Exposure to parasites increases promiscuity in a freshwater snail

Under the Red Queen hypothesis, outcrossing can produce genetically variable progeny, which may be more resistant, on average, to locally adapted parasites. Mating with multiple partners may enhance this resistance by further increasing the genetic variation among offspring. We exposed Potamopyrgus antipodarum to the eggs of a sterilizing, trematode parasite and tested whether this altered mating behaviour. We found that exposure to parasites increased the number of snail mating pairs and the total number of different mating partners for both males and females. Thus, our results suggest that, in host populations under parasite-mediated selection, exposure to infective propagules increases the rate of mating and the number of mates.

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Trematode parasites infect or die in snail hosts

Biology Letters ◽

10.1098/rsbl.2010.0857 ◽

2010 ◽

Vol 7 (2) ◽

pp. 265-268 ◽

Cited By ~ 21

Author(s):

Kayla C. King ◽

Jukka Jokela ◽

Curtis M. Lively

Keyword(s):

Immune System ◽

Digestive Tract ◽

Freshwater Snail ◽

Red Queen Hypothesis ◽

Red Queen ◽

Trematode Parasite ◽

Local Host ◽

Second Chance ◽

The Red Queen ◽

Trematode Parasites

The Red Queen hypothesis is based on the assumption that parasites must genetically match their hosts to infect them successfully. If the parasites fail, they are assumed to be killed by the host's immune system. Here, we tested this using sympatric (mostly susceptible) and allopatric (mostly resistant) populations of a freshwater snail and its trematode parasite. We determined whether parasites which do not infect are either killed or passed through the host's digestive tract and remain infectious. Our results show that parasites do not get a second chance: they either infect or are killed by the host. The results suggest strong selection against parasites that are not adapted to local host genotypes.

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Parasite Selection and the Fitness of Sexual Reproduction

IU Journal of Undergraduate Research ◽

10.14434/iujur.v1i1.13725 ◽

2015 ◽

Vol 1 (1) ◽

pp. 36

Author(s):

Samantha A. Klosak

Keyword(s):

Sexual Reproduction ◽

Natural Population ◽

Reproductive Mode ◽

Red Queen Hypothesis ◽

Red Queen ◽

Trematode Parasite ◽

Maintenance Of Sex ◽

Fitness Consequence ◽

Offspring Generation ◽

The Red Queen

Sexual reproduction is a very costly process: the growth rate of asexual lineages exceeds that of sexual lineages. Nonetheless, sex is prevalent in nature. The Red Queen Hypothesis argues that, because sex and recombination generate genetically variable offspring that may escape infection by coevolving parasites, parasites select for sex in hosts (Lively & Dybdahl, 2000). Our research directly tests if the Red Queen can explain the maintenance of sex in a natural population. This experiment focuses on a natural population of the snail Potamopyrgus antipodarum, which is native to New Zealand. Individuals of this species are either diploid and sexually reproducing or triploid and asexually reproducing (Lively & Osnas, 2006). This snail is naturally infected by the trematode parasite, Microphallus, which is sterilizing and thus exerts strong selection on its host. The Red Queen Hypothesis predicts that parasites should periodically increase the fitness of sexual relative to asexual individuals. We accordingly established mesocosms containing both sexual and asexual snails from the same natural population. Half of these mesocosms were exposed to parasites and the other half were not. These snails were then allowed to reproduce over the course of a year. We used flow cytometry to determine the frequency of diploids in the parents and the offspring, and thereby the fitness of sexual individuals in the presence and absence of parasites. Interestingly, we find that sexual individuals are currently more susceptible to parasites than are asexual individuals. In tanks in which sexual parents are relatively more infected, the frequency of sexual individuals declined significantly more in the offspring generation, indicating a fitness consequence of parasitism for sexual reproduction. Our findings suggest that parasite selection can indeed operate on reproductive mode. Moreover, our results are consistent with theory (King, Delph, Jokela, & Lively, 2009) and a prior field study in our system indicating that the direction of parasite selection is variable, such that parasites periodically select against sexual reproduction (Vergara, Lively, King, & Jokela, 2013). This current experiment will continue for multiple years in order to track the variation in parasite selection on sex through time.

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Trematode infection and the distribution and dynamics of parthenogenetic snail populations

Parasitology ◽

10.1017/s0031182001008113 ◽

2001 ◽

Vol 123 (7) ◽

pp. 19-26 ◽

Cited By ~ 24

Author(s):

C. M. LIVELY

Keyword(s):

Coefficient Of Variation ◽

Experimental Studies ◽

Reproductive Mode ◽

Freshwater Snail ◽

Red Queen Hypothesis ◽

Red Queen ◽

In The Wild ◽

Clonal Lines ◽

Asexual Populations ◽

The Red Queen

According to the Red Queen hypothesis for sex, cross-fertilization should be positively associated with the probability of exposure (risk) to virulent parasites. Unfortunately, risk is difficult to measure in the wild, and prevalence of infection is often substituted for risk. Here I suggest that prevalence of infection may not generally suffice as a surrogate for risk, since the Red Queen model can make opposite predictions depending on the distribution of risk in the wild. Specifically, the results of a matching-alleles model suggest that asexual populations should be more infected than sexual populations, when (1) the variance in risk among populations is small, and (2) the mean risk of exposure to parasites is near the point where selection switches to favouring sex over asex. If, however, the variance in risk among populations is large, sexual reproduction should be positively associated with the prevalence of infection. In addition, the coefficient of variation for reproductive mode should increase sharply at the switch point. In light of these results, I re-evaluated data from two studies on the distribution of males in 95 populations of a freshwater snail (Potamopyrgus antipodarum). Populations of these snails are often mixtures of sexual and asexual individuals, and the frequency of males is correlated with the frequency of sexual females in the population. The results show a large, highly skewed variance among populations for prevalence of infection by larval trematodes. The results also show a positive, significant relationship between prevalence of infection and the frequency of males, with a sharp increase in the coefficient of variation at intermediate prevalence. In addition, experimental studies suggest that some of the necessary conditions of the Red Queen hypothesis are also met in this system. Specifically, the most common trematode infecting these snails is (1) adapted to infecting local host populations of the snail, and is (2) more infective to clones that were common in the recent past. It is too early to know if the parasite theory is sufficient to explain the widespread distribution of sex. I suggest that the theory is not sufficient, but that parasites in combination with mutation accumulation in clonal lines may explain the maintenance of sex in species that occasionally produce apomictic mutants.

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