Psoraceae

Psora decipiens, copyright Troy McMullin.

Just a very quick one today. The photo above is of a member of the Psoraceae, a group of lichens sometimes referred to as 'fishscale lichens'. As their vernacular name indicates, Psoraceae are characterised by a scaly appearance, together with a preference for growing on soil or rock crevices (Ekman & Blaalid 2011). The scaly appearance also gives the family its botanical name: Psora comes from the Greek for 'itch'.

Psora vallesiaca, copyright Leif Stridvall.

Molecular phylogenetic analyses have supported the inclusion of three genera in the Psoraceae, Psora, Protoblastenia and Brianaria (Ekman & Svensson 2014). The last genus was only described recently to include a group of species previously included in a different genus Micarea belonging to an entirely different lichen family, the Pilocarpaceae. Micarea lichens closely resemble Brianaria species in overall appearance but differ in some features including the nature of their algal symbiont. Past authors often assumed that symbiont associations provided little guidance to lichen relationships; it was thought that a germinating lichen fungus would pretty much form a connection with whatever algal species was available. However, more recent investigations have found that the tastes of lichen fungi are more discriminating. Micarea species form associations with small algal cells, four to seven microns in diameter, with thin cell walls that are often found in pairs within the lichen thallus. Brianaria species, in contrast, have larger algal symbionts that are always isolated in the thallus (Andersen & Ekman 2005).

REFERENCES

Andersen, H. L., & S. Ekman. 2005. Disintegration of the Micareaceae (lichenized Ascomycota): a molecular phylogeny based on mitochondrial rDNA sequences. Mycological Research 109 (1): 21–30.

Ekman, S., & R. Blaalid. 2011. The devil in the details: interactions between the branch-length prior and likelihood model affect node support and branch lengths in the phylogeny of the Psoraceae. Systematic Biology 60 (4): 541–561.

Ekman, S., & M. Svensson. 2014. Brianaria (Psoraceae), a new genus to accomodate the Micarea sylvicola group. Lichenologist 46 (3): 285–294.

The Naked Ascus

Hyphae with poorly-differentiated fruiting bodies of Gymnascella marismortui, from Buchalo et al. (1998).

Three years ago, Christopher and I visited my aunt in Jordan. We spent a week visiting various parts of the country (Petra is amazing), including that most touristy of all activities, swimming in the Dead Sea. Except 'swimming' is not really the right word for what you do in the Dead Sea: with the salt content of the water and hence your own buoyancy so high, normal swimming movements are practically impossible. You can't do much more than bob along on your back*. Standing back up again is equally disconcerting: the extra force required to push your legs back down through the water is such that it is quite startling to discover that the bottom is only about two feet below you. The water has a not-particularly-pleasant greasy feel to it, and at one point I had a drop of it splash onto my lower lip. To my surprise, I could not really describe the taste of that drop as salty. Rather, I would say that what the Dead Sea tastes of is Pain.

*But not too far. One bather had a congregation of guards on the shore suddenly begin yelling at her, evidently because she was swimming too far out and was about to inadvertently invade Israel.

It's hard to believe that anything could live in such conditions, but there is life there. Salt-loving prokaryotes and unicellular dense algae can sometimes form dense blooms, and in 1998 Buchalo et al. described the filamentous fungus Gymnascella marismortui, grown from spores collected in Dead Sea water. Fungal hyphae where observed growing on wood in the Dead Sea in spots where its saltiness had been diluted, such as by the inflow of freshwater springs or rain. Gymnascella marismortui may play a significant role in breaking down wood or other plant material that has been washed into the Sea.

Reproductive structures of Kraurogymnocarpa lenticulospora, from Udagawa & Uchiyama (1999).

Gymnascella marismortui is just one species in a family of microscopic fungi known as the Gymnoascaceae. The prefix gymno- in the name means 'naked', and refers to the fact that these fungi do not have the asci (spore-packets) surrounded by a strongly differentiated fruiting-body wall. Instead, the asci are surrounded by a cluster of hyphae that are little differentiated from the remainder of the fungus, or that form a net-like arrangement called a reticuloperidium (an example of the latter can be seen in the lower left of the figure above). Greif & Currah (2003) suggested that the reticuloperidium may be an adaptation to dispersal by insects, as they became caught on the hairs of flies and were then split open to release the spores when the fly was grooming itself. The ascospores themselves are oblate in shape, with polar depressions or equatorial thickenings. Gymnoascaceae are found in such habitats as soil, rotting vegetation or dung, where they break down substances such as cellulose and keratin (Stchigel & Guarro 2007).

Apart from their largely unsung role as decomposers, Gymnoascaceae have little economic impact on humans. They are relatives of the fungi that cause ringworm and tinea (indeed, these fungi have been included in the Gymnoascaceae in the past), and there have been occasional reports of Gymnoascaceae causing similar infections. However, these infections were probably just incidental: after all, to a fungus, keratin is keratin.

REFERENCES

Buchalo, A. S., E. Nevo, S. P. Wasser, A. Oren & H. P. Molitoris. 1998. Fungal life in the extremely hypersaline water of the Dead Sea: first records. Proceedings of the Royal Society of London Series B 265: 1461-1465.

