Research report: Tetraploid European paeonies ( Paeonia ) show a homogeneous karyotype asymmetry and structure

A new study, carried out during a practical course in cytotaxonomy organized by the Italian Botanical Society in Pisa, has now shown that four tetraploid European peony taxa possess strikingly similar karyotypes, with no statistically significant differences in chromosome asymmetry or structure among them. The genus Paeonia is the sole member of the family Paeoniaceae and includes roughly forty species distributed across Eurasia, North America and western China. It is divided into three monophyletic sections: the mostly Eurasian herbaceous species of section Paeonia, the woody peonies of section Moutan endemic to China, and the North American herbaceous species of section Oanepia.

The genus has an ancestral basic chromosome number of x = 5, and three ploidy levels have been documented: diploids with 2n = 10 chromosomes found in all sections, tetraploids with 2n = 20 found only in section Paeonia, and mostly artificial hybrid triploids with 2n = 15 chromosomes. Peonies are unusually good subjects for karyomorphology, the study of chromosome form, because their chromosomes are large for flowering plants, measuring 10 to 15 micrometers in length compared with the much smaller chromosomes of most other angiosperms. This makes them an ideal model for practicing and teaching cytological techniques, which involve measuring the long and short arms of chromosomes in spread metaphase plates to characterize the physical structure of a chromosome complement.

Such approaches are simple, inexpensive and powerful sources of comparative information in systematic studies. The research emerged from a four-day course held in Pisa between February 6 and 9, 2024, organized by the Group for Floristics, Systematics and Evolution of the Italian Botanical Society. Participants analyzed permanent microscope slides prepared by one of the researchers roughly two decades earlier, between 2002 and 2005, as part of a study on Italian Paeonia species whose chromosome counts were only partially published at the time.

The slides had been made from root tips of plants cultivated in pots at the Botanical Garden of the University of Calabria, in Arcavacata di Rende, Cosenza. After hydrolysis in 1N hydrochloric acid at 60 degrees Celsius for seven and a half minutes, the tips were stained with leuco-basic fuchsine and squashed onto slides. In total, fourteen metaphase plates were analyzed by the course participants using the MATO software package.

The team focused on four quantitative traits. The first, total haploid chromosome length, serves as a proxy for genome size and is calculated by summing the lengths of all chromosomes in a plate and dividing by the ploidy level. The second, mean centromeric asymmetry, captures intrachromosomal asymmetry by measuring the average difference between the proportions of long and short chromosome arms.

The third and fourth, the coefficient of variation of chromosome length and the coefficient of variation of centromeric index, quantify interchromosomal asymmetry and heterogeneity in centromere position, respectively. The results were remarkably uniform. All four taxa displayed the expected tetraploid chromosome number of 2n = 4x = 20.

Total haploid chromosome lengths ranged from about 93 to 127 micrometers across plates, while values of the asymmetry indices fell within broadly overlapping ranges. Non-parametric Mann-Whitney pairwise tests revealed no significant differences among the four taxa for any of the karyomorphological parameters, and a principal component analysis, whose first two axes accounted for 85 percent of total variation, showed extensive overlap among all groups. One taxon offered a faint hint of divergence.

The two accessions of Paeonia mascula subsp. russoi showed slightly elevated values of chromosome length and centromeric index variation, which marginally separated them from the other taxa in the multivariate analysis. Intriguingly, this species is also considered taxonomically more distinct from P. officinalis and P. peregrina on the basis of leaf and root structure. However, the researchers caution that the very limited sampling prevents any firm conclusion, and the homogeneity observed across the sample remains the dominant signal.

To the authors' knowledge, this is the first quantitative study of karyotype asymmetry and structure in tetraploid European peonies, though the results agree with earlier qualitative observations from the 1980s and 2000s. The apparent homogeneity may extend even further: the coefficients of variation measured in the studied taxa closely resemble those reported for the tetraploid Paeonia mairei, a species endemic to China, suggesting that similar chromosome architecture could characterize large portions of section Paeonia worldwide. This uniformity also poses a methodological challenge.

Karyotype similarity can obscure the allotetraploid origin of these species, meaning their genomes may combine chromosome sets from different ancestral parents, a hypothesis advanced by earlier evolutionary studies. Traditional Feulgen staining approaches may simply be unable to reveal such hidden hybrid ancestry. The authors point to molecular cytogenetic techniques such as genomic in situ hybridization, which has proven effective in cultivated hybrid peony varieties, as a promising path forward for untangling the origins of wild tetraploid peonies and the evolutionary history written into their remarkably similar chromosomes.

Subject of Research: Karyotype asymmetry and structure of tetraploid European peonies (Paeonia sect. Paeonia)

Article Title: Tetraploid European paeonies ( Paeonia ) show a homogeneous karyotype asymmetry and structure

Article References: Peruzzi, L., Franzoni, J., Giacò, A., Abidi, E., Alù, E., Barone, G., Bianchi, E., Cataudella, C., Di Iorio, E., Guerrina, M., Mondello, F., Paino, L., Pentassuglia, M., Porrovecchio, M., Rivieccio, G., Siccardi, E., Stinca, A., Tei, A., Volanti, V., & Tiburtini, M. (2024). Tetraploid European paeonies (Paeonia) show a homogeneous karyotype asymmetry and structure. Italian Botanist, 18, 51-57. https://doi.org/10.3897/italianbotanist.18.135403

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.18.135403

Keywords: Paeonia, peony, karyotype, cytotaxonomy, tetraploid, chromosomes, karyomorphology, PCA, Italian Botanical Society, genome size, allopolyploidy, European