Shedding patterns of Daubaylia potomaca (Nematoda: Rhabditida)

Different metazoan parasites, different transcriptomic responses, with new insights on parasitic castration by digenetic trematodes in the schistosome vector snail Biomphalaria glabrata

BACKGROUND

Gastropods of the genus Biomphalaria (Family Planorbidae) are exploited as vectors by Schistosoma mansoni, the most common causative agent of human intestinal schistosomiasis. Using improved genomic resources, overviews of how Biomphalaria responds to S. mansoni and other metazoan parasites can provide unique insights into the reproductive, immune, and other systems of invertebrate hosts, and their responses to parasite challenges.

RESULTS

Using Illumina-based RNA-Seq, we compared the responses of iM line B. glabrata at 2, 8, and 40 days post-infection (dpi) to single infections with S. mansoni, Echinostoma paraensei (both digenetic trematodes) or Daubaylia potomaca (a nematode parasite of planorbid snails). Responses were compared to unexposed time-matched control snails. We observed: (1) each parasite provoked a distinctive response with a predominance of down-regulated snail genes at all time points following exposure to either trematode, and of up-regulated genes at 8 and especially 40dpi following nematode exposure; (2) At 2 and 8dpi with either trematode, several snail genes associated with gametogenesis (particularly spermatogenesis) were down-regulated. Regarding the phenomenon of trematode-mediated parasitic castration in molluscs, we define for the first time a complement of host genes that are targeted, as early as 2dpi when trematode larvae are still small; (3) Differential gene expression of snails with trematode infection at 40dpi, when snails were shedding cercariae, was unexpectedly modest and revealed down-regulation of genes involved in the production of egg mass proteins and peptide processing; and (4) surprisingly, D. potomaca provoked up-regulation at 40dpi of many of the reproduction-related snail genes noted to be down-regulated at 2 and 8dpi following trematode infection. Happening at a time when B. glabrata began to succumb to D. potomaca, we hypothesize this response represents an unexpected form of fecundity compensation. We also document expression patterns for other Biomphalaria gene families, including fibrinogen domain-containing proteins (FReDs), C-type lectins, G-protein coupled receptors, biomphalysins, and protease and protease inhibitors.

CONCLUSIONS

Our study is relevant in identifying several genes involved in reproduction that are targeted by parasites in the vector snail B. glabrata and that might be amenable to manipulation to minimize their ability to serve as vectors of schistosomes.

Integrated characterisation of Daubaylia burnupiae n. sp. (Nematoda: Daubayliidae) from a freshwater gastropod in South Africa, with comments on the biology of Daubaylia spp

Gastropod-nematode associations are underreported worldwide. In the present study, juvenile and adult nematodes were found in the freshwater gastropod Burnupia stenochorias (Melvill & Ponsonby, 1903), from the Vaal River, South Africa. The nematodes were confirmed to belong to the genus Daubaylia chitwood & chitwood, 1934 (Daubayliidae). This is the first report of Daubaylia from a snail belonging to the family Burnupiidae, and the first report of this nematode taxon in southern Africa. Like D. pearsoni and D. malayanum from Australia and Malaysia respectively, adult females of the current species possess multiple well-developed eggs in the uteri, with larvae developing in utero. Morphological and molecular characteristics showed that the nematodes are distinct from all the described species of Daubaylia. Thus, they are considered a new species, Daubaylia burnupiae n. sp. The species differs from its congeners based on spicule shape, the short tail of the male, an anal cuticular knob-like protrusion on the female, and oesophagi with short isthmi and short glandular basal bulbs in both sexes. Three club-shaped pharyngeal lobes, extending slightly above the surface of the cephalic lips in both sexes and a pre-cloacal median papilla on the male were described using scanning electron microscopy, the first of such observations for the daubayliids. Genetic analyses showed that partial sequences of D. burnupiae n. sp. differed from species for which genetic data are available, by at least 26 and 9 base pair differences for 28S and 18S rDNA, respectively. Our results show that low prevalence and abundance of nematodes in the snails, corresponded with increased pollution in the river. We suspect that exposure to pollutants reduces the viability of the infective gravid female nematode during transmission. Therefore, the nematode is a potential bioindicator for aquatic pollution.

Dispersal of a defensive symbiont depends on contact between hosts, host health, and host size

Symbiont dispersal is necessary for the maintenance of defense mutualisms in space and time, and the distribution of symbionts among hosts should be intricately tied to symbiont dispersal behaviors. However, we know surprisingly little about how most defensive symbionts find and choose advantageous hosts or what cues trigger symbionts to disperse from their current hosts. In a series of six experiments, we explored the dispersal ecology of an oligochaete worm (Chaetogaster limnaei) that protects snail hosts from infection by larval trematode parasites. Specifically, we determined the factors that affected net symbiont dispersal from a current "donor" host to a new "receiver" host. Symbionts rarely dispersed unless hosts directly came in contact with one another. However, symbionts overcame their reluctance to disperse across the open environment if the donor host died. When hosts came in direct contact, net symbiont dispersal varied with both host size and trematode infection status, whereas symbiont density did not influence the probability of symbiont dispersal. Together, these experiments show that symbiont dispersal is not a constant, random process, as is often assumed in symbiont dispersal models, but rather the probability of dispersal varies with ecological conditions and among individual hosts. The observed heterogeneity in dispersal rates among hosts may help to explain symbiont aggregation among snail hosts in nature.