Dual propagule formation, a strategy to colonize substrates that are devoid of compatible photobionts 94

Incongruent habitat requirements of associated bionts might promote the formation of dual propagules, such as sorediea, isidia, or fragmenting thalli. A corresponding correlation could be observed in several genera of the Physciaceae. Only Trebouxia taxa of subclades A5 - A11 were associated with asorediate Physciaceae growing on calcareous rock. All common, European species of Physcia, Physconia and Phaeophyscia investigated were selective for members of Trebouxia clade I. However, some species, e.g. Phaeophyscia orbicularis, Physcia caesia grow on calcareous rock. These two mycobiont species are also selective for clade I. All other European and North American species of these genera that are described to occur on calcareous rock were also described to be sorediate (Wirth 1995, Brodo et al. 2001). It is assumed that mycobionts are able to neutralize the basic pH of calcareous rock (Schöller 1997) and thereby support their compatible photobiont which alone would not be able to thrive on these substrates and therefore would not be available for germinating ascospores. Among other things, soralia formation might therefore be interpreted as a mycobiont's strategy to colonize substrates that do not bear its compatible algae free living.

Or, more generally, deviating substrate preferences of compatible bionts might promote the evolution of dual propagules.

5.4.7.6 Single Trebouxia ITS-variants might be ecologically differentiated

A wider ecological amplitude of Trebouxia subclades when compared to most of the Physciacean mycobionts is obvious from the multitude of phylogenetically and ecologically distinct mycobiont species with which they are lichenized. Even single Trebouxia ITS-variants might not be homogeneous with respect to their ecology and physiology. This assumption was implied by a study of Bačkor & Váczi (2002), who cultivated strains of “Asterochloris” erici for three years on media with increased copper concentration.

These “Asterochloris” strains achieved a significantly increased copper tolerance when compared to clones that were kept on media with low copper concentrations. Since this time span is unlikely to allow the ITS region to change noticeably, physiological plasticity is assumed to be much greater than ITS variability.

Therefore distinct ecotypes might be suspected that are characterized by identical ITS sequences. In the case of particular Trebouxia ecotypes that thrive in habitats that are unsuitable for certain mycobionts, it may be

Symbiosis in Associations of Physciaceae and Trebouxia

suspected that a given mycobiont might not be able to lichenize all ITS-variants of its compatible subclade due to an ecological restriction, rather than an instance of physiological incompatibility.

When comparing the frequencies at which certain Trebouxia ITS-variants were found on particular substrates an ecological differentiation as supposed above is supported. Of the 22 Physciacean specimens that were associated with ITS-variant I1a, 20 were collected from bark and two specimens were collected from rock (TABLE 5.7). Those two collected from rock were sorediate. The same correlation was observed in ITS-variant I1d. Twenty-seven specimens were analyzed, of which three were collected from rock and then associated with sorediate mycobiont species. As discussed above, a correlation between Trebouxia and substrate might be obscured in sorediate lichens and therefore both ITS-variants are considered to strongly prefer bark. Although preferring the same substrate, distinct ecological differences between these two ITS-variants were observed. While ITS-variant I1a appeared to occur all over Europe (Austria, Spain, Sweden) and in lichens with sexual and asexual reproduction mode, ITS-variant I1d was not observed in Spain (24 Spanish specimens of Physcia, Physconia, and Phaeophyscia analyzed) and almost exclusively found in sorediate lichens. Physconia distorta was the only asorediate lichen species in which ITS-variant I1d was found. In contrast to the two previous two ITS-variants, ITS-variant I1b was detected more often in lichens growing on rock. This ITS-variant was detected four times in asorediate, crustose specimens of the species Dimelaena oreina (three specimens) and Diplotomma alboatrum (one specimen) and once in a corticol specimen of Phaeophyscia orbicularis, a sorediate and foliose species (TABLE 5.7).

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96

TABLE 5.7: Sources from which particular Trebouxia ITS-variants of subclade I1 were obtained. *: Samples of Dahlkild et al. (2001) were included. Dual propagules are soredia or isidia that serve as propagation units.

Trebouxia

Sweden Fraxinus excelsior, Populus tremula, Ulmus glabra Physcia adscendens 3 yes Germany, Spain Acer sp., Malus domestica,

Populus sp.

Physcia aipolia 2 no Germany, Spain Juglans regia, Populus nigra

Physcia dimidiata 1 yes Austria Tuffit

Physcia semipinnata 1 no Spain bark

Physcia stellaris 3 no Germany, Spain Juglans regia, Populus sp., Quercus pyrinaicus

Physcia tenella 1 yes Spain Ailanthus sp.

Physconia distorta 3 no Spain Populus sp., Quercus pyrinaicus Physconia

enteroxantha

1 yes Spain rock

Physconia grisea 1 yes Spain bark

I1a

Physconia venusta 1 no Spain Quercus pyrinaicus

Diplotomma alboatrum

1 no Sweden On mortar

Dimelaena oreina 3 no Austria, Spain siliceous rock I1b

Phaeophyscia orbicularis

1 yes Spain Populus nigra

Buellia pulverulenta 1 yes Sweden calcareous rock

Hyperphyscia adglutinata

1 yes Germany Aesculus hyppocastanea.

Physcia adscendens 4 yes Germany Acer pseudoplatanus, Fraxinus sp., Malus domestica

Physcia tenella* 8 yes Finland, Germany,

Sweden Acer pseudoplatanus, Fraxinus excelsior, Populus tremula, Quercus robur, Salix sp., Tilia cordata

Physconia distorta* 4 no Germany, Sweden Acer platanoides, Fraxinus excelsior, Quercus robur, Tilia cordata

Physconia enteroxantha

1 yes Germany Populus x canadensis

Physconia grisea 7 yes Austria, Germany,

Italy Acer pseudoplatanus, Catalpa bignonioides, Cupressus semervirens, Juglans regia, Tilia sp., mossy slate wall, Tuffit I1d

Physconia perisidiosa

1 yes Germany Acer pseudoplatanus

Symbiosis in Associations of Physciaceae and Trebouxia