STABLE AND UNSTABLE ENVIRONMENTS INFLUENCE THE GENETIC DIVERSITY OF THE NEMATODE TELADORSAGIA-CIRCUMCINCTA, A PARASITE OF SMALL RUMINANTS

Acclimation of animal parasitic nematodes in the laboratory is the res ult of environmental disturbance; moderate numbers of infective larvae are introduced into and develop in a few naive hosts (versus many hos ts with a resistance status to parasite infection under natural condit ions), and stable conditions such as convenient moisture and temperatu re (versus the unstable climatic environment in the field) are offered to the free-living stages. The acclimation of sheep and goat lines of the nematode Teladorsagia circumcincta in lambs was arranged in the f ollowing putative order of increasing disturbance: sheep line and high success of experimental infection, sheep line and poor success of inf ection, goat line and high success of infection, sheep line with very poor success of infection, goat line and poor success of infection, an d sheep isofemale line with founder and inbreeding effects. The geneti c variability was assessed using the enzymes glucose-phosphate isomera se, lactate dehydrogenase, malate dehydrogenase, mannose-phosphate iso merase, and phosphoglucomutase, in starch-gel electrophoresis. The ran king on increasing F-st values (increasing genetic differentiation) ob served between initially introduced and twice-passaged generations ran ged from 0.003 (sheep line in sheep with high infective success) to 0. 19 (sheep isofemale line) and matched to a certain extent with disturb ance. The introduction of a goat line in sheep was a major disturbance , whereas in sheep lines the major factor of variation was due to the founder effect, i.e., the effective number of nematodes introduced to seed the acclimated lines. The deficiency in heterozygotes, which rema ins largely unexplained, was not modified during acclimation. In most cases the introduction of worms from nature resulted in lower overall genetic variability in the subsequent laboratory-reared populations.