The influence of parasite load on transcriptional activity and morphology of a cestode and its ant intermediate host
Parasites with complex life cycles are known to induce phenotypic changes in their intermediate hosts to increase transmission to the final host. The magnitude of these changes could increase with the number of parasites, which would be beneficial to co-infecting parasites. Yet, adverse effects of high parasite load (i.e., many parasites in a single host) might stress both hosts and parasites (e.g., through an increased immune response). We investigated the consequences of parasite load on the transcriptional activity and morphology of the cestode Anomotaenia brevis and its intermediate host, the ant Temnothorax nylanderi. We demonstrated that many differentially expressed host genes shifted with parasite load, and their functions indicate a stronger immune response and fight against oxidative stress in heavily infected hosts. The expression of other host genes responded to infection in an all-or-nothing manner, as did the morphology of the host workers. However, the cestodes became smaller when they competed with other parasites for resources from a single host. Their expression profile further indicated shifts in host immune avoidance, starvation resistance and vesicle-mediated transport. In summary, our study reveals clear consequences of parasite load and highlights specific processes and traits affected by this.
Steps to reproduce
Colonies of T. nylanderi were collected in November 2019 and February 2020 in the Lenneberg Forest near Mainz, Germany (50.011605” N, 8°10’48.6” E). Colonies of this ant species, whose workers are only about 2-3 mm long, include several dozen workers and inhabit dead branches and acorns on the forest floor. From the colonies we found, we selected 25 colonies, 21 containing infected workers (Table S1). Infection was initially determined by the yellow colouration of infected workers, a sign of infection (Scharf et al., 2012). The ants were transferred to slide nests consisting of a Plexiglas cavity between two slides and placed in plastered nest boxes with three chambers (Stoldt et al., 2021). Colonies were maintained at 20+/-1°C and fed with small crickets and honey twice per week. Water was provided ad libitum. The number of infected workers and queens were counted, and colonies were kept under these conditions for six months. At the end of July 2020, all infected workers and a sample of several uninfected workers per colony were collected and dissected (N of dissected, uninfected workers per colony see Table S1). Furthermore, we dissected five workers per colony from four uninfected colonies. For all ant workers that were dissected, the presence and number of cysticercoid cestode larvae was noted and their diameter was measured. Furthermore, worker head width was measured from eye to eye, as host body size was shown to decrease in T. nylanderi (Scharf et al., 2012) and other systems with increasing parasite load (Dingemanse et al., 2009). All measurements were taken under a Leica stereomicroscope 200x magnification using the LAS v4.5 software. The fat body of the ants was extracted within five minutes of starting the dissection. We focused our transcriptome analysis on this tissue because it serves as a storage organ and plays an important role in the production of proteins for immune defence, fighting aging, and fertility (Arrese & Soulages, 2010; Negroni, et al., 2019). We obtained the fat body by isolating the first cuticular segment of the abdomen and removing the gut and trachea. What remains were fat cells and the cuticular segment itself, which was promptly transferred to Eppendorf tubes containing 50µl of Trizol, frozen on dry ice and stored at -80°C until RNA extraction. The cestodes were removed from around the midgut, where they are usually located in the haemolymph and separately transferred into Eppendorf tubes containing 50µl of Trizol. They were also frozen on dry ice and stored at -80°C until RNA extraction.