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 metal contaminated soils are well known as hyperaccumulators of zinc and cadmium. The taxonomical treatment of subspecies of Thlaspi caerulescens is unsettled. We investigated the degree of genetic variation among 28 populations of Thlaspi caerulescens from Europe with isozyme analysis to compare inter- and intrapopulational diversity. British material from heavy metal contaminated environments recognized as Thlaspi sylvestre and T. occitanicum are quite similar to each other on the level of isozyme polymophisms, but they are more closely related to populations 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 possible 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 numerous relatives of Thlaspi caerulescens, and it is suggested that this trait has been established and manifested in populations from metalliferous sites during postglacial colonization. From Scandinavia only non-metallophytes are known. These populations are very similar to each other on the isozyme level. This fits to the hypothesis that Thlaspi caerulescens was introduced to Scandinavia in recent times by human activity. Despite full self-compatibility we estimated varying outcrossing rates up to 0.88 in the metallophytes and 0.658 in the non-metallophytes depending on population size and structure. (C) 1998 Elsevier Science Ltd. All rights reserved.