BACKGROUND AND AIMS: Despite the recent wealth of studies targeted at contact zones of cytotypes in various species, some aspects of polyploid evolution are still poorly understood. This is especially the case for the frequency and success rate of spontaneous neopolyploidization or the temporal dynamics of ploidy coexistence, requiring massive ploidy screening and repeated observations, respectively. To fill this gap, an extensive study of spatio-temporal patterns of ploidy coexistence was initiated in the widespread annual weed Tripleurospermum inodorum (Asteraceae). METHODS: DNA flow cytometry along with confirmatory chromosome counts was employed to assess ploidy levels of 11 018 adult individuals and 1263 ex situ germinated seedlings from 1209 Central European populations. The ploidy screening was conducted across three spatial scales and supplemented with observations of temporal development of 37 mixed-ploidy populations. KEY RESULTS: The contact zone between the diploid and tetraploid cytotypes has a diffuse, mosaic-like structure enabling common cytotype coexistence from the within-population to the landscape level. A marked difference in monoploid genome size between the two cytotypes enabled the easy distinction of neotetraploid mutants from long-established tetraploids. Neotetraploids were extremely rare (0·03 %) and occurred solitarily. Altogether five ploidy levels (2 x -6 x ) and several aneuploids were discovered; the diversity in nuclear DNA content was highest in early ontogenetic stages (seedlings) and among individuals from mixed-ploidy populations. In spite of profound temporal oscillations in cytotype frequencies in mixed-ploidy populations, both diploids and tetraploids usually persisted up to the last census. CONCLUSIONS: Diploids and tetraploids commonly coexist at all spatial scales and exhibit considerable temporal stability in local ploidy mixtures. Mixed-ploidy populations containing fertile triploid hybrids probaby act as effective generators of cytogenetic novelty and may facilitate inter-ploidy gene flow. Neopolyploid mutants were incapable of local establishment.

Department of Botany Faculty of Science Charles University Benátská 2 CZ 128 00 Prague Czech Republic

Department of Botany Faculty of Science University of South Bohemia Branišovská 1760 CZ 370 05 Ceské Budejovice Czech Republic

Department of Botany Faculty of Science University of South Bohemia Branišovská 1760 CZ 370 05 České Budějovice Czech Republic

Institute of Botany The Czech Academy of Sciences Zámek 1 CZ 252 43 Pruhonice Czech Republic

Institute of Botany The Czech Academy of Sciences Zámek 1 CZ 252 43 Průhonice Czech Republic

National Centre for Biosystematics Natural History Museum University of Oslo NO 0318 Oslo Norway

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Reproductive pathways in Hieracium s.s. (Asteraceae): strict sexuality in diploids and apomixis in polyploids