Spatial separation of diploid and triploid Butomus umbellatus in Britain and Ireland

The results of a cytological survey of British and Irish Butomus umbellatus L. are reported. With one exception, diploid (2n = 26) plants are distributed only in the south-western part of the species’ British range, whereas triploids (2n = 39) are much more widespread. Both cytotypes are present in Ireland. The cytotype of a population bears no clear relationship to its distributional history, as long-established native populations, invasive populations, and cultivated stock all occur as both diploids and triploids. However, the diploid may be more frequent in cultivation and an outlying northern population of the diploid probably originates from cultivated stock. The marked spatial separation of the cytotypes in Britain is not paralleled by the situation in the Czech and Slovak Republics, where the cytology of the species has been studied intensively. Possible explanations of the observed pattern in Britain are outlined.