Hybridization between European and Asian dandelions (Taraxacum section Ruderalia and section Mongolica): 1. Crossability and breakdown of self-incompatibility.

In order to investigate crossability between the European section Ruderalia Kirschner, H. Øllgaard & Štěpánek and the Asian section Mongolica Dahlst. of the genus Taraxacum Wigg., artificial hybridization experiments were carried out. After inter-sectional crosses on the diploid level, Mongolica plants showed high fruit-set, not strikingly different from either the control (within-populational crosses) or inter-specific crosses within the same section. However, a substantial decrease of fruit-set was detected when Ruderalia plants were the seed parents. Electrophoretic analysis revealed that Ruderalia seed parents in inter-sectional crosses showed relatively low out-crossing rates with low fruit-set, whereas Mongolica plants as seed parents showed high out-crossing rates, mostly 100%. These results indicate that the crossing-barrier is stronger in the direction from Mongolica to Ruderalia than vice versa. Crosses between diploid Mongolica (♀) and triploid Ruderalia (♂) were also successful, although fruit-set was far lower than in 2x-2x crosses, a result which confirms previous reports. Contrary to published accounts, it was found by means of electrophoretic analyses that some 87.5 % of the progeny in 2x-3x crosses were not true hybrids but sexual diploids originating from self-fertilization. Only approximately 12.5 % of the progeny were found to be true agamospermous triploid or tetraploid hybrids. A strong doubt is raised concerning reports in the literature which assume that diploid hybrids originate in 2x-3x crosses. The possible reasons why self-incompatibility was broken down by triploid pollen are discussed.