Systematics and evolution of host specificity in Erysiphe cichoracearum

The purpose of this study was to determine the number of host-limited forms and test for correlated genetic differences in the complex powdery mildew species Erysiphe cichoracearum (E.c.), and to compare patterns of parasite-host association to determine if evolution has proceeded primarily through cospeciation, or if colonization has played a more important role. E.c. has been reported from over 300 hosts distributed among at least eight plant families. However, experimental evidence indicates isolates of E.c. only infect members of single host families or genera. This evidence divides E.c. into numerous biotypes; however, genetic distances between biotypes have been difficult to quantify. Individual host-limited forms cannot be differentiated morphologically, and their obligately parasitic nature has historically made them difficult to study. Restriction fragment length polymorphisms (RFLPs) from a segment of the ribosomal DNA (rDNA) have been used to characterize genetic divergence among isolates of E.c. collected from several of its host families and genera. These RFLP profiles were also compared to profiles from other, mildew species to estimate divergence among morphological species. Comparison of RFLP types of E.c. indicate that considerable divergence has occurred within the species. E.c. is composed of at least seven statistically distinct RFLP types. Divergence between RFLP types is similar to that found between some other species or genera of fungi, and suggests that these RFLP forms may be evolutionarily separate species. The divergence between E.c. and other mildew morphological species was also extensive, and was always greater than divergence among E.c. isolates themselves. Analyses of parasite and host phylogenies indicate strong host limits for biotypes of E.c., that restricts them to sets of closely related hosts. However, other evidence suggests that evolution in E.c. has been punctuated by episodes of colonization. Thus, considerable support suggests that colonization of more distantly related hosts has been common in their evolution. Evolution within E.c. has proceeded through a process of diffuse coevolution with its hosts