Parasite spillover rather than niche expansion explains infection of host brain by diplostomid eye flukes

Parasites often occupy specific sites within their host, which has important implications for host performance and parasite transmission. Nonetheless, parasitic infections can occur beyond their typical location within a host, significantly altering host-parasite interactions. Yet, the causes behind the atypical tissue tropism are poorly understood. Here, we focus on a ubiquitous group of diplostomid parasites that form diverse communities in fish eyes. We used targeted DNA metabarcoding (COX1, 250 bp) to evaluate potential mechanisms underlying atypical tissue tropism from predominantly eye to brain infection in two widespread fish species (Eurasian perch and common roach). We found that the most common eye-infecting species (Tylodelphys clavata, Diplostomum baeri) are present in the brains of perch but not in roach. The bipartite network comprising five species and 24 mitochondrial haplotypes revealed no brain-specific haplotypes, indicating an apparent lack of genetic divergence between brain and eye-infecting parasites. Instead, the prevalence, intensity, and diversity of eye infections were positively correlated with brain infections. Thus, our results suggest that the most parsimonious mechanism underlying brain infection is density-dependent spillover rather than parasite divergence-driven niche expansion. We anticipate that “off-target” infections are likely to be severely underestimated in nature with severe ecological, evolutionary, and medical implications.

Methods
The data set was obtained by the metabarcoding characterisation of the diplostomids communties in the eyes and brain of Eurasian perch collected at seven lakes in souther Estonia. This files includes the bioinformatic pipeline to process the raw seqeunces (2 x 300 bp read length, Illumina Inc., San Diego, California, USA) and the R code for the down stream analysis.