Nematode Howardula Aoronymphium Research Articles

Olfactory Preferences of the Parasitic Nematode Howardula aoronymphium and its Insect Host Drosophila falleni.

Many parasitic nematodes have an environmental infective stage that searches for hosts. Olfaction plays an important role in this process, with nematodes navigating their environment using host-emitted and environmental olfactory cues. The interactions between parasitic nematodes and their hosts are also influenced by the olfactory behaviors of the host, since host olfactory preferences drive behaviors that may facilitate or impede parasitic infection. However, how olfaction shapes parasite-host interactions is poorly understood. Here we investigated this question using the insect-parasitic nematode Howardula aoronymphium and its host, the mushroom fly Drosophila falleni. We found that both H. aoronymphium and D. falleni are attracted to mushroom odor and a subset of mushroom-derived odorants, but they have divergent olfactory preferences that are tuned to different mushroom odorants despite their shared mushroom environment. H. aoronymphium and D. falleni respond more narrowly to odorants than Caenorhabditis elegans and Drosophila melanogaster, consistent with their more specialized niches. Infection of D. falleni with H. aoronymphium alters its olfactory preferences, rendering it more narrowly tuned to mushroom odor. Our results establish H. aoronymphium-D. falleni as a model system for studying olfaction in the context of parasite-host interactions.

A ribosome-inactivating protein in a Drosophila defensive symbiont

Vertically transmitted symbionts that protect their hosts against parasites and pathogens are well known from insects, yet the underlying mechanisms of symbiont-mediated defense are largely unclear. A striking example of an ecologically important defensive symbiosis involves the woodland fly Drosophila neotestacea, which is protected by the bacterial endosymbiont Spiroplasma when parasitized by the nematode Howardula aoronymphium. The benefit of this defense strategy has led to the rapid spread of Spiroplasma throughout the range of D. neotestacea, although the molecular basis for this protection has been unresolved. Here, we show that Spiroplasma encodes a ribosome-inactivating protein (RIP) related to Shiga-like toxins from enterohemorrhagic Escherichia coli and that Howardula ribosomal RNA (rRNA) is depurinated during Spiroplasma-mediated protection of D. neotestacea. First, we show that recombinant Spiroplasma RIP catalyzes depurination of 28S rRNAs in a cell-free assay, as well as Howardula rRNA in vitro at the canonical RIP target site within the α-sarcin/ricin loop (SRL) of 28S rRNA. We then show that Howardula parasites in Spiroplasma-infected flies show a strong signal of rRNA depurination consistent with RIP-dependent modification and large decreases in the proportion of 28S rRNA intact at the α-sarcin/ricin loop. Notably, host 28S rRNA is largely unaffected, suggesting targeted specificity. Collectively, our study identifies a novel RIP in an insect defensive symbiont and suggests an underlying RIP-dependent mechanism in Spiroplasma-mediated defense.

Defensive endosymbionts: a cryptic trophic level in community ecology

Maternally transmitted endosymbionts are widespread among insects, but how they are maintained within host populations is largely unknown. Recent discoveries show that some endosymbionts protect their hosts from pathogens or parasites. Spiroplasma, an endosymbiont of Drosophila neotestacea, protects female hosts from the sterilizing effects of parasitism by the nematode Howardula aoronymphium. Here, we show that Spiroplasma spreads rapidly within experimental populations of D. neotestacea subject to Howardula parasitism, but is neither strongly favored nor selected against in the absence of Howardula. In a reciprocal experiment, Howardula declined steadily to extinction in populations of Spiroplasma-infected flies, whereas in populations of uninfected flies, the prevalence of Howardula parasitism increased to c. 100%. Thus, Spiroplasma and Howardula exhibit effectively consumer-resource trophic dynamics. The recent spread of Spiroplasma in natural populations of D. neotestacea coincides with a decline in the prevalence of Howardula parasitism in the wild.

Ecology and Evolution of Host‐Parasite Associations: MycophagousDrosophilaand Their Parasitic Nematodes

Associations between mycophagous Drosophila and nematode parasites occur throughout the temperate and boreal regions of North America, Europe, and Asia. The nematode Howardula aoronymphium has substantial adverse effects on host survival and fertility on North American Drosophila. Long-term data show that rainy summers lead to a high prevalence of parasitism in the fall and the following spring, resulting in up to a 1-yr time lag between present rainfall and increased prevalence of H. aoronymphium parasitism. A biogeographic analysis of the relative abundance of different Drosophila species has shown that H. aoronymphium may facilitate the coexistence of different species of Drosophila that compete for larval food resources. The actual host range of parasites in nature is determined by the intrinsic suitability of potential hosts for parasite infection and reproduction and various ecological factors. For H. aoronymphium in eastern North America, intrinsically suitable hosts fall within a restricted clade within the genus Drosophila. However, the temperature sensitivity of H. aoronymphium prevents it from using several host species that occur outside the geographical range of the nematodes. Finally, the host range, virulence, and geographical range of Drosophila-parasitic nematodes appear to be highly dynamic over evolutionary timescales.

