No evidence for schistosome parasite fitness trade-offs in the intermediate and definitive host

Abstract

Background: The trematode parasite Schistosoma mansoni uses an aquatic snail intermediate and a vertebrate definitive host to complete its life cycle. We previously showed that a key transmission trait-the number of cercariae larvae shed from infected Biomphalaria spp. snails-varies significantly within and between different parasite populations and is genetically controlled by five loci. We investigated the hypothesis that the success of parasite genotypes showing high propagative fitness in the intermediate snail host may be offset by lower reproductive fitness in the definitive vertebrate host.

Methods: We investigated this trade-off hypothesis by selecting parasite progeny producing high or low number of larvae in the snail and then comparing fitness parameters and virulence in the rodent host. We infected inbred BALB/c mice using two Schistosoma mansoni parasite lines [high shedder (HS) and low shedder (LS) lines] isolated from F2 progeny generated by genetic crosses between SmLE (HS parent) and SmBRE (LS parent) parasites. We used the F3 progeny to infect two populations of inbred Biomphalaria glabrata snails. We then compared life history traits and virulence of these two selected parasite lines in the rodent host to understand pleiotropic effects of genes determining cercarial shedding in parasites infecting the definitive host.

Results: HS parasites shed high numbers of cercariae, which had a detrimental impact on snail physiology (measured by laccase-like activity and hemoglobin rate), regardless of the snail genetic background. In contrast, selected LS parasites shed fewer cercariae and had a lower impact on snail physiology. Similarly, HS worms have a higher reproductive fitness and produced more viable F3 miracidia larvae than LS parasites. This increase in transmission is correlated with an increase in virulence toward the rodent host, characterized by stronger hepato-splenomegaly and hepatic fibrosis.

Conclusions: These experiments revealed that schistosome parasite propagative and reproductive fitness was positively correlated in intermediate and definitive host (positive pleiotropy). Therefore, we rejected our trade-off hypothesis. We also showed that our selected schistosome lines exhibited low and high shedding phenotype regardless of the intermediate snail host genetic background. ​.

Keywords: Biomphalaria snail host; Genetic crosses; Positive pleiotropy; Rodent host; Schistosoma parasite; Selection; Virulence-transmission trade-offs.

Fig. 1

Outline of the parasite genetic crosses and the timeline of life history traits measured in hosts. (A) We performed two independent reciprocal genetic crosses between single genotypes of SmLE-HS and SmBRE-LS Schistosoma mansoni parasite populations exhibiting striking differences in terms of virulence and transmission stage production (number of cercariae produced) [4, 16]. For both crosses, we generated F3 progeny by selecting F2 parasites exhibiting both extreme shedding phenotypes. (B) We measured fitness and transmission traits of HS and LS parasites in the rodent host (BALB/c female mice) during the infection (growth and hematocrit rate) and at euthanasia (parasitic worm burden, parasite reproductive fitness, mouse spleen and liver weight, and liver histopathology). We also measured fitness and transmission phenotypes of HS and LS parasites infecting two different Biomphalaria glabrata inbred snails (Bg26 and Bg121) during the patent period of the infection (survival and cercarial production). We evaluated the virulence of these parasite lines by measuring the snail survival rate, total laccase-like activity and hemoglobin rate in infected snail hemolymph samples

Fig. 2

Differential impact of LS and HS parasites on rodent host life history traits and pathology. LS and HS S. mansoni lines do not differentially affect the BALB/c growth rate compared to control group (A) or hematocrit rate (B) over the course of infection. However, mice infected with HS parasites exhibit greater hepatosplenomegaly compared to rodents infected with LS lines. (C) Ratio liver/body weight (%) and (D) ratio spleen/body weight (%)]. Similarly, livers from HS-infected mice exhibited higher egg density (E) associated with the formation of bigger granulomas (F) and increased hepatic fibrosis (G) compared to livers from LS infected mice. Groups (parasite lines or control) not connected by the same letter are significantly different (post hoc test). *P < 0.05; ** P ≤ 0.01; ***P ≤ 0.001

