Topic: Ascomycete fungi: insights into convergence
Today ascomycetes are an extremely important group of fungi, and they take their name from the reproductive structures known as ascii.
The ascomycetes are an ancient group of fungi, with examples known from the Lower Devonian (Rhynie Chert). Today they are an extremely important group, and take their name from the reproductive structures known as ascii, that release gametes known as ascospores. They show many remarkable convergences, including carnivory, multiple evolution of aquatic ascomycetes, repeated association with ambrosia beetles and serial adoption of microniches in bryophytic plants.
Carnivorous fungi (nematophagy and zoophagy)
Although the basidiomycetes and zygomycetes have independently evolved the capacity to trap and feed on nematodes and other prey (carnivorous fungi) it is the group often referred to as the hypomycetes which is the most important in terms of zoophagy and enjoys not only nematodes, but rotifers, tardigrades and insects, as well as protistans. Five basic mechanisms for entrapment have evolved, which mostly depends on adhesive knots or a network, but extraordinarily some have a lasso like arrangement formed of a circle of cells. In some types this can constrict, trapping the nematode. Molecular evidence suggests that this group of nematophagous fungi are monophyletic, but interestingly examples from 100 Ma-old Cretaceous amber show the trapping ring but it is only comprised of a single cell. In addition the fungus has an associated yeast phase, which is not known in living hypomycetes. It is likely, therefore, that this is an independent lineage of ascomycetes.
Not surprisingly the zoophagy of carnivorous fungi depends on enzymes, and the proteases employ the classic aspartic acid-histidine-serine catalytic triad for binding to the substrate, which is of course a convergent molecular motif. Similar enzymes are used by animals, and also carnivorous plants. Several workers have pointed out that this zoophagy in fungi has important similarities to the carnivorous plants, and again represents a response to nutrient shortages, especially nitrogen.
Although, as noted above, the extant nematophagous hypomycetes are evidently monophyletic, the aquatic hypomycetes are evidently rampantly convergent. These typically live on leaves and other plant debris, and despite such striking similarities as spore shape (evidently connected to the need to entangle with the substrate rather than being swept away) molecular phylogeny implies multiple origins, with even the classic genus Anguillospora evidently evolving four times.
Other aquatic ascomycetes also show convergence. Particularly remarkable are those fungi that evolve flotation devices (derived from the conidia) to transport the spores in a virtually unsinkable manner. Despite these complex shapes, as in the appropriately named Helicoon, they have evolved at least five times. So too the well-known aquatic ascomycete Halosarpheia is polyphyletic, with ironically the type species being especially distant. Moreover in the important marine group known as the Halosphaeriales the move to land has occurred at least twice.
A particularly interesting example amongst the ophiostomatoid ascomycetes involves the ambrosia beetles (which independently have evolved agriculture). Not only do we see specific dispersal mechanisms, including both sticky ascospores and peritheca with long necks, that reflect this arthropod association, but in addition it is clear that the association with these ascomycetes is convergent. But it also turns out that the ophiostomatoids are convergent, with the need to adapt to the necessities of dispersal by insect determining the repeated evolution of the peritheca with long necks and modifications to the ascospores.
The primitive land plants known as the bryophytes offer a variety of microniches that are not only occupied by ascomycetes, but show a series of striking convergences, in total some 29 microniches can be identified, and as ever the convergences identified make perfectly good sense in this context. Thus one important sub-niche for the ascomycetes is the occupation of the interlamellar spaces, and not surprisingly this involves the repeated evolution of particular types of fruiting body. This example also allows one to ask the important question as to whether there are microniches on the bryophytes that are still waiting to be occupied, which “ought” to be but are not. Indeed there are, but good arguments to do with unattractiveness of the substrate or difficulties in the dispersal of spores suggest such microniches will rarely, if ever, be discovered by the ascomycetes. What we can say, moreover, is that if they ever are, it is unlikely to be achieved only once.
Good instances of convergence are found in the morphology of the ascospores in the group known as the Sordariales (recalling those seen in the aquatic hypomycetes discussed above). Finally an important genus known as Sporidesmium is polyphyletic.
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Map of Life - "Ascomycete fungi: insights into convergence"
September 19, 2021