Lobaria pulmonaria
& Symbiochloris reticulata
Being the textbook example for mutualistic symbioses, lichens
represent an association of a fungus - usually an ascomyc
ete - 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.
References
- Dal Grande F, Beck A, Cornejo C, Singh G, Cheenacharoen S, Nelsen MP, Scheidegger C (2014) Molecular phylogeny and symbiotic selectivity of the green algal genus Dictyochloropsis s.l. (Trebouxiophyceae): a polyphyletic and widespread group forming photobiont-mediated guilds in the lichen family Lobariaceae. New Phytologist 202, 455-470.
- Dal Grande F, Widmer I, Wagner HH, Scheidegger C (2012) Vertical and horizontal photobiont transmission within populations of a lichen symbiosis. Molecular Ecology 21, 3159-3172.
- Nadyeina O, Dymytrova L, Naumovych A, Postoyalkin S, Werth S, Cheenacharoen S, Scheidegger C (2014) Microclimatic differentiation of gene pools in the Lobaria pulmonaria symbiosis in a primeval forest landscape. Molecular Ecology 23, 5164-5178.
- Scheidegger C, Werth S (2009) Conservation strategies for lichens: insights from population biology. Fungal Biology Reviews 23, 55-66.
- Škaloud P, Friedl T, Hallmann C, Beck A, Dal Grande F (2016) Taxonomic revision and species delimitation of coccoid green algae currently assigned to the genus Dictyochloropsis (Trebouxiophyceae, Chlorophyta). Journal of Phycology, DOI: 10.1111/jpy.12422.
- Werth S, Scheidegger C (2012) Congruent genetic structure in the lichen-forming fungus Lobaria pulmonaria and its green-algal photobiont. Molecular Plant-Microbe Interactions 25, 220-230.
- Widmer I, Dal Grande F, Excoffier L, Holderegger R, Keller C, Mikryukov VS, Scheidegger C (2012) European phylogeography of the epiphytic lichen fungus Lobaria pulmonaria and its green algal symbiont. Molecular Ecology 21, 5827-5844.