Effectively vertical transmission of aDrosophila‐parasitic nematode: mechanism and consequences

Summary1. Long‐term control of insects by parasites is possible only if the parasite populations persist. Because parasite transmission rate depends on host density, parasite populations may go extinct during periods of low host density. Vertical transmission of parasites, however, is independent of host density and may therefore provide a demographic bridge through times when their insect hosts are rare.2. The nematodeHowardula aoronymphium, which parasitises mycophagous species ofDrosophila, can experience both horizontal and effectively vertical transmission, relative rates of which depend, in theory at least, on the density of hosts at breeding sites.3. A nine‐generation experiment was carried out in which nematodes were transmitted either exclusively vertically or primarily horizontally. This experiment revealed that these parasites can persist and exhibit positive population growth even when there is only vertical transmission.4. Assays at the end of the experiment revealed that the vertically transmitted nematodes had suffered no inbreeding depression and that they were similar to the horizontally transmitted nematodes in terms of virulence, infectivity, within‐host growth rate, and fecundity. Thus, vertical transmission ofH. aoronymphiumdid not appear to compromise the ability of these parasites to controlDrosophilapopulations.

GENERAL-PURPOSE GENOTYPES FOR HOST SPECIES UTILIZATION IN A NEMATODE PARASITE OF DROSOPHILA.

The nematode Howardula aoronymphium parasitizes several species of mushroom-feeding Drosophila. A survey of isofemale strains of H. aoronymphium and a 25-generation selection experiment revealed that this species does not comprise host races, and that it harbors little heritable variation for adaptation to specific hosts No tradeoffs in performance on the different host species were evident. General-purpose genotypes, which can utilize all host species, characterize H. aoronymphium. An important feature of the natural history of these nematodes-correlated epidemiology across host species-is postulated to be both a cause and a consequence of the evolution of general-purpose genotypes in this species.

Lack of Behavioral Fever in Nematode-Parasitized Drosophila

The parasitic nematode Howardula aoronymphium causes severe fertility reduction and sterility in females of several species of mycophagous Drosophila. Infected females of Drosophila putrida recover their fertility when kept at a postemergence temperature of 29 C but not when kept at 27 C or below. When given a choice of temperatures in a thermal gradient, neither this species, Drosophila falleni, nor Drosophila neotestacea manifested adaptive behavioral fever in response to parasitism. However, our conclusions concerning D. neotestacea and D. falleni remain tentative because of small sample sizes.

Parasite‐Induced Mortality in Mycophagous Drosophila

Reductions in host survival due to parasitism have immediate effects on the population dynamics of both parasites and hosts. Despite its potential importance, parasite—induced host mortality has rarely been studied experimentally under natural conditions. We conducted a series of field releases of Drosophila putrida and Drosophila neotestacea that had been parasitized in the laboratory by the nematode Howardula aoronymphium. In 11 out of 12 independent tests, the proportion of flies that were parasitized declined significantly through time, suggesting that parasites cause host mortality. Independent tests showed that parasites have no detectable effect on dispersal of flies and that motherworms do not disappear from surviving hosts, thus supporting the conclusion that parasites cause significant host mortality in the field. Host mortality rate increased with worm burden, significantly so in four independent tests. This is a possible mechanism for density—dependent regulation of these parasite populations. Parasite—induced mortality was considerably greater among flies that had been released in the field than among those kept concurrently in the laboratory. Thus, laboratory or clinical traits may not provide an accurate assessment of parasite virulence in natural populations. These results support a central assumption of many recent ecological and evolutionary models, i.e., that parasites are harmful to their hosts under natural conditions.

Aggregations of nematode parasites within Drosophila: proximate causes

Macroparasites almost invariably exhibit overdispersed distributions of parasites/host, yet the specific causes of such aggregations remain poorly understood. The present study focused on the distribution of the parasitic nematode Howardula aoronymphium among its hosts, several species of mycophagous Drosophila. The distribution of parasites/host is close to random for cohorts of flies of a given host species emerging from single mushrooms. At the level of Howardula populations, overdispersion of parasites among hosts results primarily from variation among subgroups of hosts in their exposure to infective-stage nematodes. The sources of variation identified in this study include Drosophila host species, the site where flies bred, mushroom species within sites, and, most importantly, individual mushrooms within mushroom species at a site. For the mean intensity of parasitism observed in this study, the degree of aggregation is typical of macroparasites in general. Combinations of random distributions with different means, resulting from variation among groups in exposure to parasites, may be a common cause of the overdispersion of macroparasites among hosts.

