Should we conserve pure species or hybrid species? Delimiting hybridization and introgression in the Iberian endemic Centaurea podospermifolia

Natural hybridization can be a serious threat to rare and endangered species because of the risk of extensive genetic swamping or assimilation. Hybridization, however, can be beneficial for rare species as it increases their genetic diversity, which in turn may improve their fitness and confer adaptive potential. Centaurea podospermifolia is a narrowly endemic, threatened, tetraploid species occurring only in two mountain ranges of northeastern Spain. This species may produce a highly unstable homoploid hybrid (Centaurea x loscosii) with the more widespread Centaurea cephalariifolia. Allozymes and nuclear DNA sequences (ETS and Agt1) were used to test whether C. podospermifolia populations have undergone hybridization or introgression with C. cephalariifolia, which should have left an imprint in the genetic structure of the former. Nuclear DNA sequencing indicates that all the populations from one mountain range (the Ports Massif) are introgressed, whereas those from the second (the Cardó Range) should be regarded as ‘pure’. Genetic variability detected in C. podospermifolia is low for a tetraploid species, and the occurrence of population bottlenecks may have played a role in this lack of diversity. Populations from the Ports are more variable than those from Cardó, the lower genetic variability observed for Cardó populations being unequivocally related to a lack of genetic material from C. cephalariifolia. We suggest that a management plan for C. podospermifolia should address the conservation of both the pure-bred and the introgressed populations.