Date of Award:

5-1-2006

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Life Sciences: Biology

Committee Chair(s)

Donald W. Roberts

Committee

Donald W. Roberts

Committee

Edward W. Evans

Committee

Anne J. Anderson

Committee

Darwin L. Sorensen

Committee

Diane G. Alston

Abstract

The fungus Metarhizium anisopliae is an important agent for biological control of insects and has great advantages over chemical insecticides. However, ultraviolet radiation and heat from sunlight are serious obstacles for the use of M. anisopliae in agriculture, where the conidia may be impaired in a matter of a few hours in the field. Therefore, isolate selection and/or generation of conidia with increased tolerance to UV radiation and heat are a major goal of this dissertation. High variability in heat and UV-B tolerance was found among 17 isolates of M. anisopliae from different geographic origins, but there was very low variability in tolerance among 30 morphologic mutants of two wild-type isolates; indicating that tolerance to UV-B and heat is largely regulated by genotype. Nevertheless, significant, phenotype alterations in UV-B and heat tolerance were obtained with individual isolates by use of stress conditions before conidiogenesis. In many organisms, pre-adaptation to one stress may induce cross-protection to other stresses, and this was found true for M. anisopliae when conidia were produced under nutritive stress (carbon starvation), osmotic stress (NaCl or KC1), and heat shock stress. Conidia produced under nutritive and osmotic stresses were 2-fold more heat and UV-B tolerant than conidia produced under optimal conditions on rich PDAY medium. Thermotolerance of conidia produced on heat-shocked mycelium was increased 2-fold, but there was little cross-protection to UV-B radiation. Conidia produced under oxidative stress (i.e. hydrogen peroxide, UV-A radiation or menadione) increased thermotolerance only when produced on medium containing menadione. Conidial production, however, was significantly reduced with most of the pre-adaptation treatments. Four levels of conidial UV-B tolerance were observed when conidia of M. anisopliae (ARSEF 2575) were produced on different media or substrates. Conidia produced on insects had the lowest and next-to-lowest UV-B tolerance, conidia produced on rich PDAY medium had slightly higher UV-B tolerance, and conidia produced on minimal medium (= MM = carbon starvation) had the highest UV-B tolerance. In addition, conidia produced on MM had higher virulence to the insect Tenebrio molitor than conidia produced on PDAY medium. In conclusion, this research pointed out that not only the genetic makeup of M. anisopliae isolates is important to their variability in stress tolerance and virulence, but exposure to various stresses before conidiation improved phenotypic tolerances of individual isolates to stress and virulence.

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