Genetic Structure Of Fungal Entomopathogens Causing Natural Epizootics In Some Insect Populations

Entomopathogenic fungi are the entomopathogen recognized earliest and some ofthem play an important role in regulating the insect populations in the nature. Up to now,much has been achieved in epizootiology of insect diseases by use of traditionalmicrobiological and insect ecological techniques, and insect ecologica theories, which ofthem have been applied in controlling pests. In order to further reveal the nature of theoccurrence and dynamics of natural epizootics caused by entomopathogenic fungi,genetical study of populations of the causal agents in relation to genetic differentiation of apopulation is much needed to help more rational regulation of the entomopathogenic fungiand associated arthropods in pest control and beneficial insects protection. In the presentdissertation, genetic structures of10populations of entomopathogenic fungi causingnatural epizootics were studied by molecular ecological methods. The impacts of geneflow and other estimates on genetic differentiation of the fungal populations were analyzedwith the theories of molecular ecology.1. The results of ISSR-PCR analyses on the different entomopathogenic fungi showedthat there existed rich polymorphism in the isolates of the10populations of fungalentomopathogens and showed good reproducibility, indicating that the ISSRs are goodmarkers for evaluation of the genetic diversity of the entomopathogenic fungi. TheShannon index and Nei gene diversity index resulted in similar population orders, with theformer more sensitive. The differentiations among strains and within a population andbetween populations were well identified by UPGMA clustering and principal coordinateanalysis, and substantial differences were revealed by genetic differentiation index（0.1137＜Gst＜0.9101）within population.2. The genetic structure of Beauveria bassiana populations causing epizootics inpraying mantid populations of Anhui were analysed. Molecular evidence suggested thatthere might be extensive gene exchange and long-distance gene flow between the B.bassiana populations causing epizootics in September and October,2010, respectively atQianshan, southwest Anhui and Huangcangyu, north Anhui; the white muscardine ofmantids was a panzootic consisting of a wide spreading epizootic and many local enzootics;the conidia of B. bassiana might have transferred among praying mantids of successivehabitats of from south to north with the aid of monsoon. All the data revealed that theepizootic of mantids in Anhui province was a panzootic made up with several epizooticsand enzootics simultaneously. The heterogeneity of B.bassiana populations was due toobvious host specificity, temporal and spacial heterogeneity. 3. The genetic structure of B. bassiana causing an epizootic in the population of a truebug, Cyclopelta obscura, in Qiandaohu, Zhejiang, was analysed. The epicenter wasrevealed by UPGMA clustering, with the reason for the high density of infected cadaverswas due to swarming behaviors of C. obscura nymphs before scattering. Molecularevidence revealed a diffusing trend of the B. bassiana population outwards from the center.4. The result of genetic differentiation of a B. bassiana population causing epizooticsof the Masson’s pine caterpillars, Dendrolimus punctatus in Mingguang, east Anhui indifferent time showed that gene differentiation index Gstwas very small, varying between0.0986and0.0818, and gene flow Nmwas very strong, varying between2.2857and2.8048.Among the time groups, the Groups of13-04-2006and28-04-2007and91.9%isolates ofthe Group22-06-2006clustered together, with genetic similarity over80%. The general Gstwas0.1137among the three time groups, suggesting a light differentiation. However, thegenetic differentiation between each of the three groups and Bb13, the isolate which werereleased in this area, was all over0.6, suggesting a heavy differentiation. It is henceconfirmed that the natural epizootics of2006and2007in Mingguang were not caused byBb13, but by indigenous strains including the three groups and3other isolates whichclustered far from them. The indigenous strains keep the caterpillars sustainably in check.5. The genetic structure of Metarhizium anisopliae var. anisopliae populations wereanalysed, which caused an epizootic of a true bug, Schiodtella formosana, at Jingtingshan,southeast Anhui. UPGMA clustering displayed two main clades at the similarity of83%,which differentiated over70%strains of different time groups. All the above data revealeda considerably distinct temporal heterogeneity. Further identification of obviousdifferentiation within the population was done with help of AMOVA. The heterogeneouspopulations of M. anisopliae var. anisopliae will benefit sustainable control of Schiodtellaformosana.6. By UPGMA clustering and primary coordinate analysis were run with fiveepizootic populations, including the population of Isaria tenuipes causing epizootics oflepidopteran larvae in Manshuihe, west Anhui and lepidopteran pupae in Huangcangyu,north Anhui, population of I. farinosa causing epizootic of lepidopteran pupae in Xishan,west Anhui, B. bassiana causing epizootic of a homopteran insects in Xinhuang, westHunan and epizootic of the wood borer, Holcocerus hippophaecolus in zhungeer, southInner Mongolia. All the analyses supported the clustering of two main clades within eachpopulation and categorized the five populations as a clearly differentiated heterogenouspopulation even though the investigated plots were very limited habitat. 7. The genetic structure of ten I. cicadae populations, which cause depizootic innymphs of a cicada at jingtingshan, southeast Anhui, was analysed. The clustering resultsshowed that47of all53isolates clustered together at a similarity of0.8(?), while the other2isolates diverged and clustered into an isolated group, suggesting that the population wasa comparatively low heterogenous one. The clustering was not associated to the geographicorigin, sampling time and isolating substrate.Based on the integration of the15cases studied, it can be concluded that all theentomopathogenic populations analysed were heterogeneous populations, but theirheterogeneity varied much. The most heterogenous is Manshuihe population of I. tenuipeswith Gst=0.9101, and the least heterogenous is Mingguang population of B. bassiana with Gst=0.1137,but Gstof the most populations were above0.25, the threshold of heavy differentiation. There coexistedseveral or a single predominant subpopulation and some non-predominant subpopulations or scatteredstrains with most populations. Such differentiation enhances population heterogeneity and increasedenvironmental adoptability of the populations. The predominant subpopulation helps rapid increase ofincidence so as to form an epizootic, while the non-predominant subpopulation or scattered strainimpact their host population as enzootic, which benfit long-term ecological balance. The heterogenouspopulations of the entomopathogenic fungi are characteristic of the coexistence of epizootic andenzootic. The heterogenous populations may be necessary for occurrence, persistence and the nextoutbreak.