AOBPreview originally published online on February 14, 2005
Annals of Botany 2005 95(5):807-815; doi:10.1093/aob/mci084
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Triploidy in Equisetum subgenus Hippochaete (Equisetaceae, Pteridophyta)
1 Spezielle Botanik, Ruhr-Universität Bochum, D-44780 Bochum, Germany, 2 Wodantal 28, 45529 Hattingen, Germany and 3 Partec GmbH, Otto-Hahn-Str. 32, D-48161 Münster, Germany
* For correspondence. E-mail wilfried.bennert{at}rub.de
Received: 22 July 2004 Returned for revision: 13 October 2004 Accepted: 20 December 2004 Published electronically: 14 February 2005
• Background and Aims The genus Equisetum is cytologically uniform, having a base chromosome number of x = 108. All previously known species and hybrids that have been counted represent diploids with a sporophytic chromosome number of 2n = 216. Biosystematic studies on Equisetum subgenus Hippochaete revealed evidence that triploids occur in nature. The objective of this study was to confirm that triploid plants exist in the natural environment.
• Methods Flow cytometry was used to establish nuclear DNA values and cytological investigations of meiosis were carried out to obtain information on chromosome number and pairing behaviour.
• Key Results Triploidy exists in three morphologically different hybrid taxa. Two of these are morphologically intermediate between a primary diploid hybrid and a parent, while the third apparently combines genomes from all three Central European Hippochaete species. Nuclear 1C DNA values for the four European Hippochaete species range from 21·4–31·6 pg. For the hybrids, the 1C DNA values not only occupy the same range as the species, but their total DNA amounts agree closely with values predicted by adding the 1C DNA values of each parental genome. Chromosome counts confirm diploidy in the species E. hyemale and E. variegatum and in the hybrid E. xtrachyodon (= E. hyemale x E. variegatum). For the triploids (2n 
324), cytological information is presented for the first time.
• Conclusions Triploid taxa may have originated by backcrossing or by crossing of a diploid hybrid with an unrelated diploid species. As tetraploid plants are unknown, these crossings probably involve diploid gametophytes that developed from unreduced diplospores. By repeated crossing events or backcrossing, reticulate evolution patterns arise that are similar to those known for a number of ferns and fern allies.
Key words: Equisetum, subgenus Hippochaete, flow cytometry, nuclear DNA content, triploidy, chromosome numbers, hybridization, reticulate evolution