Plant genomes are highly diverse in size and repetitive DNA composition. In the absence of polyploidy, the dynamics of repetitive elements, which make up the bulk of the genome in many species, are the main drivers underpinning changes in genome size and the overall evolution of the genomic landscape. The advent of high-throughput sequencing technologies has enabled investigation of genome evolutionary dynamics beyond model plants to provide exciting new insights in species across the biodiversity of life. Here we analyze the evolution of repetitive DNA in two closely related species of Heloniopsis (Melanthiaceae), which despite having the same chromosome number differ nearly twofold in genome size [i.e., H. umbellata (1C = 4,680 Mb), and H. koreana (1C = 2,480 Mb)]. Low-coverage genome skimming and the RepeatExplorer2 pipeline were used to identify the main repeat families responsible for the significant differences in genome sizes. Patterns of repeat evolution were found to correlate with genome size with the main classes of transposable elements identified being twice as abundant in the larger genome of H. umbellata compared with H. koreana. In addition, among the satellite DNA families recovered, a single shared satellite (HeloSAT) was shown to have contributed significantly to the genome expansion of H. umbellata. Evolutionary changes in repetitive DNA composition and genome size indicate that the differences in genome size between these species have been underpinned by the activity of several distinct repeat lineages.

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The newly described diploid species Cirsium greimleri (2n = 34; 2C = 1929.1±60.6 Mbp) belongs to Cirsium sect. Cirsium and is distributed sparsely throughout the Eastern Alps and Dinarides, whereas the closely related tetraploid vicarious species C. waldsteinii (2n = 68; 2C = 3682.3±69.8 Mbp) is endemic to the south-eastern Carpathians. The ploidy, genetic and morphological separation of both taxa is confirmed using flow cytometry, AFLP and morphometric analyses of 169 plants from 27 populations covering representatively distribution ranges of the respective species. The species differ in flower colour, with those of C. greimleri ruby red to brownish-purple and those of C. waldsteinii pinkish-purple to purple. The colour difference remains consistent when both species are cultivated together under the same conditions. Differences between the species in the size of the stomata, achenes, corollas, styles and pappus are statistically significant and congruent with differences in the ploidy level. Because both species are gynodioecious (their populations contain female and hermaphrodite plants), the generative features should be compared carefully with respect to gender (e.g. females with females) because the between-gender differences within the same species could be larger than the between-species differences. The basal and median cauline leaves of C. waldsteinii are narrower and more deeply lobed than those of C. greimleri. A slight difference in the flowering period is detected when both species are cultivated together, with C. waldsteinii flowering two weeks earlier than C. greimleri. Both species share ecological/habitat preferences for subalpine woodland tall-forb vegetation. While C. waldsteinii hybridizes extremely rarely with co-occurring diploid congeners, C. greimleri produce hybrids very often, which increase the risk of its extinction via genetic erosion. K e y w o r d s:

In this study we describe a new nothospecies, Cirsium ×sudae Michálková et Bureš, a homoploid hybrid between two rare Alpine species, C. carniolicum and C. greimleri. Hybrid status was confirmed for four morphologically intermediate individuals, found in the Ennstal Alps, Austria (three of them were F1 hybrids and one a backcross with C. carniolicum). We used amplified fragment length polymorphism (AFLP) to confirm affiliation to the parental species and exclude the potential contribution of other sympatric species. In addition, we used flow cytometry to confirm the diploid status of this hybrid. The newly assessed genome size of this hybrid is 2C = 1.99±0.03 pg, and for C. carniolicum 2C = 2.03±0.04 pg.

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.

We examined populations of North Apennine stenoendemics C. 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 we confirmed by flow cytometry, morphometrics and amplified fragment length polymorphism (AFLP) and interpreted them 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 and 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 previously reported 2n = 12 needs to be considered erroneous. We found Cirsium bertolonii to be gynodioecious species (similarly as many other species of Cirsium sect. Cirsium) that is not reproductively isolated by different 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 or backcrosses similarly as hybrids of other Cirsium-species. Genetic erosion/swamping via interspecific hybridization can therefore pose a serious risk to genetic integrity of C. bertolonii like in the case of some diploid narrowly endemic high mountain Cirsium species in Europe.

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