Schistocephalus solidus: a molecular test of premature gamete exchange for fertilization in the intermediate host Gasterosteus aculeatus.

We tested the hypothesis that the cestode Schistocephalus solidus is capable of premature gamete exchange as a plerocercoid in the last intermediate stickleback host. The existence of such a reproductive mode is suggested by the highly advanced gonadal development in the plerocercoid and the large fitness gain of outcrossing. In addition, eggs from selfing cestodes have a higher hatching rate when the cestode originated from a doubly infected stickleback than when it came from a singly infected fish. We hatched eggs from 10 singly breeding cestodes that originated from doubly infected sticklebacks with the prediction that some should be outcrossed and share alleles with both the breeding cestode and the second cestode in the coinfection if the hypothesis is correct. However, all of the 430 tested larvae matched only the alleles of the breeding cestode. It is therefore very unlikely that S. solidus engaged in gamete exchange in its fish host. We suggest an alternative hypothesis to explain the higher hatching rate of eggs produced by cestodes from doubly infected fish as compared to those from single infections.