Transcriptional patterns of sexual dimorphism and in host developmental programs in the model parasitic nematode Heligmosomoides bakeri

Background

Heligmosomoides bakeri (often mistaken for Heligmosomoides polygyrus) is a promising model for parasitic nematodes with the key advantage of being amenable to study and manipulation within a controlled laboratory environment. While draft genome sequences are available for this worm, which allow for comparative genomic analyses between nematodes, there is a notable lack of information on its gene expression.

Methods

We generated biologically replicated RNA-seq datasets from samples taken throughout the parasitic life of H. bakeri. RNA from tissue-dwelling and lumen-dwelling worms, collected under a dissection microscope, was sequenced on an Illumina platform.

Results

We find extensive transcriptional sexual dimorphism throughout the fourth larval and adult stages of this parasite and identify alternative splicing, glycosylation, and ubiquitination as particularly important processes for establishing and/or maintaining sex-specific gene expression in this species. We find sex-linked differences in transcription related to aging and oxidative and osmotic stress responses. We observe a starvation-like signature among transcripts whose expression is consistently upregulated in males, which may reflect a higher energy expenditure by male worms. We detect evidence of increased importance for anaerobic respiration among the adult worms, which coincides with the parasite’s migration into the physiologically hypoxic environment of the intestinal lumen. Furthermore, we hypothesize that oxygen concentration may be an important driver of the worms encysting in the intestinal mucosa as larvae, which not only fully exposes the worms to their host’s immune system but also shapes many of the interactions between the host and parasite. We find stage- and sex-specific variation in the expression of immunomodulatory genes and in anthelmintic targets.

Conclusions

We examine how different the male and female worms are at the molecular level and describe major developmental events that occur in the worm, which extend our understanding of the interactions between this parasite and its host. In addition to generating new hypotheses for follow-up experiments into the worm’s behavior, physiology, and metabolism, our datasets enable future more in-depth comparisons between nematodes to better define the utility of H. bakeri as a model for parasitic nematodes in general. 

Methods
Adult worms were removed from the intestinal tract day 10 post-infection and isolated using a modified Baerman apparatus. They were washed three times with water and placed in Dulbecco’s modified eagle’s medium–high glucose (Sigma cat. D5796) where they were sexed and counted. Worms were either used immediately for feeding assays. Mixed sex groups of worms were incubated with or without beads ((108 beads per 5mL of media, Polysciences Inc., Cat. 18859-1for two days. Worms were euthanised with 70% ethanol. Images were acquired using the Zeiss Axio ZoomV.16 stereoscopic microscope (Bio Core Facility, Department of Biological Sciences, University of Calgary). Brightfield and fluorescence images were taken either under the the PlanNeoFluar Z 1x/0.25 FWD 56mm objective objective with the AxioCam High Resolution colour (HRc) and High-Resolution mono (HRm) cameras. Beads were counted manually by zooming into images by two independent observers.