Euploid and aneuploid evolution in Potamogeton (Potamogetonaceae): a factual basis for interpretation

In a review of chromosome numbers in the genus Potamogeton, we highlight numerous errors that have crept into the literature. These have resulted chiefly from reliance on abstracts in chromosome number indices and compilations, rather than on the original publications, but partly also because of misleading summaries even in the primary literature. We present a list of counts that we believe are original and genuine, and a list of those that were never made but which nevertheless appear in the literature. Scrutiny of the list of accepted counts indicates that aneuploidy is widespread in the genus and that transition between the two common chromosome numbers (2 n=26 and 2 n=28) has occurred several times. Currently available data are insufficient to resolve the question of the ancestral base number. We also present details of the first chromosome counts from English populations of five taxa: P. polygonifolius Pourr. (2 n=28), P. pectinatus L. (2 n=ca. 78), P. perfoliatus L. (2 n=ca. 52), P.× nitens Weber ( P. gramineus× P. perfoliatus) (2 n=ca. 52) and P.× salicifolius Wolfg. ( P. lucens× P. perfoliatus) (2 n=ca. 52).