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 stag es) or/and environmentally (on the free-living stages) induced. In the present study the role a host (susceptible to the establishment of th e parasite and paltry resistant ones) and environmental changes expres sed by season (Autumn versus Spring) or year of sampling (1990 or 1992 ) were investigated. Five polymorphic enzymes were studied glucose-pho sphate-isomerase (GPI) lactate dehydrogenase (LDH), malate dehydrogena se (MHD), mannose-phosphate isomerase (MPI), and phosphoglucomutase (P GM). No significant difference in allelic frequencies was recorded bet ween 4 susceptible lambs from a flock grazed in 1990 as well as in 3 l ambs selected on their susceptibility or resistance to natural infecti on in a flock grazed in 1992. The mode of infection (natural - frequen tly repeated small infections, versus experimental - 3 repeated large infections) did not modify the allelic frequencies of the nematodes. T he allelic frequencies remained stable along the seasons and during th e 2 years investigated. The largest distance of Rogers (0.07) and F-st (0.012) were recorded between worms originating from susceptible and resistant lambs or in Autumn versus Spring populations of worms (0.08 and F-st 0.010). A deficiency in heterozygotes of the same magnitude w as recorded in the worm populations obtained from the different infect ions. The stability of allelic frequencies and genetic structure are s urprising when one considers the evolution of the same parasite in sev eral generations of laboratory conditions. This stability could be rel ated to the fact that levels of disturbance on parasitic and free-livi ng stages are neither too low (as in laboratory conditions) not too hi gh (as in farms when anthelmintic treatment are used regularly and des troy the adult stages).