In the polymorphic Teladorsagia circumcincta (morphs circumcincta and trifurcata), a sheep and goat line (SGL) and a goat line (GL) have been previously described on the basis of the malate dehydrogenase allozyme polymorphism (MDH-2) and of the morphology of the dorsal ray. The GL were never found alone in 1 host, so the status of species was not given to these 2 lines. To investigate further whether there are other genetic markers that will delineate them, we collected T. circumcincta worms from goat and sheep at 8 farms in Touraine (west-central France). The worms were identified individually as being SGL or GL on the basis of MDH-2 polymorphism. This distinctiveness was corroborated by sequences of the β-tubulin isotype 1 gene, the second internal transcribed spacer (ITS2) of their rDNA, and the nicotinamide dehydrogenase (ND4) gene of their mDNA. The extent of the divergence in the 3 additional genetic markers was such that SGL and GL may be considered as 2 species. A third putative species was found in the SGL line based exclusively on the ND4 gene. These findings suggest that T. circumcincta is a species complex and that further investigation is required on a wider geographic scale.

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