Host, season and year do not play an important role on genetic variability in a trichostrongyle nematode as assessed from allozymes.

The isolates of the trichostrongyle nematode Teladorsagia circumcincta in sheep may vary from one site to another as assessed from allozyme studies. This genetic variability could be host (on the parasitic stages) or/and environmentally (on the free-living stages) induced. In the present study the role of host (susceptible to the establishment of the parasite and partly resistant ones) and environmental changes expressed by season (Autumn versus Spring) or year of sampling (1990 or 1992) were investigated. Five polymorphic enzymes were studied: glucose-phosphate-isomerase (GPI), lactate dehydrogenase (LDH), malate dehydrogenase (MHD), mannose-phosphate isomerase (MPI), and phosphoglucomutase (PGM). No significant difference in allelic frequencies was recorded between 4 susceptible lambs from a flock grazed in 1990 as well as in 3 lambs selected on their susceptibility or resistance to natural infection in a flock grazed in 1992. The mode of infection (natural--frequently repeated small infections, versus experimental--3 repeated large infections) did not modify the allelic frequencies of the nematodes. The allelic frequencies remained stable along the seasons and during the 2 years investigated. The largest distance of Rogers (0.07) and Fst (0.012) were recorded between worms originating from susceptible and resistant lambs or in Autumn versus Spring populations of worms (0.08 and Fst 0.010). A deficiency in heterozygotes of the same magnitude was recorded in the worm populations obtained from the different infections. The stability of allelic frequencies and genetic structure are surprising when one considers the evolution of the same parasite in several generations of laboratory conditions. This stability could be related to the fact that levels of disturbance on parasitic and free-living stages are neither too low (as in laboratory conditions) nor too high (as in farms when anthelmintic treatment are used regularly and destroy the adult stages).