Ascosphaera aggregata

Ascosphaera aggregata is a species of fungus.

Ascosphaera aggregata
Scientific classification
Kingdom:	Fungi
Division:	Ascomycota
Class:	Eurotiomycetes
Order:	Onygenales
Family:	Ascosphaeraceae
Genus:	Ascosphaera
Species:	A. aggregata
Binomial name
Ascosphaera aggregata Skou, JP (1975)[1]

History and taxonomy

Ascosphaera aggregata, discovered in 1975 by Jens-Peder Skou^[1] is a fungus that is related to Ascosphaera apis.^[2]

Habitat and ecology

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]

Growth, morphology and pathobiology

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]

Sexual development

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:

1. The vegetative hyphae tips swell and form a thallus^[6]
2. The middle of the thallus grows and forms a nutriocyte (previously referred to as an archicarp^[8])
3. The apical portion differentiates into the trichogyne cell.^[6]
4. Compatible trichogyne fuse and initiate plasmogamy.^[6]
5. Resulting dikaryotic fungal protoplasm then enters the nutriocyte and causes enlargement of the nutriocyte.^[6]
6. Nutriocyte growth causes the integument to rupture and initiate development of a fragile spherical structure without a cell wall.^[6]
7. Individual spores then pack together into a seemingly membrane-less spore ball.^[11]
8. Multiple spore balls then join and form a spore cyst.^[11]
9. Cell wall deposition changes spore colour from opaque white to grey to dull black^[3][6]

Physiology

Ascosphaera aggregata has been found to be unable to break down chitin.^[6][3]

Diagnostic considerations

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]

Economic importance

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]