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.

Two morphological types of the viviparous oxyurid nematode Gyrinicola were recovered from waterfrog tadpoles in France (Ain department, Rhône-Alpes region) and assigned to two species: G. tba (Dinnik 1930), redescribed, and G. chabadamsoni n. sp. Gyrinicola tba is close to G. batrachiensis (Walton 1929) and G. chabaudi Araujo and Artigas 1982 in that the female possesses a long tail and few cuticular buccal flaps. The new species, type host Rana kl. esculenta, is distinct by several characters: in females, by the 12 cuticular buccal flaps with internal crests, short caudal filament, short ovary producing embryonated eggs and large uterine pouch containing males and embryos, like in G. japonica Yamaguti 1938, and narrower thick-shelled eggs with subapical elongated operculum; in males, by the round genital cone, the third caudal pair of papillae at mid tail, length of the body and caudal part. Gyrinicola tba and the new species were found in R. lessonae and in the hybrid R. kl. esculenta but each with different seasonal infection rates. Comparative analyses of the data from the five Gyrinicola species were done. All females have one genital tract producing thick-shelled female eggs. The second genital tract is one of four types corresponding to different reproductive patterns: i) it forms a uterine pouch containing males in G. chabadamsoni and G. japonica; ii) it forms a slender uterus containing numerous autoinfective larval stages in G. batrachiensis from R. clamitans; iii) it forms a slender uterus containing only a few embryos in G. tba and G. chabaudi; and iv) it has regressed in G. batrachiensis from Bufo americanus. Haplodiploidy is expected or proved for types i) and ii) and thelytoky for types iii) and iv).

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