The probable causes for the high degree of seed sterility often reported forLysimachia nummulariaare discussed. It seems that meiotic irregularities, i.e. lack of bivalent formation, result in the formation of a high percentage of non‐viable pollen grains and pollen grains of unequal chromosome number. As demonstrated by the existence of individuals showing seed set, these meiotic irregularities do not lead to total sterility. In a given population, the self‐incompatibility of the species and the clonal nature of the population due to its vegetative propagation seem to be a further cause for the sterility observed. The frequency of fertile individuals is comparatively high mainly in Southeast Europe. Probably this area served as a refugium in glacial times. It seems possible that the spread ofL. nummulariainto northern latitudes resulted in the degeneration of the sexual system. This might have been caused either by the participation of only one or few genotypes spreading mainly vegetatively ‐ in this situation the frequency of seed set would have decreased due to the self‐incompatibility of the species ‐ or from the absence of a pollinator, also leading to decreased
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