Saturday, October 2, 2010

A lichenicolous fungus Illosporiopsis (syn. Hobsonia) christiansenii

I am an amateur naturalist trying to learn something about everything living in my garden.

No, not another lichen posting! Instead, the star of today's posting - the pink blobs in photo 1 - is a fungus. Specifically a lichenicolous fungus (from the Latin colous = living amongst [lichen]).

The lichen being infested here is our old friend Physcia tenella.

Photo 2 shows the rather lumpy, 'coralloid' texture of the fungal blobs close up.

I first noticed pink blobs of this type some years ago on a country walk. I struggled to identify them for a long time but an acquaintance suggested the fungus Marchandiomyces. Searching the internet for more information led me to the very nice website of Alan Silverside. There I found pictures of two species: the rich-pink blobs of M. corallinus and the orangey-pink blobs of M. aurantiacus. I was ready to settle for one of these, but then I noticed a comment alluding to yet another pink-blob species called Illosporiopsis christianesnii. (There is yet another called Hobsonia christiansenii - but as far as I can tell this and Illosporiopsis are the same).

Distinguishing between these various blobs seemed a forlorn hope. As it said in a paper by Sikaroodi (Mycological Reserach, April 2001) I came across during my searches

"These [species] are frequently misidentified because of a paucity of morphological characters"

I was about to quit, but then I caught sight (here and in a paper by Lowen Mycologica 78(5), p.842) of a mention that the 'conidia' (= asexual spores) of I. christiansenii had a characteristic 'spiral' appearance. I took a tiny part of my fungus in a drop of water, squashed it between a slide and cover slip and viewed it with my trusty student microscope. The result is shown in photo 3 (click to enlarge).

I'm not expert enough to be confident of what I'm really looking at here. Furthermore, working at x1000 magnification is a tricky and frustrating business - there's hardly any depth of focus and the slightest knock sends things scudding out of the field of view. Nevertheless I was left pretty confident there were indeed some spiral 'objects' in my sample (the object in the photo inset for example, and another in the main image above the number '3'). On that basis I'm identifying my fungus as Illosporiopsis (syn. Hobsonia) christiansenii.

Searching more generally for information about lichenicolous fungi I was rewarded by finding the splendid review article by Lawry and Diederich here. From this I learn that the whole research topic of lichenicolous fungi is enjoying a purple (pink?!) patch. From a single illustrated species (a gall on the lichen 'Usnea') in 1792, the number of known species grew steadily to reach around 686 in 1989. Over the past 10-years however, as scientists around the world have started look in earnest for such lichen-loving fungi, the number of known species has more than doubled.

With this explosion in species-count is coming a growing appreciation of just how rich a field-of-enquiry the lichenicolous fungi represent. Take the task of unravelling and understanding the interactions between the attacking fungus and its target lichen. Some fungi are very unfussy, being adaptable to a wide range of lichens. Others have a very intimate and specific relationship with only one or two hosts. Some invaders aggressively attack and kill their target lichen. Others are parasitic, insinuating their hyphae (=the long tube-like cells that make up a fungus) into the cells of their host to suck the juices, vampire-like, from their cells. Some lichenicolous fungi even stage a 'take-over' bid: A lichen is basically a fungus that is 'farming' a crop of algae (see my post here). The game plan of some lichenicolous fungi is to kill the 'farmer'-fungus' in order to acquire his algae.

There are more questions over how lichenicolous propogate and spread themselves. It's hypothesised that some may hitch a lift with roving, lichen-feeding mites. But generally not much seems to be known. There are unanswered questions about the sensitivity of lichenicolous fungi to air quality. Certainly some lichens are incredibly sensitive to impure air, unable to survive even trace amounts of pollution. Whether this holds for their attackers isn't known.

There are further questions about how lichenicolous fungi affect the ecology of a region. It's been argued by biologists that having a lot of parasites in some eco-system ought to encourage a lot of species diversity. Whether this is born out in regions where parasitic lichenicolous fungi are prevalent however is not well studied however.

These topics (and a lot more) are discussed in the review above. All in all, I suspect that any amateur naturalist hoping to make some genuine and lasting contribution to scientific understanding could do worse than to cultivate an interest in lichenicolous fungi!

To return to the pink blob species M. corallinus and I. christiansenii, and the paper I mentioned above by Sikaroodi, a remarkable thing to learn was that these two nominally identical blobby fungi in fact represent two entirely separate fungal kingdoms. There are millions of species of fungi, but (crudely) they can be split into two huge groups. There is a huge group of fungi that grow their spores inside little sausage-shaped bags called asci (see photo 4 on my posting here). Such fungi are termed ascomycetes. The other group grow their spores, not inside asci, but on the ends of sausage-shaped protruberences called basdia. Such fungi are termed basidiomycetes. From everything I've read this is a very deep and ancient division, the ascomycota and basidiomycota representing an ancient 'parting on the ways' in the evolution of fungi. What's surprises me therefore, is that whilst M. corallinus and I. christiansenii seem almost indentical in every regard (both are small pink blobs, and both grow on the same types of lichens), whilst the former is a basidiomycete the latter is an ascomycete. Now, sometimes, entirely different lifeforms can end up evolving very similar bodies simply because these are the best bodies for the life they're trying to live ('convergent evolution'): Think 'whales' and 'fishes' or 'birds' and 'bats'. Have two very distant fungal cousins independently evolved the conclusion that if you want to survive on lichen, being a small pink blob is a good way to go? The plot only thickens when you learn that although DNA testing shows the species above to be members of the basidiomycota and ascomycota respectively (and therefore that they should grow their (sexual) spores in quite different ways) in fact for neither species has this (sexual) fruiting stage ever actually been seen! (Though it should be remarked that the same was true of the blob M. aurantiacus until recently when a fruit body ('teleomorph') was discovered by Diederich and co workers).

So there we have it. A tiny inconspicuous fungus occupying the (to our human eyes) minute and obscure niche of subsisting in the crevices of a lichen. And yet what a rich and unexplored natural history awaits.

"Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum"
( Augustus de Morgan, 1806-1871)

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