SYSTEMATICS AND EVOLUTIONARY HISTORY OF HEAVY-METAL TOLERANT THLASPI-CAERULESCENS IN WESTERN-EUROPE - EVIDENCE FROM GENETIC-STUDIES BASED ON ISOZYME ANALYSIS

Thlaspi caerulescens is distributed in Europe on metalliferous and not metalliferous soils. Individuals from populations growing on heavy me tal contaminated soils are well known as hyperaccumulators of zinc and cadmium. The taxonomical treatment of subspecies of Thlaspi caerulesc ens is unsettled. We investigated the degree of genetic variation amon g 28 populations of Thlaspi caerulescens from Europe with isozyme anal ysis to compare inter- and intrapopulational diversity. British materi al from heavy metal contaminated environments recognized as Thlaspi sy lvestre and T. occitanicum are quite similar to each other on the leve l of isozyme polymophisms, but they are more closely related to popula tions from non-contaminated stands from Scandinavia and Middle Europe than to metallophytes distributed in Continental Europe. Our findings indicate that a taxonomical subdivision of T. caerulescens is not poss ible and, furthermore, heavy metal tolerance might have evolved twice in populations of Thlaspi caerulescens from different areas. The trait of zinc tolerance and hyperaccumulation is frequently found in numero us relatives of Thlaspi caerulescens, and it is suggested that this tr ait has been established and manifested in populations from metallifer ous sites during postglacial colonization. From Scandinavia only non-m etallophytes are known. These populations are very similar to each oth er on the isozyme level. This fits to the hypothesis that Thlaspi caer ulescens was introduced to Scandinavia in recent times by human activi ty. Despite full self-compatibility we estimated varying outcrossing r ates up to 0.88 in the metallophytes and 0.658 in the non-metallophyte s depending on population size and structure. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.