Tracking a heterosis effect in the field: tadpole resistance to parasites in the water frog hybridogenetic complex

Depending on the extent of evolutionary divergence among parent taxa, hybrids may suffer from a breakdown of co-adapted genes or may conversely exhibit vigour due to the heterosis effect, which confers advantages to increased genetic diversity. That last mechanism could explain the success of hybrids when hybridization zones are large and long lasting, such as in the water frog hybridization complex. In this hybridogenetic system, hybrid individuals exhibit full heterozygosity that makes it possible to investigate in situ the impact of hybridization. We have compared parasite intensity between hybrid Rana esculenta and parental R. lessonae individuals at the tadpole stage in two populations inhabiting contrasted habitats. We estimated intensity of Gyrinicola sp. (Nematoda) in the gut, Echinostome metacercariae in the kidneys and Haplometra cylindracea in the body cavity (both species belong to Trematoda). Despite high sampling effort, no variation in parasite intensity was detected between taxa, except a possible higher tolerance to H. cylindracea in hybrid tadpoles. The low effect of hybridization suggests efficient gene co-adaptation between the two genomes that could result from hemiclonal selection. Variation in infection intensity among ponds could support the Red Queen hypothesis.