Allozyme segregation and inter-cytotype reproductive barriers in the polyploid complex Centaurea jacea

In eastern Belgium. diploid and tetraploid knapweeds (Centaurea jacea L. s. l.) show a parapatric distribution with a contact zone. To compare the suc cess of inter- and intra-cytotype crosses, and to investigate the pattern o f allozyme segregation, controlled crosses involving plants from this conta ct zone were performed. Polysomic inheritance at two loci was observed in t etraploids, suggesting an autopolyploid origin. Two crosses allowed the det ection of double reduction events in tetraploids at one locus. but no such event was detected among the 217 progenies. Null alleles were detected in t etraploids at two loci. Both cytotypes were highly self-incompatible. Inter -cytotype crosses were much less successful than intra-cytotype crosses in terms of seed set (five-fold reduction) and germination rates (three-fold r eduction), suggesting a 'triploid block' effect. Progenies from inter-cytot ype crosses most often had the maternal cytotype and resulted from selfing or pollen contamination. However. six triploids were detected. The signific ance of these results for the understanding of the geographical distributio n of the cytotypes and inter-cytotype gene flow is discussed.