SPECIATION IN LICHENS OF THE RAMALINA-SILIQUOSA COMPLEX (ASCOMYCOTINA, RAMALINACEAE) - GENE FLOW AND REPRODUCTIVE ISOLATION

The lichens of the rock-inhabiting Ramalina siliquosa complex of maritime cliffs of western Europe are differentiated into six chemotypes characterized by a replacement series of beta-orcinol depsidones. These chemotypes are sharply zoned from the most sheltered to the most exposed positions in the supralittoral zone. Since artificial crosses in lichens are impossible, the breeding system was investigated by an analysis of the progeny of maternals from nature-some growing in pure zones and others growing intermixed with different chemotypes where zones overlap. Overall, the chemotypes are highly reproductively isolated, the progeny tending to be chemically identical to their respective maternal individuals. Only one pair of chemotypes appears to represent a clear polymorphism; the remainder are apparently ecologically differentiated sibling species. Two chemotypes, which interbreed where their zones overlap, are apparently kept distinct by strong selection against unadapted sporelings in the ''wrong'' niche. Spermatia function over only short distances, also contributing to the reproductive isolation.