Abstracts (first author)
Host-specificity in hybrids of two Schistocephalus species with different stickleback hosts
Schistocephalus solidus is a highly specific tapeworm that only infects the three spined stickleback Gasterosteus aculeatus as a second intermediate host. The closely related S. pungitii uses the nine-spined stickleback Pungitius pungitius as second intermediate host, showing the same host specificity at this level. Both parasites potentially share the same final hosts – piscivorous birds - and can occur in sympatry. It is therefore possible that natural hybridization takes place but they have not been detected. We used an in vitro breeding system to hybridize S. solidus and S. pungitii and quantified the hybridization rate 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. 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. This suggests that the hybrids have given up the host specificity and raises the question why natural hybrids have not been identified in the wild. We are currently investigating pre- and postzygotic mechanisms that might prevent natural hybridization. Our results furthermore indicate a co-dominance of the genes responsible for infection of different fish hosts. This system offers the unique possibility to investigate the genetic mechanisms underlying this host specificity using backcrosses and a QTL approach.