Disease susceptibility is influenced by intestinal morphology and gut passage time in Daphnia dentifera

Background/Question/Methods

Interactions between invading pathogens and their hosts’ innate immune system are key determinants of infection success. When pathogens infect via the intestine, immune factors associated with intestinal morphology and physiology could be sources of variation in host susceptibility. We examined how intestinal characteristics of Daphnia dentifera relate to its susceptibility to the fungal pathogen Metschnikowia bicuspidata, a pathogen which D. dentifera acquire while filter feeding. First, we examined the relationship between gut passage time and pathogen susceptibility, and asked whether D. dentifera with longer gut passage times were more susceptible to infection than those with shorter gut passage times. Second, we examined the relationship between intestinal morphology and infection success. M. bicuspidata spores are thought to pierce through the host’s intestinal membrane, and we asked whether D. dentifera with thinner and/or more porous intestinal membranes were more susceptible to infection than those with thicker membranes and/or smaller intercellular spaces. We examined gut passage time by visually assessing passage of dye particles through D. dentifera gastrointestinal tracts, and examined intestinal morphology using tissue histology. Variation in traits was compared across D. dentifera genotypes and was related to infection status for genotypes challenged with M. bicuspidata genotypes and unchallenged controls.

Results/Conclusions

Across D. dentifera genotypes we found significant variation in gut passage time, intestinal thickness, and intestinal porosity, with each trait strongly linked to rates of infection with M. bicuspidata. We found a positive relationship between gut passage time and M. bicuspidata prevalence; those genotypes with the longest gut passage times exhibited the highest susceptibility to infection. Intestinal thickness was negatively correlated with pathogen prevalence, and the average intercellular space of epithelial cells was positively correlated with prevalence. Therefore, hosts with thinner and more porous intestinal membranes suffered the highest rates of infection with M. bicuspidata. These results suggest that, for pathogens that are acquired while feeding, host intestinal physiology and morphology can play important roles in pathogen resistance.