Population Heterogeneity Of Three Entomogenous Fungi Caused Natural Epizootics Of Spiders And Cicadas In Forests

Cicadas are an important component of insect fauna of decidous broad-leave forests of North Subtropic, and spiders are important predators of global forests. Their populations rae often regulated by natural epizootics caused by entomopathogenic fungi. The knowledge of population heterogeneity of these fungi benefits understanding prevelence of the epizootics and their role in natural regulation of insect populations.Natural fungal epizootics of spiders and cicadas were investigated at Langyashan Forest Park, Huangfushan Nature Reserves in East Anhui, and Guniujiang Nature Reserve in Southern Anhui, revealing that Nomuraea atypicola is the main causal agent for spider epizootics, Metarhizium cylindrosporae for adult cicadas, and Isaria cicadae for nymphal cicadas. The molecular marks of ISSR were used for determination of genetic diversity of the 3 fungi to study population heterogeneity of the 3 fungi with relation to prevelence of their respective epizootics. Different genetic diversity and heterogeneity of interpopulation and intrapopulation were found.1. N. atypicola and epizootics in spiders Twenty-two isolates were obtained at Langyashan and Guniujiang. By 9 ISSR primers selectd, 117 bands were amplfied, 111 (94%) of which showed polymorphism. A hirerarchical dendrogram constructed with the UPGMA method revealed a high differentiation among the 22 isolates, with a Jaccard similarity coefficient at 0.48-0.88 (average 0.71). The 22 isolates were clustered into two groups at the coefficient = 0.60. The first group included 10 isolates from different spider species of Langyashan, with an average genetic similarity at 0.74, while the second group included 12 isolates from different spiders of Guniujiang, with average genetic similarity at 0.76. Intrapopulation similarity was higher than that of interpopulation, but none of the isolates were identical or highly similar, suggesting that no predominant epizootic strain existed in either population. The epizootics at both nature reserves were caused by a highly heterogenetic Nomuraea atypicola population. The genetic similarity of Nomuraea atypicola isolates was associated with their geographical origin instead of taxonomical host origin. 2. M. cylindrosporae and epizootics in cacadas By 10 ISSR primers selected, 103 DNA bands were amplified in 35 isolates from Langyashan and Huangfushan, in which 77 (75%) showed polymorphism. In general, the isolates from different spider hosts in the same place had low genetic differentiation, while the isolates from different places showed high differentiation. The 35 isolates had a Jaccard similarity coefficient at 0.29-1.00, They were clustered into two groups geologically at the coefficient = 0.33. The first group included 17 isolates from different cicada hosts of Langyashan, with an average genetic similarity at 0.98, and there were several identical isolates in this group. The second group included 18 isolates from different spiders of Guniujiang, with the similarity at 0.97, and there were also several identical isolates in the group. The similarity coefficient between the two group was small, only at 0.31. The above analyses suggested that M. cylindrosporae isolates from the same place had high genetic similarity, and low population heterogeneity. The two groups both owned several identical isolates, indicating that there were predominant epizootic strains existing in either population. The epizootics at both places were caused by a lowly heterogenetic population of M. cylindrosporae, with evident predominance.3. Isaria cicadae and epizootics in nymphal cacadas With 9 selected ISSR primers, a total of 97 bands from 48 isolates of Guniujiang were amplified, in which 77 (69%) showed polymorphism. Based on collection time, 52 I. cicadae isolates were divided into 4 subpopulations. The isolates collected in July of 2007 had the highest Nei's genetic diversity index (H) at 0.1868, Shannon's information index (I) at 0.2848, suggesting that genetic diversity of the isolates collected in summer was higher than those in any other season. Genetic distance ranged within 0.0503 and 0.1864, with the shortest between May, 2008 and July, 2008, and the longest between July, 2007 and May, 2008. The total genetic diversity (Ht) was 0.1740, the intrapopulation genetic diversity (Hs) 0.0926, the intrapopulation differentiation (Gst) 0.4679, and the gene flow (Nm) 0.5685. A Nm <1 suggests that the causal pathogen of an epizootic is genetically poorly differentiated and abundant identical predominant epizootic strain may result in reduction of genetic diversity because of genetic drift. It will go against adaptability of a pathogen to the environment. A poorly heterogenetic pathogem population with prominent predominance may cause a very transitory epzootic, but does not favor self maintenance of a pathogen and long-term suppression of a pest in ecosystem.4. Sequence analysis of ITS By the sequence analyse of internal transcribed spacers (ITS) , genetic diversity of some isolates of M. cylindrosporae and I. cicadae was tested. The results showed that there were no sequence difference of ITS among I. cicadae isolates, and there are only 2 base mutation appearing in M. cylindrosporae isolates. It suggested information deficiency for ITS used as a marker for detection of traces of genetic diversity difference.