Greif, M. D., & R. S. Currah. 2003. A functional interpretation of the role of the reticuloperidium in whole-ascoma dispersal by arthropods. Mycological Research 107 (1): 77-81.

Stchigel, A. M., & J. Guarro. 2007. A reassessment of cleistothecia as a taxonomic character. Mycological Research 111 (9): 1100-1115.

Udagawa, S., & S. Uchiyama. 1999. Taxonomic studies on new or critical fungi of non-pathogenic Onygenales 1. Mycoscience 40: 277-290.

Learning to Like Lichen

The lichen Parmelia saxatilis. The red cups at the top of the photo are the lichen's fruiting bodies (apothecia) that produce fungal spores. Photo from here.

We all know what lichens are. They're the standard example of a mutualistic association that we were all presented with in high school, an association of fungus and unicellular alga allowing both to survive long-term in situations that would normally be fatal for them both. More than 15,000 species of lichen have been described—or, rather, species of lichenised fungi, as names applied to lichens technically apply to the fungal member of the association (only a relatively small number of algae form lichen associations). Though these species can all be attributed to the Ascomycetes among the main fungal subdivisions*, they do not form a single clade within the asomycetes. Instead, it appears that the lichen lifestyle has been gained and/or lost on numerous occasions.

*Lichen-like associations are sometimes formed by other fungi such as Basidiomycetes but they lack the integrity of the ascomycetous examples. Lab workers have even been able to induce lichen-like associations between unicellular algae and colonial or hyphal bacteria such as myxobacteria and streptomycetes (Ahmadjian 1965).

Parmelia is a genus of foliose lichens which is found worldwide but has its highest diversity in Asia (Molina et al. 2004). Well over 1000 species have been assigned to the genus over the years but many (though not all) recent authors have tended towards a much more restricted circumscription of about forty species. True Parmelia, in this sense, is distinguished from other genera in the lichen family Parmeliaceae by its linear pseudocyphellae (pore-like structures in the upper-surface of the lichen's cortex) and its particularly small spores and conidia (conidia are reproductive structures like spores but produced asexually rather than sexually) (Elix 1993). ITS rDNA phylogeny is mostly consistent with many of the proposed segregate genera, including the restricted Parmelia, though it provides little information on their higher relationships (Crespo & Cubero 1998).


Another view of Parmelia saxatilis. As well as the spore-producing apothecium, this photo also shows numerous isidia, the small finger-like protrusions covering the thallus. Containing both fungal and algal cells, the isidia can break off to form new lichens. Photo by Stephen Sharnoff.

Parmelia achieves its highest diversity in temperate or boreal regions. The type species, P. saxatilis, is one of the world's most widespread lichen species, found in both the Arctic and the Antarctic, as well as cooler localities in between (Molina et al. 2004). Lichens can reproduce in one of two ways: small pieces of the thallus containing both algal cells and fungal hyphae may break off to grow directly into a new thallus elsewhere, or the lichen can release spores and/or conidia in the manner of other fungi. A germinating lichen spore will grow extremely slowly: even in laboratory cultures on agar, some lichen fungi will only reach a diameter of 1 mm within the course of a year when grown without algal symbionts(Ahmadjian 1965). Formation of the lichen association is dependent on the fungus randomly coming into contact with an alga, and growing lichen fungi will form exploratory hyphae around anything (even grains of sand) that they touch that might turn out to be an alga (Ahmadjian 1960). The low variety of algal species occuring in lichens appears to be dependent not on any direct attraction of the alga for the fungus, but on the alga's ability to resist digestion by the fungus' hyphae. Lichens are famed for their slow growth even after an association is established, and may increase in diameter by only a millimetre a year*, but the limiting factor is probably not so much their inherent growth abilities as that their favoured environments such as exposed on rocks may only allow growth for a minute part of the year.

*If you're thinking that that doesn't sound any greater than the rate for symbiont-less fungi that I cited above, remember that the latter rate applies to growth in the laboratory under theoretically optimal conditions; growth in the natural environment would be much, much slower.

REFERENCES

Ahmadjian, V. 1960. The lichen association. Bryologist 63 (4): 250-254.

Ahmadjian, V. 1965. Lichens. Annual Review of Microbiology 19: 1-20.

Crespo, A., & O. F. Cubero. 1998. A molecular approach to the circumscription and evaluation of some genera segregated from Parmelia s. lat. Lichenologist 30 (4-5): 369-380.

Elix, J. A. 1993. Progress in the generic delimitation of Parmelia sensu lato Lichens (Ascomycotina: Parmeliaceae) and a synoptic key to the Parmeliaceae. Bryologist 96 (3): 359-383.

Molina, M. del C., A. Crespo, O. Blanco, H. T. Lumbsch & D. L. Hawksworth. 2004. Phylogenetic relationships and species concepts in Parmelia s. str. (Parmeliaceae) inferred from nuclear ITS rDNA and β-tubulin sequences. Lichenologist 36 (1): 37-54.