Mites and reproduction in Ceratocystis ulmi and other fungi

Abstract

Adults of the mite Tyrophagus putrescentiae were isolated from diseased elm bark and maintained on agar cultures of the aggressive or non-aggressive strains of Ceratocystis ulmi. T. putrescentiae was used in fertilization experiments to carry spores from aggressive or non-aggressive isolates of B mating-type growing on elm bark pieces to the protoperithecia (♀) of an aggressive A-type on adjacent elm twigs. Numerous perithecia developed on the twigs in the aggressive (twig) × aggressive (bark) combinations but very few in the aggressive (twig) × non-aggressive (bark). This confirms the existence of a mechanism inhibiting hybridization between the aggressive and non-aggressive strains of C. ulmi . T. putrescentiae ate the mycelium of C. ulmi leaving the protoperithecia and perithecia untouched, suggesting that these and similar structures of other fungi may be tough and pigmented to deter predation. Over 90 % of mites from diseased elm bark in nature were found to be carrying C. ulmi when plated on a medium selective for the fungus. Microscopic examination revealed probable ascospores as well as mycelial and synnematal conidia of C. ulmi on their surfaces. It is suggested that C. ulmi is commonly fertilized by mites in nature, and that mites disperse genetic variants of C. ulmi within bark and introduce potential antagonists of C. ulmi from without. Mites from outer elm bark and others from beech bark were found to be carrying a variety of fungal spore types. The potential role of mites in the dispersal, fertilization and adaptation of fungal fruiting structures is discussed.