ХОЗЯЕВА ИЛИ ПАРАЗИТЫ? ПАРАЗИТИЧЕСКИЕ ОБРАЗОВАНИЯ ЛИШАЙНИКОВ НА СУХИХ СКАЛАХ КАК ЭФФЕКТИВНЫЙ СПОСОБ ЛИХЕНИЗАЦИИ МОЛОДЫХ МИКОБИОНТОВ

HOSTS OR PARASITES? NON-SPECIFIC LICHENICOLOUS GROWTH OF LICHENS IN DRY ROCK COMMUNITIES AS AN EFFECTIVE TOOL FOR LICHENIZATION OF YOUNG MYCOBIONTS

Ян Вондрак¹, Ондржей Пекса², Ольга Вондракова³

Jan Vondr?k¹, Ond?ej Peksa², Olga Vondr?kov?³

¹Институт Ботаники ЧАН, (Замок-1, 25243, Пругонице, Чешская Республика) и Университет Южной Богемии, отделение ботаники факультета биологических наук, (Бранишовска 31, 370 05, Ческе Будеёвице, Чешская Республика)

¹Institute of Botany, Academy of Sciences, (Z?mek 1, CZ–252 43 Pr?honice, Czech Republic) & Department of Botany, Faculty of Biological Sciences, University of South Bohemia, (Brani?ovsk? 31, 370 05, ?esk? Bud?jovice, Czech Republic)

e-mail: j.vondrak@seznam.cz

²Западночешский Плзеньский музей (Сады Копецкего 2, 30100, Плзень. Чешская Республика)

²Z?pado?esk? muzeum v Plzni (Kopeck?ho sady 2, CZ-30100, Plze?, Czech Republic)

e-mail: opeksa@zcm.cz

³Федеральное государственное бюджетное учреждение науки Институт степи Уральского отделения Российской академии наук (460000 Россия, Оренбург, ул.Пионерская, 11)

³Institute of Steppe of the Ural Brunch of the Russian academy of Sciences(460000, Russia, Pionerskaya str, 11)

e-mail: mer.os@mail.ru

Эпилитные лишайники в аридных условиях часто образуют очень богатые сообщества, где неспецифично нарастают друг на друге. Интересным является то, что первоначально лишайники ведущие себя как паразиты, вскоре сами становятся хозяевами для других лишайников. Изучение этого явления представляет большой интерес, поскольку оно может показать, каким образом начинается процесс первичной лихенизации (поиска водоросли грибом) в аридных условиях.

In dry conditions, epilithic lichens often form abundant populations and individuals of various lichens overgrow each others. The fascinating fact is that lichens starting as parasites become hosts for other starting lichens in later stages. A question for the future research is: Why these "parasite becomes host" cycles are pronounced especially in arid regions?

Lichens are basically dual organisms [5] containing fungus (mycobiont) and alga / cyanobacterium (photobiont). This makes understanding their biology fairly difficult and many questions are still unsufficiently answered. For instance, we still know a little about the establishment of a new thallus.

In vegetatively reproducing lichens, where both partners are distributed together in large multi-cellular diaspores (e.g. soredia = globose units of several tens of micrometers in diameter containing colony of photobiont cells intermixed and coated by cells of mycobiont), establishment of the new lichen is no mystery. Quick mobilization of both lichen partners will result in the new thallus. Nevertheless, situation is not always so straightforward - algal population, which was transferring with the mycobiont in the soredium, may be replaced by the local population of a new photobiont [8].

Many lichens are known to reproduce exclusively by small fungal diaspores: by sexual ascospores and asexual conidia. Whether mycobiont in such lichens may be easily dispersed, the algal partner does not produce and diaspores for effective dispersal. Its mobility is thus restricted, although algal cells or colonies of cells may be removed from thalli and distributed by themselves.

The green algae from the genus Trebouxia represent frequently detected photobionts. Although Trebouxia has been found in a free-living state [e.g. 1, 4], these findings are rather rare. It is almost certain, that Trebouxia algae do not form larger and permanent free-living colonies. They are predominantly found in lichens, in their vegetative diaspores (mainly soredia) or in decaying lichen thalli.

Restricted source of available photobionts must often cause obstacles in lichenization of germinating fungal ascospores / conidia. Selectivity of mycobionts for only particularTrebouxia lineages [2] makes their situation even worse. Germinating mycobionts may solve this problem by taking photobiont from foreign soredia [3]. It is suggested that detached soredia of various lichens are abundant in most lichen-inhabited habitats and they may densely covered substrata colonized by young mycobiont mycelia.

Another possibility for the young mycelium to obtain the photobiont is starting on other lichen. Lichenicolous growth is not considered as the main way to obtain the photobiont partner in most habitats; lichenicolous lichens are in low minority, when taken data from e.g. the main European lichen floras [6, 7]. Nevertheless, "cryptic" lichenicolous growth in early stages of lichens, which are soon free-living, is very probable. The well known example is a lichen Xanthoria parietina which obtains its photobiont by invading soredia, pre-thalli or thalli of Physcia species [3].

In steppes of southern Russia, we have found lots of lichen communities on limestone rocks, where lichenicolous growth is strongly pronounced in most of lichen species (Tab. 1).

Each species often behave as lichenicolous lichens at the beginning but later becomes the host for other starting lichen mycobionts. The obviously big frequency of these "parasite becomes host" cycles in very dry continental areas is probably not random. We have no doubts that photobionts are transferred in these cycles from hosts to young thalli, but why is this strategy enormously developed in the dry areas?

Hypotheses for future investigations

(1) Non-specific lichenicolous growth of lichens increases towards dry habitats.

(2) Biodiversity of photobionts decreases towards dry habitats.

(3) Lichen mycobiont diversity decreases towards dry habitats.

(4) Biodiversity and abundance of vegetatively distributed species decreases towards dry habitats.

(5) Surviving of detached vegetative diaspores is limited in dry habitats.

(6) "parasite becomes host" cycles exist in various habitats with high frequencies. They are more obvious in dry areas, because of slow growth and long life of lichens in dry habitats.

Our research was supported by the Visegrad Fund (51100753) and Institute of Botany AS ?R (AV0Z60050516).

REFERENCES:

1. Bubrick P., Galun M. & Frensdorff A. (1984): Observations on free-living Trebouxia de Puymaly and Pseudotrebouxia Archibald, and evidence that both symbionts fromXanthoria parietina (L) Th. Fr. can be found free-living in nature. New Phytologist 97: 455–462.
2. Nyati S. (2007): Photobiont diversity in Teloschistaceae (Lecanoromycetes). Dissertation, Universit?t Z?rich, Switzerland.
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