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A Case of Reversal: The Evolution and Maintenance of Sexuals from Parthenogenetic Clones in Hieracium pilosella

We provide evidence for the origin of sexual individuals from parthenogenetic progenitors in natural populations. We demonstrate that this reversal has occurred independently in three geographically separated populations of the Asteraceous polyploid, Hieracium pilosella. We used chromosome counts and flow cytometry to determine ploidy and crossing experiments and flow cytometry to confirm sexuality. Inter–simple sequence repeat and allozyme markers demonstrated that the sexuals at each site were more closely related to their parthenogenetic neighbors than to sexuals at other sites. The same markers were used to estimate levels of ramet diversity, which were equally high among the parthenogens and sexuals. The observation that sexuals were always tetraploid is possibly explained by their having arisen through a rare sexual event, the fusion of two reduced (2x) gametes from pentaploid, facultatively apomictic parents. Such a reversal from almost total clonality to obligate sexual reproduction is unusual, and further work will determine whether the sexuals are in evolutionary equilibrium, are increasing at the expense of asexuals, or are simply surviving because of a lack of negative selection pressure.