Résumé
Temporal isolation is probably the least studied speciation mechanism, in particular in fungi. We studied whether temporal isolation could be responsible for the maintenance of genetic differentiation among sympatric populations of Ampelomyces, widespread intracellular mycoparasites of powdery mildew (PM) fungi, whose life cycle is tied to that of their host PM species. Both ITS sequence and microsatellite analyses showed that European Ampelomyces populations collected from apple PM (Podosphaera leucotricha) are genetically highly differentiated from Ampelomyces populations sampled from many other PM species infecting plant hosts other than apple. While P. leucotricha starts its life cycle in spring, and apple PM epidemics mainly occur before mid-summer, the mycohosts of the other Ampelomyces strains cause epidemics essentially in autumn. When we experimentally exposed two PM species to Ampelomyces strains naturally occurring in apple PM in spring and to strains naturally present in other host PM species in autumn, cross infections always occurred, even with PM species Ampelomyces is unlikely to encounter in nature. Thus, the host-related genetic differentiation in Ampelomyces cannot be explained by a narrow mycohost specialization, but could instead result from temporal isolation.