Effects of host outcrossing on the interaction between an aquatic snail and its locally adapted parasite

Abstract

In this study, we investigated the interaction between host outcrossing and infection in the Biomphalaria glabrata-Schistosoma mansoni system. Snails collected from three susceptible isofemale lines were mated with either siblings or snails recently derived from a field site in Brazil. Resulting inbred and outcrossed progeny were then exposed to S. mansoni larvae and monitored for a 10-week period. Interestingly, all snails exhibited equal susceptibility whether they were the result of inbreeding or outcrossing. However, further examination of both host and parasite life-history traits uncovered significant differences between the groups. In uninfected snails, outcrossed progeny tended to exhibit greater fitness relative to inbred progeny. When snails were parasitized, these differences were magnified in certain life-history traits, particularly host reproduction and survival. As an extension of the work, we also investigated virulence within this host-parasite system. Estimates of parasite reproduction and host size were combined to generate a novel "exploitation index," and these indices were regressed with host survivorship. As predicted, there was a significant and negative correlation between the variables, but this was restricted to a single snail line. Results from this study demonstrate that infection outcomes (as measured by prevalence) may not differ between inbred and outcrossed hosts. However, outcrossing may enhance snail fitness through life-history trait expression.

Female biased sex-ratio in Schistosoma mansoni after exposure to an allopatric intermediate host strain of Biomphalaria glabrata

“…Many authors have observed differences in infectivity when the parasite is exposed to sympatric or allopatric snails (Woolhouse 1989;Webster & Woolhouse 1999;Sandland et al 2009;Jones-Nelson et al 2011). Geographic patterns of compatibility between schistosomes and snails suggest that there is a tendency for sympatric combinations to be more compatible than allopatric combinations (Sandland et al, 2009). This pattern is in agreement with the patterns reported for other parasite-host systems (Sandland et al, 2009).…”

“…The impact of allopatric vs sympatric associations on host-parasite compatibility has been extensively studied in schistosome-snail interactions. Many authors have observed differences in infectivity when the parasite is exposed to sympatric or allopatric snails (Woolhouse 1989;Webster & Woolhouse 1999;Sandland et al 2009;Jones-Nelson et al 2011). Geographic patterns of compatibility between schistosomes and snails suggest that there is a tendency for sympatric combinations to be more compatible than allopatric combinations (Sandland et al, 2009).…”

Transmission dynamics of two strains of Schistosoma mansoni utilizing novel intermediate and definitive hosts

“…Together with the fact that outcrossed parasites produced more cercariae than inbred parasites, this could be evidence of a more successful diversion of host resources by outcrossed parasites. Furthermore, increased mortality of outcrossed-infected snails is also consistent with increased exploitation and exhaustion of host energetic reserves (Woolhouse 1989;Dekock 1992;Webster and Woolhouse 1999;Sandland et al 2009). …”

“…Previous works by Sandland et al (2007Sandland et al ( , 2009) and Zavodna et al (2008) have made considerable headway into investigating the effects of host genetic variation on infection outcomes and host and parasite life history strategies. The study presented here builds upon that foundation by including parasite genetic variation and extending the work to examine how genetic background of all three species involved in the complex life cycle of S. mansoni can affect parasite transmission dynamics.…”

“…To induce shedding of schistosome larvae (cercariae), snails were placed individually into 10-mL plastic wells with approximately 7 to 8 mL of well water and placed under fluorescent lights for 5 h. Snails were classified as infected or uninfected based on presence or absence of cercariae. As in Sandland et al (2007Sandland et al ( , 2009 and Zavodna et al (2008), after parasite shedding, snails were removed from their wells, and additional water was added until each well contained approximately10 mL total. Well contents were mixed to ensure uniform suspension of cercariae and a 1 mL sample was taken from each well and added to a Petri dish with 70% ethanol to preserve cercariae for counting.…”