Symbiotic lifestyle and phylogenetic relationships of the bionts ofMastodia tessellata(Ascomycota,incertae sedis)

Abstract

The biological nature of some symbioses is unclear because it is often not easy to discern whether the symbionts obtain any benefits from the association. Mastodia tessellata, a symbiosis between a leafy green alga and a fungus of uncertain phylogenetic position, is among the most investigated, controversial, and poorly understood associations. Because it has been difficult to determine whether this association is mutually beneficial or parasitic, not all scientists accept M. tessellata as a true lichen symbiosis. Mastodia tessellata is thus an interesting model to illustrate the interactions and processes that occur in fungal-algal symbioses. To improve our understanding of this association, we address the phylogenetic positions of the bionts involved and examine their interactions at the ultrastructural level. Examining the nuLSU and nuSSU gene regions of the mycobiont and the rbcL gene region of the photobiont, we found the fungus to be related to a group of marine species in the genus Verrucaria, family Verrucariaceae, despite its present ascription to the family Mastodiaceae. In addition, the photobiont of the symbiosis emerged as closely related to the North American species Prasiola borealis. Our electron microscopy observations provide new information on the process of fungal colonization of the algal thalli, as well as on relationships between the symbionts during different stages of colonization. The special features of this lichen symbiosis are discussed and compared with other examples of fungal symbioses in nature.

Revisiting photobiont diversity in the lichen family Verrucariaceae (Ascomycota)

“…4) The multigene phylogeny (RPB1-nuLSU-nuSSUmtSSU) was similar to previous studies (Gueidan et al, 2007Savic´et al, 2008;Muggia et al, 2009Muggia et al, , 2010Pe´rez-Ortega et al, 2010). Additionally, the phylogenetic position of several freshwater species was demonstrated here: Verrucaria elaeina was sister to V. praetermissa,…”

“…In most lichens, the symbiotic alga is embedded in a fungal tissue forming a lichen thallus that is dominated by the mycobiont. However, in the Verrucariaceae several fungal species are found in association with thalloid algae, such as the brown algae Pelvetia and Ascophyllum (Kohlmeyer & VolkmannKohlmeyer, 1998), and the green alga Prasiola crispa (Lightfoot) Ku¨tzing (Pe´rez-Ortega et al, 2010). In these associations the mycobiont grows within the independently formed algal thallus.…”

“…In these associations the mycobiont grows within the independently formed algal thallus. Although the Prasiola photobiont of the species of Verrucariaceae Mastodia tessellata (Hooker f. & Harvey) Hooker f. & Harvey has recently been characterized (Pe´rez-Ortega et al, 2010), little is known about the photobiont identity of its close relatives found in typical fungus-dominated lichen thalli.…”

“…Finally, DNA sequence data provide a phylogenetic framework for algal classification (Friedl & Bü del, 2008). In the Verrucariaceae, however, very few studies have included molecular data for the photobionts (Friedl & Zeltner, 1994;Peters & Moe, 2001;Nyati et al, 2007;Pe´rez-Ortega et al, 2010).…”

Interface between fungi and green algae in lichen associations

“…In most lichens, the fungal hyphae do not pierce the cell wall of the alga where they form peg-like structures called appressoria. Appressoria may then develop into intraparietal haustoria (Honegger 1985(Honegger , 1986a(Honegger , 1991a where they may dissolve the inner algal wall layer (Pérez-Ortega et al 2010). These haustoria are found in lichens with algae belonging to the genera Coccomyxa, Elliptochloris (Brunner and Honegger 1985;Honegger 1991b), Myrmecia and Dictyochloropsis (Honegger 1990(Honegger , 1991b(Honegger , 1992.…”