Abstract
When parasites invade paired structures of their host non-randomly, the resulting asymmetry may have both pathological and ecological significance. To facilitate the detection and visualisation of asymmetric infections we have developed a free software tool, Analysis of Symmetry of Parasitic Infections (ASPI). This tool has been implemented as an R package (https://cran.r-project.org/package=aspi) and a web application (https://wayland.shinyapps.io/aspi). ASPI can detect both consistent bias towards one side, and inconsistent bias in which the left side is favoured in some hosts and the right in others. Application of ASPI is demonstrated using previously unpublished data on the distribution of metacercariae of species of Diplostomum von Nordmann, 1832 in the eyes of ruffe Gymnocephalus cernua (Linnaeus). Invasion of the lenses appeared to be random, with the proportion of metacercariae in the left and right lenses showing the pattern expected by chance. However, analysis of counts of metacercariae from the humors, choroid and retina revealed asymmetry between eyes in 38% of host fish.
Highlights
Bilateral asymmetry arises when parasites infect paired structures non-randomly
One side is consistently favoured in all hosts and examples include: echinostome trematodes preferentially invading the right kidney of their larval amphibian intermediate host (Thiemann and Wassersug 2000, Johnson et al 2014); and the monogenean Entobdella soleae migrating to the lower surface of its flatfish host, Solea solea (Linnaeus)
The second is an inconsistent bias, with the left side favoured in some hosts and the right in others, as observed in the distribution of metacercariae of species of Diplostomum von Nordmann, 1832 (Trematoda) in the eyes of fish (Rau et al 1979, Dwyer and Smith 1989, Graczyk 1991, Rintamäki-Kinnunen et al 2004, Karvonen and Seppala 2008) and larvae of the dipteran Protocalliphora avium Shannon et Dobroscky in the ears of the hawk, Buteo lineatus (Gmelin)
Summary
Bilateral asymmetry arises when parasites infect paired structures non-randomly. Two types of asymmetry may be observed. The second is an inconsistent bias, with the left side favoured in some hosts and the right in others, as observed in the distribution of metacercariae of species of Diplostomum von Nordmann, 1832 (Trematoda) in the eyes of fish (Rau et al 1979, Dwyer and Smith 1989, Graczyk 1991, Rintamäki-Kinnunen et al 2004, Karvonen and Seppala 2008) and larvae of the dipteran Protocalliphora avium Shannon et Dobroscky in the ears of the hawk, Buteo lineatus (Gmelin) (see King et al 2010). Bilateral asymmetry may have adaptive significance; through localising the effects of infection it may minimise the pathogenicity of the parasite, with benefits for the fitness of both host and parasite (Johnson et al 2014). ASPI is demonstrated here using observations on the distribution of metacercariae of Diplostomum spp. in the eyes of ruffe, Gymnocephalus cernua (Linnaeus) from Llyn Tegid, a mesotrophic lake in north Wales
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