Diversity and phylogeography in Alisma (Alismataceae), with emphasis on Northern European taxa

Abstract: In the present thesis the phylogeny and patterns of genetic diversity of the plant genus Alisma (Alismataceae) were investigated by means of genetic markers (RAPDs, allozymes, and ITS and trnL sequences). Additionally, morphological studies were carried out in paper IV. All species in the genus were found to be closely related to each other and two major groups of diploid taxa were found: one containing Alisma gramineum and A. wahlenbergii and the other containing A. plantago-aquatica, A. subcordatum, A. orientale, A. juzepczukii and the not yet described taxon “A. bottnica”. The last two species are very similar to A. plantago-aquatica. The polyploid taxa (A. lanceolatum, A. triviale, A. rariflorum and A. canaliculatum) differed from each other as well as from the diploid taxa and may thus be the result of ancient hybridization events where nowadays-extinct taxa have been involved. The Baltic endemic A. wahlenbergii are most similar to populations of the probable ancestral species A. gramineum from Lake Peipsi in Estonia and the Gulf of Finland. Thus, it is likely that the ancestor of A. wahlenbergii entered the Baltic Sea from the east, presumably via the Gulf of Finland. There are indications of a multiple origin of A. wahlenbergii. The regional populations in Lake Mälaren and the Gulf of Bothnia seems to have a common origin, while the regional population in the Gulf of Finland may have originated from other populations of A. gramineum. The genetic markers revealed a weak geographic pattern in the common, widespread species A. plantago-aquatica, while the much more uncommon species A. gramineum showed a quite distinct pattern with strong differentiation between regions. The observed pattern of genetic variation indicates that A. plantago-aquatica in Europe has had a wide distribution south of the land ice even during maximum glacial. Alisma gramineum has probably also existed with several but nonetheless more isolated populations during the ice age. Furthermore, two major refugial sources were found for this species: one in the east (presumably Russia or Ukraine) and one in central and W Europe. These two genetic forms have probably been isolated from each other for a long period of time. The studied populations of A. gramineum from the St. Lawrence River in SE Canada most likely originate from recent introduction by man of W European material of the species. The two narrow-leaved forms of A. plantago-aquatica from the Baltic Sea region (A. juzepczukii and “A. bottnica”) were found to have different origins, despite a similar leaf shape. “Alisma bottnica” has probably originated as a locally adapted ecotype of A. plantago-aquatica in situ in the northern parts of the Gulf of Bothnia. This taxon should probably be regarded as a subspecies of A. plantago-aquatica. The origin of A. juzepczukii, which is found mainly in the St. Petersburg region in Russia, is less certain and this taxon seems to be more variable and less distinct than “A. bottnica” and should probably be regarded as a variety or forma of A. plantago-aquatica.