The prevalence of infection varies dramatically on a fine spatial scale. Many evolutionary hypotheses are founded on the assumption that this variation is due to host genetics, such that sites with a high frequency of alleles conferring susceptibility are associated with higher infection prevalence. This assumption is largely untested and may be compromised at finer spatial scales where gene flow between sites is high. We put this assumption to the test in a natural snail-trematode interaction in which host susceptibility is known to have a strong genetic basis. A decade of field sampling revealed substantial spatial variation in infection prevalence between 13 sites around a small lake. Laboratory assays replicated over 3 years demonstrate striking variation in host susceptibility among sites in spite of high levels of gene flow between sites. We find that mean susceptibility can explain more than one-third of the observed variation in mean infection prevalence among sites. We estimate that variation in susceptibility and exposure together can explain the majority of variation in prevalence. Overall, our findings in this natural host-parasite system argue that spatial variation in infection prevalence covaries strongly with variation in the distribution of genetically based susceptibility, even at a fine spatial scale.
Gibson2016_AmNat_InfectionPrevalence
Individual-based data for analysis of infection prevalence. Each row gives collection and dissection data for an individual female snail. Columns: 1) “year” gives year of collection (2006-2015); 2) “sites” gives site of collection (13 sites); 3) length” gives the shell length of the female in millimeters, a proxy for age; and 4) “mic” gives the infection status of the female (0 = healthy, 1 = Microphallus infection).
Gibson2016_AmNat_ArtificialInoculations
Infection data for exposed and control replicates in 2013, 2014, and 2015 artificial inoculations. Each row gives identifying information and infection data for an experimental replicate. Columns: 1) “year” gives the year of the artificial inoculation experiment (2013-2015); 2) “site” gives the site from which experimental juveniles were collected (13 sites); 3) “treatment” gives experimental treatment. Control replicates did not receive parasites and are marked as 0. Infections in control replicates were acquired prior to collection of juvenile snails from the field and reflect early-life exposure to parasites. Exposed replicates received field-collected parasites and are marked as 1. Infections in exposed replicates are the sum of infections acquired in the field and after exposure to high doses of parasites. Susceptibility is defined as the infection rate in exposed replicates; 4) “source” applies to the 2014 artificial inoculation experiment in which two distinct sources of parasites were used for inoculations. Replicates exposed to source 1 (southern end of lake) are marked as 1, replicates exposed to source 2 (northeastern site, JMS) are marked as 2, and control replicates are marked as 0; 5) “replicate” indicates the experimental replicate, nested with year*site; 6) “uninfected” gives the number of female snails that were healthy in the experimental replicate; and 7) “infected” gives the number of female snails infected with Microphallus in the experimental replicate.
Gibson2016_AmNat_GPSCoordinates
GPS coordinates used in spatial analyses of infection prevalence and susceptibility. Latitude and longitude given in decimal degrees. Coordinates obtained in Google Earth.
Gibson2016_AmNat_GeographicDistance
Geographic distances used in Mantel tests for infection prevalence and susceptibility. Straight line distances given in meters. Distances obtained in Google Earth.
Gibson2016_AmNat_Mortality
Mortality data for 2013, 2014, and 2015 artificial inoculations. Each row gives identifying information and infection data for an experimental replicate. Columns: 1) “year” gives the year of the artificial inoculation experiment (2013-2015); 2) “site” gives the site from which experimental juveniles were collected (13 sites); 3) “treatment” gives experimental treatment. Control replicates did not receive parasites and are marked as 0. Infections in control replicates were acquired prior to collection of juvenile snails from the field and reflect early-life exposure to parasites. Exposed replicates received field-collected parasites and are marked as 1; 4) “source” applies to the 2014 artificial inoculation experiment in which two distinct sources of parasites were used for inoculations. Replicates exposed to source 1 (southern end of lake) are marked as 1, replicates exposed to source 2 (northeastern site, JMS) are marked as 2, and control replicates are marked as 0; 5) “replicate” indicates the experimental replicate, nested with year*site; 5) “total” gives the total number of female snails in the experimental replicate at initiation of the experiment; 6) “present” gives the number of female snails that were present (i.e. alive) at the time of dissection of the experimental replicate; and 7) “absent” is “total” minus “present” – the number of female snails missing (i.e. presumed dead) at the time of dissection of the replicate.
