Hybridization may endanger the rare North Apennine endemic Cirsium bertolonii

Abstract

We examined populations of North Apennine stenoendemics Cirsium bertolonii in the Apuan Alps and Tuscan-Emilian Apennines and found individuals morphologically shifted to co-occurring C. acaulon or C. erisithales. Hybrid status of these intermediates was confirmed by flow cytometry, morphometrics and amplified fragment length polymorphism (AFLP). We interpreted these hybrids taxonomically as Cirsium ×sagrense (C. acaulon × C. bertolonii) and C. ×abetonense (C. bertolonii × C. erisithales). Estimated genome size (2C) was 2244 ± 31 Mbp for C. ×sagrense and 2152 ± 99 Mbp for C. ×abetonense. Their genomic GC content was 38.95 ± 0.35% and 38.77 ± 0.26%, respectively. Diploid chromosome number 2n = 34 was counted for C. bertolonii, and the previously reported 2n = 12 needs to be considered erroneous. We found C. bertolonii to be a gynodioecious species (like many other Cirsium species) that is not reproductively isolated by ploidy level or chromosome number from co-occurring congeners. The relatively frequent occurrence of C. ×sagrense in Monte Sagro (locus classicus of C. bertolonii) and the prevalence of C. ×abetonense in Alpe Tre Potenze suggest that hybridization occurs repeatedly in C. bertolonii, as also confirmed by older herbarium specimens. C. ×abetonense produces ripe achenes and F2 hybrids or backcrosses, as some other Cirsium hybrids do. Genetic erosion/swamping via interspecific hybridization can therefore pose a risk to the genetic integrity of C. bertolonii, as it does for some other narrowly endemic high-mountain Cirsium species in Europe.