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Biological interrelationships between the Ambrosia beetle Xyleborus dispar with its symbiotic fungus Ambrosiella hartigii Public Deposited

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  • Biological interrelationships between the ambrosia beetle Xyleborus dispar (F.) (Coleoptera:Scolytidae) with its symbiotic fungus, Ambrosiella hartigii Batra (Fungi Imperfecti) were investigated in western Oregon. Postdiapause adults of X. dispar collected in March through June with rotary nets, and excised from overwintered and newly attacked host material, produced a single generation when the beetle was reared in vitro with A. hartigii. Diapause beetles excised from host materials in the fall failed to oviposit. This study is the first record of rearing a temperate zone scolytid in vitro and on a fungus in the genus Ambrosiella. Such in vitro rearing allowed controlled studies of ecological, behavioral, developmental and physiological aspects of this ectosymbiosis. Comparisons were made between wild and in vitro populations. Seasonal variations of the fungus within the female beetles mesonotal mycangium, and the synchronization of ovariole development were demonstrated. Comparative concentrations of solube proteins and free amino acids suggested that the fungus in the mycangia was built up from free amino acids of the insects. At the period of emergence, flight and attack of new hosts, the females were found to have a concentration of soluble proteins more than double that found in the beetles during the remainder of the year. Whereas, the free amino acids were the lowest values recorded during this period (March-October). Ovariole development and oviposition only occurred after the post-diapause female had fed on the ambrosial form of A. hartigii. These beetles attacked a new host with empty intestinal tracts. The relationship between the number of progeny and the volume of the galleries was linear. Experiments were conducted in an attempt to terminate maturation diapause in this univoltine species of ambrosia beetle using temperature, Juvenile Hormone Analogs (JHA) and an olfactometer. Termination of the maturation diapause was not achieved with these procedures. Qualitative and quantitative analyses of the major nitrogenous excretory products were made on the various life stages of X. dispar. The main nitrogenous product found in excreta and hindguts of beetles, larvae and pupae, was uric acid (range 7.6-14.8 μg uric acid/beetle). No ninhydrin-positive compounds were located in excreta of the beetles. The concentration of ammonia-nitrogen in the various life stages averaged between 0.70-1.13 μg NH₃-N/beetle. Total nitrogen determinations were made on sapwood samples of Malus sylvestris (0.34 ± 0.005% N by dry weight), attacked wood, "pre-brood" (0.31 ± 0.005% N by dry weight) and attacked wood-"post-brood" (0.17 + 0.02% N). Similar determinations of the artificial medium (L-asparagine) indicated that a nitrogen requirement of about 0.08 -0.1% N by dry weight was necessary before oviposition could occur. Fixation of atmospheric nitrogen by individual X. dispar beetles in vitro was not indicated using the acetylene ethylene reductase method. In vivo situations may be different, but were not investigated. Proteolytic enzyme activity was not found on examination of dispause beetles, their excreta, larval and pupal excreta, and the ambrosial and mycelial forms of A. hartigii. Bioassays were used to study the interactions and effects of the various life stages of X. dispar on the induction of the ambrosial form of its symbiotic fungus, A. hartigii. Postdiapause adults and pupae of X. dispar were able to cause a change from the mycelial to the ambrosial form of A. hartigii in culture. Larvae fed on the mycelia' form in vitro, but ambrosia is required by larvae to develop and pupate. One of the main factors inducing the ambrosial form of A. hartigii is probably a secretory product of X. dispar. Nitrogenous compounds are considered necessary for the cause of ambrosial induction, but not the primary factor alone for the continued growth of the ambrosia. Morphogenic compounds that caused ambrosial induction in vitro responded negatively to the Azocoll procedure, indicating no proteolytic activity. The symbiotic association of X. dispar and its sole food fungus, A. hartigii is a reciprocal biochemical alliance based on carbon and nitrogen metabolic integration. The beetles contribute their free amino acids and nitrogenous excretions, and the fungus con - tributes its cellulose-degrading enzymic ability to the association, plus synthesizing proteins, sterols, vitamins and other growth factors. The beetle-fungus symbiosis has been depicted in the form of an abstract pattern.
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