The Centaurea alba complex in the Iberian Peninsula: gene flow, introgression, and blurred genetic boundaries

Using microsatellites and cpDNA sequences, we have analyzed genetic diversity and structure and evaluated levels of introgression of 19 populations from the Centaurea alba complex from subsection Phalolepis and six taxa from the closely related subsections Centaurea and Willkommia from the Iberian Peninsula. Genetic differentiation among the different populations of the C. alba complex was lower than those found in the Italian/northern-central Balkans, Greek, and Anatolian populations of subsect. Phalolepis. The lower genetic differentiation among the populations of the Iberian Peninsula compared to the other Mediterranean peninsulas may be due to intraspecific higher levels of admixture but also to a considerable degree of introgression with species that grow in close proximity. Although there is a significant pattern of isolation by distance within the C. alba complex, we have found considerable levels of gene flow between taxa, both recent and historical. Hybridization and introgression were also evident, as it is often observed within the genus. Patterns of genetic structure support the recognition of C. alba and C. costae as different species.