host cell and parasite membranes appeared fused at some places on the epimerite. Hoplorhynchus acanthatholius organisms attached by means of an ovoid epimerite with backward-pointing digitations; the entire epimerite was embedded in a host cell, and host cell microvilli surrounded the stalk. Steganorhynchus dunwoodyi organisms attached by means of an ovoid stalk papilla enclosed in a retractable globular sheath; the papilla was covered with epicytic folds, but the sheath was not, and the sheath had a single membrane, whereas the epicytic folds had 2 or 3 membranes. The entire apparatus was inserted between epithelial cells, and the sheath was highly folded at its surface. The ultrastructural observations suggest that actinocephalid gregarines have evolved 2 general strategies for attaching to the host epithelium, that is, suckerlike protomerites, as in the case of N. nebraskensis, and deeply embedded epimerites inserted within or between host cells, as in the other species studied.

The response of Gregarina niphandrodes (Apicomplexa: Eugregarinida: Septatina) to host starvation in Tenebrio molitor (Coleoptera: Tenebrionidae) adults.

Numerous studies of host starvation have emphasized pathological effects of parasites on their insect host, but little attention has been focused on the effects of host starvation on the parasites. This study addressed the possibility that parasite life-cycle events could be manipulated by withholding food from the host. The system used was Gregarina niphandrodes (Apicomplexa: Eugregarinida) in Tenebrio molitor (Coleoptera: Tenebrionidae) adults. Gregarine gametocyst formation and shedding ceased after 1 day in starved beetles but continued in fed controls. There were no statistically significant differences between total lengths of associated (3 of 5 trials) or unassociated (5 of 5 trials) gregarines found between experimental and control groups, but average numbers of the 2 life cycle events were generally higher in fed hosts than in starved ones. If infected, fed control beetles continued to form gametocysts throughout the 7-day trial periods, and gametocysts could be observed in the gut. Starved experimental beetles had no gametocysts in their guts. Refeeding of starved beetles after 4 days resulted in resumption of gametocyst formation and shedding. The studies demonstrated that the gregarine life cycle could be stopped and then started at the gametocyst formation stage like an off/on switch, simply by withholding food from, then refeeding, the host.

Host specificity among Ancyrocephalinae (Monogenoidea) of Nebraska sunfish.

This study addressed the problem of local patterns of host specificity among Ancyrocephalinae (Monogenoidea) on bass and sunfish species, when the hosts occur in different species combinations in separate ponds. One hundred fifty-three fish of the Centrarchidae, from 4 study sites in Nebraska, were collected. Host species included bluegill (Lepomis macrochirus), green sunfish (L. cyanellus), largemouth bass (Micropterus salmoides), black crappie (Pomoxis nigromaculatus), white crappie (P. annularis), and rock bass (Ambloplites rupestris). These fish occurred in different species combinations, depending on the pond sampled. Results indicated that several centrarchid species could inhabit the same pond and yet support distinct monogene communities. Clavunculus bursatus, Onchocleidus helicis, O. principalis, and Syncleithrum fusiformis were found only on largemouth bass, regardless of what other centrarchids were present in a particular pond. Haplocleidus dispar occurred on green sunfish, bluegill, largemouth bass, and black crappie, and H. furcatus occurred on both bluegill and largemouth bass. Onchocleidus cyanellus and O. ferox were found on both bluegill and green sunfish. Rock bass were present in only 1 of the 4 ponds, but were not infected with any monogenes, even though co-occurring centrarchids were often heavily infected. Largemouth bass had the most diverse ancyrocephaline communities. The degree of parasite host specificity among these monogenes was inversely related to the diversity of host species present in a particular pond. In general, the parasites were more host specific than might be inferred from the literature; parasite species did not necessarily colonize supposedly receptive host species even when the latter were present, and host relatedness was the major factor in determining whether host species shared a common parasite species.

Parasite community structure in Pimephales promelas (Pisces: Cyprinidae) from two converging streams.

Parasites of the fathead minnow, Pimephales promelas, were examined in fish collected from Elk Creek (40.88534 degrees N, 96.83366 degrees W) and West Oak Creek (40.90821 degrees N, 96.81432 degrees W), Lancaster County, Nebraska. These 2 streams are part of the Salt Valley watershed and flow together approximately 2 km downstream from the collection sites to form Oak Creek. This study examined the extent to which the 2 tributaries constitute a continuous habitat with respect to fish hosts. The parasite community included Trichodina sp., Myxobolus sp., Dactylogyrus simplex, D. bychowskyi, and D. pectenatus (all on gills); Gyrodactylus hoffmani (gill and body surface); Posthodiplostomum sp. (neascus, body cavity); and Uvulifer ambloplitis (encysted in skin). Among 46 fish from Elk Creek and 56 fish from West Oak Creek taken on 5 dates during April-July 1998, U. ambloplitis was found in Elk Creek fish at prevalences of 44-100% but in only 2 West Oak fish on 1 date. Prevalence and mean abundance of D. simplex also differed between the 2 sites. On the basis of these observations, fish populations in the 2 streams were considered to be distinct, with little or no fish movement between the tributaries.

