The role of prezygotic isolation mechanisms in the divergence of two parasite species

Abstract

BackgroundThe formation of reproductive barriers in diverging lineages is a prerequisite to complete speciation according to the biological species concept. In parasites with complex life cycles, speciation may be driven by adaptation to different intermediate hosts, yet diverging lineages can still share the same definitive host where reproduction takes place. In these cases, prezygotic isolation mechanisms should evolve very early and be particularly strong, preventing costly unfavourable matings.In this study, we investigated the importance of prezygotic barriers to reproduction in two cestode species that diverged 20–25mya and show an extraordinary degree of specificity to different intermediate hosts. Both species share the same definitive hosts and hybridize in the laboratory. Yet, natural hybrids have so far not been detected.MethodsWe used a combination of different experiments to investigate the role of prezygotic barriers to reproduction in the speciation of these parasites. First, we investigated whether hybridization is possible under natural conditions by exposing lab-reared herring gulls (Larus argentatus, the definitive hosts) to both parasites of either sympatric or allopatric combinations. In a second experiment, we tested whether the parasites prefer conspecifics over parasites from a different species in dichotomous mate choice trials.ResultsOur results show that the two species hybridize under natural conditions with parasites originating either from sympatric or allopatric populations producing hybrid offspring. Surprisingly, the mate choice experiment indicated that both parasite species prefer mates of the different species to conspecifics.ConclusionsNeither fundamental constraints against hybridization in a natural host nor assortative mate choice sufficiently explain the persistent segregation of the two tapeworm species in nature. Hence, postzygotic ecological selection against hybrids is presumably the more important driving force limiting gene flow between the two parasite sister species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0799-5) contains supplementary material, which is available to authorized users.

The cestode parasite Schistocephalus pungitii: castrator or nutrient thief of ninespine stickleback fish?

“…Identification of S. pungitii was based on the reported host specificity of Schistocephalus in species of sticklebacks (Bråten 1966; Orr et al . 1969) and genetic and experimental analyses supporting the status of this cryptic species (Nishimura et al 2011; Henrich and Kalbe, 2016).…”

“…The blotted wet mass of each eviscerated fish carcass (body mass, BM) was measured to the nearest 0•001 g following dissection and evisceration of all internal organs except the kidney. Identification of S. pungitii was based on the reported host specificity of Schistocephalus in species of sticklebacks (Bråten 1966;Orr et al 1969) and genetic and experimental analyses supporting the status of this cryptic species (Nishimura et al 2011;Henrich and Kalbe, 2016).…”

“…The diphyllobothriidean parasite S. pungitii is a trophically transmitted parasite of the ninespine stickleback P. pungitius (Nishimura et al 2011; Henrich and Kalbe, 2016). The life cycle of S. pungitii includes a free-living coracidium larva, copepod (first intermediate host), ninespine stickleback (second intermediate host) and aquatic, piscivorous bird (definitive host), with almost all growth of the parasite occurring within the body cavity of the fish host (Smyth, 1962).…”

Characterisation and evaluation of interspecific cross-incompatibility in two Delphinium species

“…The examination of pollen viability was conducted with three plants of each species during anthesis based on in vivo germination method (He et al, 2017). In a full flowering stage, fresh pollen was collected every 2 h from three flowers per plant and was incubated for 4 h at 25°C in the liquid medium (5% sucrose, 150 mg • L −1 boric acid and 20 mg • L −1 CaCl 2 ) from 9:30 to 15:30.…”

Molecular data exclude current hybridization between iguanas Conolophus marthae and C. subcristatus on Wolf Volcano (Galápagos Islands)

“…However, evolution may also promote prezygotic and/ or postzygotic reproductive isolation mechanisms (RIMs) that prevent hybridization. The formation of reproductive barriers can be either adaptive or a by-product of genetic differentiation between lineages (Henrich and Kalbe 2016). Additionally, hybridization may also cause genetic introgression and contamination of pure populations, endangering natural populations and species.…”