Biodiversity and Conservation 10: 1543–1554, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Genetic variation in the horsetail Equisetum variegatum Schleich., an endangered species in the Parisian region NATHALIE MACHON 1,∗ , JEAN-MICHEL GUILLON 1,2 , GAUTHIER DOBIGNY 3 , SOLENN LE CADRE 1 and JACQUES MORET 1 1 Conservatoire Botanique National du Bassin Parisien, Muséum National d’Histoire Naturelle, 61, rue Buffon, F-75005 Paris; 2 Laboratoire Ecologie, Systématique et Evolution, UPRESA 8079 CNRS, Bâtiment 362, Université Paris-Sud, F-91 405 Orsay cedex; 3 Present adress: Laboratoire de Zoologie Mammifère et Oiseau, Muséum National d’Histoire Naturelle, 55, rue Buffon, F-75005 Paris, France; *Author for correspondence (e-mail: machon@mnhn.fr; fax: +33-1-40793553) Received 10 May 2000; accepted in revised form 26 October 2000 Abstract. Equisetum variegatum Schleicher is a circumboreale species of horsetail. In France, it typically grows at high elevations but is very rare in lowlands. The genetic variation of these populations is described using isozyme electrophoresis and PCR-RFLP of chloroplast DNA. Sampled sites were chosen to represent central vs. marginal and/or endangered parts of the distribution area. Extensive clonal multiplication of plants together with the absence of local recruitment by sexual reproduction seem to be responsible for the low genetic diversity observed within populations. Since adaptive response to environmental changes ultimately relies on the presence of genetic variability, clonal populations of E. variegatum may be partic- ularly vulnerable to disturbance. Moreover, in lowland populations, isolation gives no chance to recover new genotypes through migration events. The preservation of the two endangered populations is proposed by propagation by cuttings of all extant genetic individuals. In the case of a disappearance of one genotype in the field, a replacement will be possible. This plan may be sufficient to preserve E. variegatum in the French lowland for several years. Key words: chloroplast DNA, clonal reproduction, conservation, Equisetum variegatum, isozymes Introduction Genetic studies are more and more employed for plant conservation purposes: first to identify and evaluate the threats that endanger a species and secondly to help design optimal conservation programs. The lack of genetic variability, even when estimated at selectively neutral genetic markers, such as allozymes, would be able to decrease population viability (Vrijenhoek 1994). Indeed, a parameter such as allelic richness is highly dependent on effective population size (Nei et al. 1975) and should be a good indicator of past demographic changes (Petit et al. 1998). Loveless and Hamrick (1984) showed how the mode of reproduction of the species had an effect on genetic structure within and among populations. For species reproducing mainly asexually, the distribution of genetic markers can show how clonal a population is (Ellstrand and