Patterns of allozyme variation in diploid and tetraploid Centaurea jacea at different spatial scales

The extent and spatial patterns of genetic variation at allozyme markers we re investigated within and between diploid and autotetraploid knapweeds (Ce ntaurea jacea L. sensu late, Asteraceae) at contrasted geographic scales: ( 1) among populations sampled from a diploid-tetraploid contact zone in the northeastern part of the Belgian Ardennes, and (2) within mixed populations from that zone where diploids and tetraploids coexist. Our data were also compared with a published dataset by Sommer (1990) describing allozyme vari ation in separate diploid and tetraploid knapweeds populations collected th roughout Europe. Genetic diversity was higher in tetraploids. In the Belgia n Ardennes and within the mixed populations, both cytotypes had similar lev els of spatial genetic structure, they were genetically differentiated, and their distributions of allele frequencies were not spatially correlated. I n contrast, at the European scale, diploids and tetraploids did not show di fferentiated gene pools and presented a strong correlation between their pa tterns of spatial genetic variation. Numerical simulations showed that the striking difference in patterns observed at small and large geographic scal es could be accounted for by a combination of (1) isolation by distance wit hin cytotypes; and (2) partial reproductive barriers between cytotypes and/ or recurrent formation of tetraploids. We suggest that this may explain the difficulty of the taxonomic treatment of knapweeds and of polyploid comple xes in general.