Behavioral basis of second intermediate host specificity among four species of Haematoloechus (Digenea: Haematoloechidae).

Cercarial behavior patterns were examined in 4 species of frog lung flukes (Haematoloechus spp.). Cercariae of Haematoloechus complexus, Haematoloechus medioplexus, Haematoloechus longiplexus, and Haematoloechus varioplexus were exposed to 3 species of experimental arthropods and an inanimate control. The number of cercariae attached to an experimental host at 5 min postexposure was recorded. Haematoloechus longiplexus and H. complexus cercariae attached to experimental hosts at higher rates than cercariae of H. medioplexus and H. varioplexus. Cercariae of H. longiplexus attached to experimental hosts in approximately the same numbers as H. complexus, but H. longiplexus penetrated only damselfly naiads, and only at the base of the zygopteran caudal gills. Cercariae of H. complexus, a second intermediate host generalist, were able to penetrate and enter several arthropod species at the intersegmental membranes. Haematoloechus medioplexus and H. varioplexus are restricted to development in dragonfly naiads and cercariae rarely attached to and never penetrated experimental hosts. These behavioral patterns dictate the range of hosts suitable for metacercarial development of H. complexus, H. longiplexus, H. medioplexus, and H. varioplexus. The evolution of disparate patterns of behavior among the cercariae of these 4 congeners has directly affected subsequent patterns of transmission to the definitive host.

Inhibition of in vitro macrophage digestion capacity by infection with Leishmania donovani (Protozoa: Kinetoplastida).

Promastigotes of Leishmania donovani transformed into amastigotes and survived in cultured macrophages. Promastigotes of Leptomonas costoris, a kinetoplastid parasite of water striders, transformed into amastigotes but did not survive in cultured macrophages. A significant difference in size between the two species of parasitic protozoa allowed development of a bioassay in which the in vitro digestion rate of L. costoris was taken as a measure of macrophage digestive capacity following preinfection with L. donovani. In a 12- to 24-hr period, macrophage cultures infected with the 2S strain of L. donovani retained 50% more L. costoris than did control macrophages infected with L. costoris alone. The 2S strain has been in culture since 1967 and has retained its infectivity for hamsters, although its virulence is attenuated. In similar experiments with promastigotes of the Khartoum strain of L. donovani, a strain but evidently has lots its infectivity for hamsters, no such suppression of macrophage ability to digest L. costoris was observed. The results suggests that infective strains of Leishmania donovani suppress macrophage function beyond the time at which initial lysosomal fusion with the parasitophorous vacoule occurs.

Host stadium specificity in the gregarine assemblage parasitizing Tenebrio molitor.

Reciprocal cross-stadia experimental infections were used to demonstrate stadium specificity within the gregarine assemblage parasitizing Tenebrio molitor, the yellow mealworm. Gregarina cuneata, Gregarina polymorpha, and Gregarina steini are characteristic parasites of larval T. molitor. Gregarina niphandrodes is a characteristic parasite of adult T. molitor. Experimental infections were produced in all homologous host-parasite combinations. No infection was produced in heterologous or cross-stadia combinations. This study introduces the concept of separate, distinct parasite niches corresponding to separate life cycle stages and established by known, predictable life cycle events within a single host species.

The roles of ecological and evolutionary influences in providing structure to parasite species assemblages.

Parasite species assemblages currently are thought to range from isolationist to interactive, their dynamic properties being related to the number of species and types of hosts involved. The literature contains few experimental tests of this concept, however, and many of the host/parasite systems studied to date are not amenable to experimental manipulation. In this review, the presence of a parasite species, in a sample of host individuals, is considered to be an evolutionary phenomenon, but the parasite's population structure is considered to be an ecological one. Studies that allow evaluation of these 2 influences are comparative in nature and include data from a series of homogeneous samples of host populations. A lottery model is presented, in which hosts acquire their assemblages of parasites by Monte Carlo type sampling from multiple kind arrays; the major structuring influence is the relative probability of becoming infected by various parasite species. Claims of parasite species interaction need to be supported by studies showing departures from the predictions of this model. The species density and infraassemblage diversity index distributions are recommended as quantitative tools useful in such work.

The niche of Salsuginus thalkeni, a gill parasite of Fundulus zebrinus.

Distribution of a monogenean parasitic helminth Salsuginus thalkeni on the gills of the fish Fundulus zebrinus is described by calculation of mean positions and niche breadths on the linear spatial resource gradients gill filament length, gill arch length and arch number. All distributions are given for parasites in the presence and absence of various combinations of potential competitors, namely the 6 other parasite species that occupy the same host species. Filament niche breadth was narrowest in the absence of potential competitors; breadth on arch was widest in the presence of potential competitors. Breadth on both resources was correlated positively with mean number of parasites per individual. Arch breadth exhibited cyclic seasonal changes, being lowest in early to mid-summer. Mean position exhibited no repeated pattern of variation on either resource. The results are considered consistent with predictions about the niche structures of species in unsaturated noninteractive specialist communities.