Use of RAPD analysis for in situ identification of Ascosphaera aggregata and Ascosphaera larvis in larval cadavers of the alfalfa leafcutting bee, Megachile rotundata

A multi-gene phylogeny provides additional insight into the relationships between several Ascosphaera species

Ascosphaera fungi are highly associated with social and solitary bees, with some species being pathogenic to bees (causing chalkbrood) while others are not, and proper identification within this genus is important. Unfortunately, morphological characterizations can be difficult, and molecular characterizations have only used one genetic region. We evaluated multiple phylogenies of the Ascosphaera using up to six loci: the Internal Transcribed Spacer (ITS) region, 18S rRNA, 28S rRNA, Elongation Factor-1α (EF-1α) the RNA polymerase II largest subunit (RPB1), and the second largest subunit (RPB2). The ITS sequence alone produced an inadequate phylogeny, and the addition of both the 18S and 28S rRNA loci to the ITS sequence produced a phylogeny similar to that based on all six genetic regions. For all phylogenies, Ascosphaera torchioi was in a separate clade that was the most basal, with a strong genetic similarity to Eremascus albus, introducing the possibility of paraphyly within Ascosphaera. Also, based on this new phylogeny, we now suggest that the Apis mellifera (honey bee) pathogens arose within a group of saprophytes, and the Megachile (leafcutting bees) pathogens arose separately.

PCR diagnostic methods for Ascosphaera infections in bees

Fungi in the genus Ascosphaera are the causative agents of chalkbrood, a major disease affecting bee larval viability. Identification of individual Ascosphaera species based on morphological features has been difficult due to a lack of distinguishing characteristics. Most identifications are based on the size and shape of the ascomata, spore balls and conidia. Unfortunately, much overlap occurs in the size of these structures, and some Ascosphaera species will not produce sexual structures in vitro. We report a quick and reliable diagnostic method for identifying Ascosphaera infections in Megachile bees (leafcutting bees) using PCR markers that employ genus-specific primers for Ascosphaera, and species-specific primers for species known to be associated with Megachile spp. Using these methods, species identifications can be performed directly on bees, including asymptomatic individuals. Furthermore, the PCR markers can detect co-infections of multiple Ascosphaera species in a single host. We also identified a marker for Ascosphaera apis, the predominant cause of chalkbrood in Apis mellifera, the honey bee. Our diagnostic methods eliminate the need for culturing samples, and could be used to process a large number of field collected bee larvae.