Mutate, mate, sporulate!

Back in the mists of time when my blog had just been born, I wrote a piece about Entomophthora - a fungal genus that parasitises flies, changing their behaviour to aid its own spore dispersal before killing the host. On that occasion, I had found dung-flies (genus Scathophaga, probably S. stercoraria) attached to the top of grass stems - the usual location for flies infected by this fungus. However, a couple of days ago, while putting some post-Christmas stuff into the attic, I noticed something quite different - two flies in the typical Entomophthora posture (proboscis attched to the substrate, abdomen lifted, wings spread), but indoors, attached to a skylight frame.

Fly attached to skylight frame.

Even in this photo, you can see the reddish fungal mass spreading out from between the abdominal segments, the proboscis tightly stuck to the frame and some powdery white spores on the legs and abdomen. However, these features are clearer if we look more closely. It's also helpful to identify the fly - it's one of the house-flies (Muscidae), but to separate the genera, a key feature is the wing venation.

Fly wing - the red arrow points to the bend in the discal vein.

The bend in the discal vein mean this is either Orthellia or Musca, but at the fly is clearly not metallic green, it is Musca, in this case M. domestica, the common house-fly. I don't want to go into muscid identification in any more detail here, but if you are interested, a key work on British species is d'Assis Fonseca (1968) which is available second-hand or by inter-library loan. So, returning the fungus and its fly host...

Side view of the abdomen - note the white spores stuck to hairs near the rear.

The underside of the abdomen, again showing the fungal mass and white spores.

Close-up showing individual spores attached to hairs.

Outdoors, this would be the typical method of spore dispersal as the spores are actively released and blow away or attach to other nearby flies, especially if there is direct physical contact. However, being indoors there is no wind and the spores have simply attached to the flies' own bristles, although it is of course possible that other house-flies have been infected (window-frames are a coomon place to find infected flies though I have not previously noticed them). My previous post covers infection routes and methods to some extent, as well as a little on taxonomy, so here I want to look at the basic structure and life cycle of the fungus in a little more detail.

Microscope 'squash' preparation of the fungal mass (magnification x400)

Several structures can be seen here as indicated by the coloured arrows:

• Green: these are the asexual spores (conidia, singular = conidium) covered in a gelatinous coating that allows them to stick to flies once released, and are then seen as white powder attached to hairs and bristles.
• Red: these small round structures are the spores themselves without the gelatinous coating.
• Blue: this is a coated spore attached to one of the elongate conidiophores, stalked structures which produce spores by mitosis.

In Entomophthora muscae (which is almost certainly the species of fungus seen here), once the host fly dies, the conidiophores emerge from between the abdominal segments (forming the fungal mass seen in the photos above), and produce primary spores. If, once released, they encounter a suitable host, the spores germinate quickly (within a few hours ), a germ tube penetrating the insect's cuticle. Once the tube reaches the heamocoel (fluid-filled cavity around the insect's organs), the cytoplasm grows through the tube and into the haemolymph (effectively the mixture of 'blood' and other fluids that fill the haemocoel). Fungal hyphae then grow into the nervous system (as well as the rest of the body) causing the change in behaviour that induces host flies that are near death to climb to high points and adopt the typical posture mentioned before. The fungus also digests the fly's gut causing death after around five to seven days. New conidiophores develop around three hours later and the cycle of infection continues. However, if there is no suitable host, spores may develop into smaller secondary conidiophores which produce secondary spores.

So, a jolly start to 2012 with a gut-eating parasitic fungus - I will undoubtedly return with some small shiny beetles soon, but until then, Happy New Year :)


Reference

d'Assis Fonseca, E.C.M. (1968). Muscidae. Royal Entomological Society Handbooks for the Identification of British Insects 10(4b): 1-119.