Hybridization may endanger the rare North Apennine endemic Cirsium bertolonii

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 xsagrense (C. acaulon x C. bertolonii) and C. xabetonense (C. bertolonii x C. erisithales). Estimated genome size (2C) was 2244 +/- 31 Mbp for C. xsagrense and 2152 +/- 99 Mbp for C. xabetonense. 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. xsagrense in Monte Sagro (locus classicus of C. bertolonii) and the prevalence of C. xabetonense in Alpe Tre Potenze suggest that hybridization occurs repeatedly in C. bertolonii, as also confirmed by older herbarium specimens. C. xabetonense 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.

Automated summary

We studied the populations of Cirsium bertolonii in different locations and identified individuals that showed morphological shifts towards co-occurring C. acaulon or C. erisithales. These intermediate individuals were confirmed to be hybrids through various methods. The genetic erosion caused by interspecific hybridization poses a risk to the genetic integrity of C. bertolonii and other narrowly endemic high-mountain Cirsium species in Europe.