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Lobaria pulmonaria
Closeup of a fertile thallus of Lobaria pulmonaria. Photo credit: Christoph Scheidegger
Symbiochloris reticulata
Symbiochloris reticulata (SAG 53.87). (m) Chloroplast structure of mature cells, maximum projection of multiple serial confocal sections. Note large chloroplast pores (n) Young autosporangium. (o) Mature aplanosporangium. (p) Young aplanosporangium showing inner thickening of the cell wall. Scale bars = 5 μm. From Škaloud et al 2016 Journal of Phycology DOI: 10.1111/jpy.12422.

Lobaria pulmonaria & Symbiochloris reticulata

Being the textbook example for mutualistic symbioses, lichens represent an association of a fungus - usually an ascomycete - with one or several photosynthetic partners, the photobionts. Photobionts can be green algae or cyanobacteria. In lung lichen (Lobaria pulmonaria), the primary photobiont is the green alga Symbiochloris reticulata, but the lichens also contain Nostoc cyanobacteria in internal structures called cephalodia. These internal cyanobacteria fix atmospheric nitrogen, thus contributing to lichen growth and to the ecosystem nitrogen balance. Some consider today's populations of L. pulmonaria to be relicts of a time when the species was very abundant in the far moister Tertiary climate. The lichen species is threatened and continuously declining because of complex interactions between the lichen's spatial distribution, abundance and genetic diversity of populations, and the type, spatial extent, frequency and severity of disturbance of the lichen habitat i.e. natural and managed forests (Scheidegger & Werth 2009).
     Lobaria pulmonaria's green-algal photobiont Symbiochloris reticulata has a spherical chloroplast located close to the cell wall (parietal), forming an intricate, net-like, structure (Škaloud et al. 2016); this chloroplast shape also explains the species' name ("reticulata" meaning net-shaped). Asexual reproduction occurs by 4-8(-16) autospores, stages of sexual reproduction are so far unknown. The majority of lichen photobionts including S. reticulata belongs to the class Trebouxiophyceae, and this green alga teams up with the ascomycete family Lobariaceae. In nemoral and boreal ecosystems of Europe, other lichen fungi pairing up with this photobiont are rare (Lobaria virens, Ricasolia amplissima) (Dal Grande et al. 2014). In subtropical forests, S. reticulata is shared among several species of lichen-forming fungi of the Lobariaceae.
     For most lichens, an individual green algal photobiont species associates with many lichen-forming fungi, and a single lichen-forming fungus partners with several green algal species. However, Lobaria pulmonaria pairs up with only a single green-algal species, and is therefore one of the most specific symbioses known in lichens to date. The association is mainly transmitted vertically with some episodes of horizontal transmission (Dal Grande et al. 2012; Werth & Scheidegger 2012). In Europe, phylogeographic structures are highly similar for Lobaria pulmonaria and Symbiochloris reticulata (Widmer et al. 2012). Moreover, a high frequency of geographically restricted alleles indicates that Southern Italy and the Balkans are primary refugial areas, characterized by long-term historical continuity of the symbiotic association, possibly over several glacial-interglacial cycles.
     Genetic structures are highly similar among the two symbionts also on smaller spatial scales (Werth & Scheidegger 2012; Nadyeina et al. 2014). One of the most striking population genetic patterns found in lichens to date is that of Lobaria pulmonaria where two fungal-algal gene pools showed climatic associations, one occurring on mountain ridges and the other in valleys and slopes of the Carpathian mountains (Nadyeina et al. 2014). This pattern may have been shaped by natural selection.