Revision of chromosome numbers of Potamogetonaceae: a new basis for taxonomic and evolutionary implications.

In order to establish a sound basis for systematic and evolutionary research, we determined the chromosome numbers of 181 samples of 47 species and 32 hybrids of Potamogetonaceae from 27 countries and areas, ranging from Greenland in the north to New Zealand in the south and reevaluated previously published counts. The first counts are reported here for 10 species and 25 hybrids of Potamogeton and for 1 species and 3 hybrids of Stuckenia. Both homoploid and heteroploid hybrids were identified, as well as hybrids resulting from the fusion of reduced and unreduced gametes. Three previously undetected hybrids of Potamogeton are described and validated as P. ×drepanoides, P. ×luxurians and P. ×serrulifer. The extensive within-species variation in chromosome numbers sometimes reported in the literature was not confirmed. Chromosome numbers appeared to be generally species-specific in Potamogetonaceae; the only exceptions were two sterile autotriploid plants detected within two otherwise fertile diploid Potamogeton species. Furthermore, chromosome numbers were often uniform even within species groups or genera and to some degree also clade-specific in phylogenetic trees based on nuclear ribosomal markers (ITS and 5S-NTS regions). In the largest genus, Potamogeton, there are two base numbers for diploids (x = 13 and x = 14) and three ploidy levels in species (diploids, tetraploids and octoploids; all polyploids were based on x = 13), in Stuckenia only hexaploids (also based on x = 13) occur and Groenlandia is monotypic with x = 15. A critical evaluation of the published counts revealed three major sources of error: (i) methodological problems due to difficult karyotypes, (ii) approximations based on wrong preconceptions and (iii) poor taxonomic treatments, misidentified species or unrecognized hybrids. We estimate that about 24% of the counts in original publications and up to 41% in chromosome atlases and indices are doubtful or demonstrably erroneous. Most of these were from a relatively few dubious sources whereas the majority of counts reported in the literature correspond to our findings. Two alternative scenarios for the evolution of chromosome numbers in this family are discussed in a phylogenetic context, with either x = 13 or x = 14 as the base chromosome number in the family; the base number of x = 7 suggested by some authors is refuted. In both scenarios, several aneuploid transitions between these karyotypes and a single change towards x = 15 have to be assumed. Polyploidizations are rare in this family and mostly associated with major evolutionary events. A single or a very few events led to a large species group of tetraploids in Potamogeton, and two subsequent rounds of polyploidization can explain the cytotype of Stuckenia, in which speciation took place entirely at the hexaploid level. Three octoploid species of Potamogeton had allopolyploid origins. This study gives an example of how the careful re-examination of chromosome numbers can substantially ameliorate interpretations of systematic and phylogenetic patterns.