Fig. 3

Increased reproductive fitness of HS parasite line in rodents. (A) The ability of cercarial larvae to infect their rodent definitive host is similar for LS and HS lines, in both genetic crosses, as a comparable number of worms were recovered from mice infected with HS and LS parasites after being exposed to the same number of cercariae. (B) In both crosses, the HS parasite line produced significantly more live miracidia (recovered from rodent infected livers) compared to the LS parasites, and therefore (C) HS reproductive fitness (total number of miracidia recovered/total number of female worms) is higher than in LS parasites. NS: No significant difference between the considered groups; *P < 0.05; **P ≤ 0.01; ***P ≤ 0.001

Fig. 4

Differences between LS and HS parasite life history and transmission traits in different snail lines. HS and LS lines were founded by selecting F2 parasites exhibiting extremely low and extremely high shedding phenotypes for both genetic crosses (A, B) between SmBRE-LS and SmLE-HS. (A–B: F2 parents) For both crosses, F2 LS parents (in Bg26 snails) produced fewer cercariae (average over 4 weeks, one shedding/week) compared to F2 HS parents (in Bg26 snails) (Cross A: Wilcoxon test, W = 100, P = 1.083 × 10^–5; Cross B: Welsh t-test, t = 10.933, df = 35.838, P = 1.334 × 10^–8). Similarly, for both crosses (A–B: F3) and snail genetic background (Bg26 or Bg121), F3 progeny HS parasites produced significantly more cercariae (average over 4 weeks, one shedding/week) than F3 LS parasites (Cross A, F3-Bg26 snails: Welsh t-test: t = 5.8502, df = 80.313, P = 1.019 × 10^–7, F3-Bg121 snails: Welsh t-test: t = 10.933, df = 35.838, P = 5.76 × 10^–13; Cross B, F3-Bg26 snails: Wilcoxon test: Z = 3084, P = 8.88 × 10^–15, F3-Bg121 snails: Welsh t-test: t = 15.557, df = 39.28, P < 2.2 × 10^–16). *P < 0.05; **P ≤ 0.01; ***P ≤ 0.001

Fig. 5

Differential impact of HS and LS parasites on snail host life history traits and physiology. (A) We did not detect significant differences in the survival of the infected snails between populations (Bg26 or Bg121) or between infection groups (HS or LS schistosome parasites selected lines). (B, C) There was a strong impact of the infection group on the snail physiological parameters. (B) laccase-like activity (Welsh t-test: Cross A: F3-Bg26 infected with HS vs. LS, t = −11.126, df = 16.846, P = 3.50 × 10^–9; F3-Bg121 infected with HS vs. LS, t = −9.3019, df = 12.838, P = 4.57 × 10^–7; Cross B: F3-Bg26 infected with HS vs. LS, t = −11.329, df = 13.79, P = 2.28 × 10^–8; F3-Bg121 infected with HS vs. LS, t = −9.6534, df = 25.15, P = 6.11 × 10^–11) and (C) hemoglobin rate (Welsh t-test: Cross A: F3-Bg26 infected with HS vs. LS, t = −7.8325, df = 12.101, P = 4.429 × 10^–6; F3-Bg121 infected with HS vs. LS, t = −6.4313, df = 12.051, P = 3.185 × 10^–5; Cross B: F3-Bg26 infected with HS vs. LS, t = −6.0988, df = 11.636, P = 6.099 × 10^–5; F3-Bg121 infected with HS vs. LS, t = −7.3274, df = 14.353, P = 3.238 × 10^–6). Snails infected with HS schistosome parasites lines consistently exhibit lower laccase activity and hemoglobin rates compared to those infected with LS parasite lines. NS: No significant difference between the considered groups; *P < 0.05; **P ≤ 0.01; ***P ≤ 0.001

Fig. 6

Differential impact of schistosome selected lines on the phenotype of their intermediate and definitive hosts. Summary table of all the phenotypes measured for the selected HS and LS Schistosoma mansoni lines and the differential impact of these selected lines on the phenotype of their snail intermediate hosts and rodent definitive host