Rapid evolution of host specificity in a parasitic nematode

In eastern North America, the nematodeHowardula aoronymphium parasitizes four species of mushroom-breedingDrosophila:D. falleni andD. recens of the quinaria species group, andD. putrida andD. testacea of the testacea group. One strain ofH. aoronymphium, designated Mendon-87, was initially capable of infecting all four of these host species. After less than 3 years in laboratory culture usingD. falleni as the sole host, this strain had completely lost the ability to infectD. putrida. Two other nematode strains parasitizedD. falleni andD. putrida at equal rates. These results demonstrate the existence of genetic variation for host specificity within this nematode species. More importantly, they show that host specificity can evolve rapidly when only one host is available for parasitization. Ecological conditions are such that natural populations ofH. aoronymphium may comprise numerous host races, lineages incapable of parasitizing the full range of host species. However, I argue that such host races are probably ephemeral and thus unlikely to persist long enough to undergo speciation.

Dynamics of Host-Parasite Interactions: The Drosophila-Howardula System

Several species of mycophagous Drosophila in eastern North America are parasitized by the nematode Howardula aoronymphium. The flies become infected as larvae when feeding in mushrooms. The nematodes, which are obligate parasites that reproduce only within adult flies, colonize mushrooms when they are expelled by the adult flies. Parasitized female flies suffer greatly reduced fertility. Thus, in order to understand the population dynamics of these host-parasite interactions, one must ask what governs levels of parasitism among adult flies in natural populations. Here we examine how processes occurring at or within individual mushrooms affect rates of parasitism in adult Drosophila. At normal intensities of parasitism, these nematodes do not reduce pre-adult survival of their hosts. Field cage experiments revealed that level of infection in emergent flies increases with the ratio of adult flies to mushrooms in a habitat. Thus, high population densities of Drosophila tend to bring on high rates of parasitism. This could contribute to seasonal variation in prevalence of parasitism observed in this and other insect-nematode systems and provide a mechanism by which nematodes can regulate insect populations below levels set by their resources. The field cage experiment also revealed that the numbers of worms per fly were randomly distributed among flies emerging from individual mushrooms, but that the mean intensity of parasitism varied among mushrooms. This suggests a simple mechanism for the generation of aggregated distributions of worms among hosts at the population level.

Mycophagous Drosophila and Their Nematode Parasites

The nematode Howardula aoronymphium (Allantonematidae) parasitizes several mushroom-feeding species of Drosophila of the quinaria and testacea species groups in eastern North America. The principal effect of nematodes on their hosts is a reduction in female fertility: females of Drosophila putrida and Drosophila testacea are sterilized by these parasites. The prevalence of parasitism varies substantially among host species and through time, showing evidence of seasonal as well as year-to-year variation. Because the principal effect of these parasites is on host reproduction and because they are not highly aggregated among hosts, this parasite-host system may be prone to unstable population dynamics. Some field evidence suggests that H. aoronymphium may influence the outcome of competition among its host species Because these parasites and their hosts have similar population sizes, generation times, and rates of gene flow, they may interact evolutionarily in a dynamically interesting manner

Aggregation and the Coexistence of Mycophagous Drosophila

SUMMARY (1) Intraspecific aggregation has been postulated to facilitate the coexistence of competing species that share patchy resources. Data on adult emergence numbers of four mycophagous species of Drosophila from field-collected mushrooms show that these species exhibit highly aggregated distributions, both in collections comprising several naturally occurring species of mushrooms and in those of a single species for which mushroom size, condition, exposure time, and location in the field were controlled. (2) The relation between the mean and the variance in the numbers of flies emerging per mushroom reveals that the mean level of larval crowding increases with overall population density. Aggregation of larvae of each species is due to (i) females laying eggs in clutches of more than one egg, and (ii) non-random distributions of ovipositing females across breeding sites. (3) The emergence number of different species of Drosophila tend to be positively correlated across mushrooms, especially between species belonging to the same species group. In fact, within species groups, intraspecific and interspecific aggregation of ovipositing females contribute about equally to the proportional increase in the number of competitors an individual larva shares a mushroom with. Thus, aggregation does not appear to play an important-role in the coexistence of species belonging to the same species group. (4) The probability of parasitism of emergent Drosophila by the nematode Howardula aoronymphium Welch is generally independent of emergence numbers. Thus, although density-dependent parasitism could promote coexistence of species that have independent aggregated distributions, we find little evidence of such density dependence in these species.