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Abstract:
Background: Many parasites show an extraordinary degree of host specificity, even though a narrow range of host
species reduces the likelihood of successful transmission. In this study, we evaluate the genetic basis of host specificity
and transmission success of experimental F1 hybrids from two closely related tapeworm species (Schistocephalus solidus
and S. pungitii), both highly specific to their respective vertebrate second intermediate hosts (three- and nine-spined
sticklebacks, respectively).
Methods: We used an in vitro breeding system to hybridize Schistocephalus solidus and S. pungitii; hybridization rate
was quantified using microsatellite markers. We measured several fitness relevant traits in pure lines of the parental
parasite species as well as in their hybrids: hatching rates, infection rates in the copepod first host, and infection rates
and growth in the two species of stickleback second hosts.
Results: We show that the parasites can hybridize in the in vitro system, although the proportion of self-fertilized
offspring was higher in the heterospecific breeding pairs than in the control pure parental species. Hybrids have a lower
hatching rate, but do not show any disadvantages in infection of copepods. In fish, hybrids were able to infect both
stickleback species with equal frequency, whereas the pure lines were only able to infect their normal host species.
Conclusions: Although not yet documented in nature, our study shows that hybridization in Schistocephalus spp. is in
principle possible and that, in respect to their expanded host range, the hybrids are fitter. Further studies are needed to
find the reason for the maintenance of the species boundaries in wild populations.
