| Summary: | <p>Abstract</p> <p>Background</p> <p>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 F<sub>1</sub> hybrids from two closely related tapeworm species (<it>Schistocephalus solidus</it> and <it>S. pungitii</it>), both highly specific to their respective vertebrate second intermediate hosts (three- and nine-spined sticklebacks, respectively).</p> <p>Methods</p> <p>We used an <it>in vitro</it> breeding system to hybridize <it>Schistocephalus solidus</it> and <it>S. pungitii</it>; 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.</p> <p>Results</p> <p>We show that the parasites can hybridize in the <it>in vitro</it> 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.</p> <p>Conclusions</p> <p>Although not yet documented in nature, our study shows that hybridization in <it>Schistocephalus</it> 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.</p>
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