Identification of genes associated with the Nosema ceranae infection tolerance in the honey bee
Nosema is a Microsporidan parasite of the honey bee (Apis mellifera), which infects the epithelial cells of the gut. Nosema lives as an obligate intracellular parasite. The infection starts from the ingestion of the environmental spores, which germinate in the mid-gut and extrude the polar tubes that penetrate the epithelial cells to release the sporoplasm into the cytoplasm. The infected cells will eventually burst and release a new generation of primary spores. The primary spores can either germinate and infect new host cells (proliferative phase) or expelled as environmental spores through the feces (sporogonic phase). However the mechanisms to prevent the germination of the primary spores remain unclear.
Nosema infection affects honey bees in multiple ways including reducing the life span. Two Nosema species are known to infect the honey bee A. mellifera: Nosema apis and N. ceranae. N. apis is an evolutionarily old pathogen of A. mellifera with a moderate virulence. N. ceranae is a newly established parasite of A. mellifera and has been reported to be highly virulent causing large colony losses. However, there are increasing reports suggesting a moderate virulence of N. ceranae similar to N. apis.
Irrespective of the level of virulence, it is clear that Nosema adds to the pathogen load in honey bee colonies and reduce their productivity. We compared the immune response towards the N. ceranae infection of a Nosema resistance selected and unselected honey bee strains. Bees of the selected strain showed significantly higher survival and higher expression levels of the immune genes, which gave evidence the selection program is successful. We then used drones of a hybrid queen resulting from a cross of queens of the selected with drones of an unselected honey bee strain to identify QTLs reducing N. ceranae spore load. A major QTL was identified and two candidates genes were significantly differentially expressed between bees with high and low spore load. Moreover, the expression levels of the two candidate genes were strongly correlated with the immune gene expression levels.
If they are the genes responsible for the tolerance, then the positive selection should have generated a strong selective sweep in the corresponding genome region. We used markers within and flanking the QTL region to screen for a reduction of genetic variability in the selected honeybee strain. Within the QTL region, the genetic diversity was strongly reduced and a swept locus was identified, which confirmed the significance of the QTL for the reduction of the Nosema spores.
Currently, we are fine mapping the previously identified QTL region and interfering the expression of the candidate genes of the selected bees to acquire the susceptible trait.