| Ascosphaera aggregata | |
|---|---|
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Class: | Eurotiomycetes |
| Order: | Onygenales |
| Family: | Ascosphaeraceae |
| Genus: | Ascosphaera |
| Species: | A. aggregata |
| Binomial name | |
| Ascosphaera aggregata Skou, JP (1975) [1] | |
Ascosphaera aggregata is a species of fungus.
Ascosphaera aggregata, discovered in 1975 by Jens-Peder Skou [1] is a fungus that is related to Ascosphaera apis . [2]
Ascosphaera aggregata is an obligate parasite [3] that causes chalkbrood in bees, [4] symptom manifestations differ depending on age of the larva. [5] It primarily infects alfalfa leafcutting bees, Megachile rotundata . [2] [6] [7] [3] [8] Megachile rotundata infected with A. aggregata have been detected in the United States, Canada, [9] and South America. [5] Other bee species that A. aggregata has been seen to infect include the red mason bee ( Osmia rufa [8] ), the patchwork leafcutter bee ( Megachile centuncularis [8] ), Megachile pugnata and Megachile relativa . [2] [10] [8]
Ascosphaera aggregata is an obligate parasite that can cause chalkbrood by the fifth instar. [5] The majority of the life cycle and growth of A. aggregata occurs in M. rotundata larvae. Infection of bee larvae occurs only via ingestion of resting spores, [3] and is not possible via spore inhalation nor contact with the fungal vegetative form. [6] [5] Spores develop in the larva and cause it to swell, bursting the larval integument (giving the dead larvae a ragged appearance) [1] and furthering the spread of the fungus. Buildup of larval cadavers traps the unaffected emerging bees, forcing them to chew through the cadavers and be covered in spores. [7] Bees covered in spores then contaminate food provisions for other broods [7] and spread the infection.
Early vegetative growth utilizes gut lumen nutrients. [6] A. aggregata grows through the midgut wall to the hemocoele (event trigger is unknown, not because of lack of space nor food) [6] eventually replacing larval tissue. [3] Resulting larvae are filled with a mycelial mat comprising two layers: a dense inner layer and a less dense outer layer. [6]
Ascospore morphology consists of two layers: an inner chitinous and smooth layer, and an outer layer that is rough, spotted, [1] and not composed of chitin nor cellulose.; [6] ) Ascospore development in A. aggregata is unique and the resulting structure is referred to as a "spore cyst", or "ascocyst" or "synascus". [8] Sexual development occurs on the outer mycelial mat in the subcuticular region, [3] [6] and is documented to proceed as follows:
Ascosphaera aggregata has been found to be unable to break down chitin. [6] [3]
Although ascospore development is very unique, it is very hard to identify A. aggregata because the spore balls and conidia tend to resemble other species. [12] Recent investigations by James and Skinner (2005) [12] have discovered that PCR of the ITS domain of ribosomal DNA with species specific primer sets allows the detection of fungal DNA (working, even, in asymptomatic individuals). [12] The PCR technique can also be used on hair and honey samples to avoid the difficulty of culturing spores, [12] as spore were shown before to only germinate well in lipids. [13] Storage of the fungus has also proven to be difficult as it collapses after 1–2 months during normal culture passaging. [14] However, Jensen et al. (2009) found that spores could be preserved via cryopreservation or freeze-drying whereas hyphae unfortunately could not be preserved. [14]
Megachile rotundata is the primary pollinator of the commercially grown alfalfa seed, [11] [7] accounting for 46,000 metric tonnes of North American alfafa seed (two-thirds the global production) in 2004. [7] M. rotundata is also the second most valuable field crop pollinator, behind the honey bee, because of the value of alfalfa in animal feed and hay. [7] A. aggregata has been killing this economic pollinator in the US since 1972 [10] and has been reported to be able to kill greater than 50% of a population. [8]
Effective management of the fungus has yet to be discovered, as the current registered treatment in Canada (paraformaldehyde fumigation of spores [7] ) involves a carcinogen and other treatment options (heat and chloride treatments) are expensive and labour-intensive. [7]
