Conservation Genetics 2: 271–277, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 271 A study of genetic variation in the threatened hepatic Petalophyllum ralfsii (Wils.) Nees. and Gottsche (Fossombroniaceae) Frederick J. Rumsey ∗ , Johannes C. Vogel & Stephen J. Russell The Natural History Museum, Cromwell Road, London, SW7 5BD, U.K. ( ∗ Author for correspondence: E-mail: F.Rumsey@nhm.ac.uk) Received 28 December 2000; accepted 17 February 2001 Key words: allozyme, monomorphic, Petalophyllum Abstract Genetic diversity of the dioicous thallose hepatic Petalophyllum ralfsii was investigated and its significance for the species’ conservation assessed. This globally threatened hepatic apparently reaches its greatest abundance in the British Isles. Samples were taken from throughout the British range; 178 individual thalli from 24 colonies in nine localities were scored for variation in 11 enzyme systems and 16 putative loci. No variation was observed. Many studies demonstrating allozymic monomorphism in bryophytes have been of bisexual, or predominantly vegetatively propagating taxa. Lack of variability in an obligately outbreeding taxon, with no specialised vegetative dispersal mechanism, lends support to the view that hepatic taxa, at least within Europe, routinely support little allozymic variation. Unlike flowering plants, life history and breeding system do not appear to have significant effects on the extent and partitioning of allozymic diversity. Introduction Petalophyllum ralfsii (Wils.) Nees and Gottsche is a small thallose liverwort, the only European represen- tative of an unmistakeable genus of bryophytic plants uniquely possessing parallel photosynthetic lamellae on the upper thallus surface. The most widespread of the five described species, P. ralfsii is scattered throughout coastal areas in the Mediterranean region eastwards to Turkey, reappearing further north along the Atlantic coasts of Europe, as far north as N.W. Scotland. Elsewhere it is reported only in the southern U.S.A in Texas, Louisiana and Arkansas (Schuster 1992) but doubts have been expressed as to the conspecificity of New-World material (Porley pers. comm.). P. ralfsii is dioicous, i.e. has a life-cycle in which the dominant haploid gametophyte phase bears male and female gametes on separate individuals. It is a perennial species without any specialized asexual means of propagation, but which can over time, through bifurcation and die-back of older portions, fragment to form clonal patches many metres in extent; individual thallus rosettes being typically less than 10 mm in diameter. The species is restricted to a limited and widely but irregularly distributed habitat which is transient in nature. In Britain, P. ralfsii only occurs in open, seasonally-inundated calcareous depressions in maritime dune systems. This distinctive species’ rarity, and concerns that it was in widespread decline, lead to its legal protection at both European and national levels, e.g. inclusion on Appendix 1 of the Bern Convention on the conservation of European wildlife and natural habitats (Anon. 1979) and Annex II of the EC Habitats Directive (Anon. 1992) and to protection within Britain under Schedule 8 of the Wildlife and Countryside Act, 1981. It was among the first “lower” plants to be considered for such protection and accordingly has been seen as some- thing of a flagship for cryptogamic conservation. The majority of its known extant sites occur within the British Isles, fittingly recognised by its inclusion as a “Priority species” within the UK Biodiversity Action Plan process (Anon. 1995). An allozymic study was undertaken in conjunc- tion with a long-term population monitoring exercise,