Ecological, morphological and allozymic differentiation between diploid and tetraploid knapweeds (Centaurea jacea) from a contact zone in the BelgianArdennes

In the northeastern part of Belgium, the Centaurea jacea complex shows exte nsive morphological variation and is represented by a diploid (2n = 22) and a tetraploid (2n = 44) cytotype. Polysomic inheritance of allozyme markers in the tetraploids, suggesting autopolyploidy, is here demonstrated for th e first time. In order to test whether the tno cytotypes occupy distinct ha bitats and possess different gene pools, patterns of allozymic and morpholo gical variation were investigated in relation to ploidy level and site char acteristics in 26 populations from the Belgian Ardennes. The two cytotypes showed a parapatric distribution, the diploids occurring at higher elevatio ns (mostly above 500 m) than the tetraploids (mostly below 500 m). Three mi xed populations were found near the contact zone of the two cytotypes. With in the mixed populations no triploid plant and no evidence for gene flow be tween cytotypes were found, despite widely overlapping flowering periods. T he two cytotypes can be distinguished on the basis of morphological traits and enzymatic gene pools. The congruence of morphological and allozymic var iation with chromosome numbers suggests a secondary contact between the two cytotypes with limited gene flow between them. The origin and persistence of the parapatric distribution are discussed.