Morphological, Pathogenic And Genetic Variability Among Isolates Of The Nematophagous Species Of Hirsutella Spp.

Plant parasitic nematodes cause great losses in agricultural production. Hirsutella species colonize nematodes at high rate in nature and are considered as important fungal biological control agents. However, variability among isolates in morphology and pathogenicity were noted. This study is to demonstrate the variation among isolates of Hirsutella species in morphology, pathogenicity and genetics.Morphological comparison showed that there was distinct difference among species of Hirsutella, and isolates within one species are also a little bit different from each other. Isolates of H. rhossiliensis were different in lengths of conidiophores regardless of their host or geographical origin. ARSEF2789, isolated from male of Heterodera schatchi, had a whitish colony, indicating its similarity to H. rhossiliensis. But its short conidiophores and globosal conidia suggested that it was closer to H. minnesotensis. ITS sequence analysis indicated that it belonged to H. rhossiliensis.Pathogenicity tests showed that H. rhossiliensis isolates WT29-2 from saprophytic nematode, ARSEF2894 from Heterodera humili, ARSEF 3755 from Rotylenchus robusta, H. minnesotensis isolate ARSEF2799 from springtail of insect, and H. thompsonii isolates had no or week parasitism to the 6 tested nematodes. Parasitism of H. minnesotensis isolates was high to cyst nematodes (Heterodera glycines and H.avenae) but week to other 4 nematodes (Meloidogyne sp., Bursaphelenchus xylophilus. Heterorhabdus bacteriphora, Steinernema sp.). Other isolates parasitized all of 6 nematodes at a moderate level. It was concluded that there was host specificity among Hirsutella isolates, but races or varieties could not be distinguished.Genetic variation among 29 isolates of Hirsutella spp. was studied via ITS sequences analysis. The 3 species were clustered into their own corresponding clades. Two H. rhossiliensis isolates, WT29-2 and JA16-1 obtained from saprophytic nematodes, was clustered in one clade. ARSEF2789, which was distinctly different from other isolates in morphology, constituted another clade. All the remaining isolates were clustered into another one clade. The 7 isolates of H. minnesotensis formed two clusters, which was closely related with their geographical origins. One cluster contained isolates from China, the other cluster contained isolates from North America. H. thompsonii formed anindependent cluster separable from that of H. rhossiliensis and H. minnesotensis.Two previously unidentified isolates, ARSEF2799 and ARSEF2795, were classified into H. rhossiliensis and H.minnesotensis, respectively, based on their morphology, pathogenecity and ITS sequence, even though their morphology and parasitism might not match their corresponding species characteristics so well.