First chromosome counts unlock secrets of Italy’s endemic knapweeds and beach catchflies

Botanists in Italy have published a new installment of an ongoing effort to chart the chromosome numbers of the Italian flora, and this time the results concern some of the country’s most localized endemic plants. In a contribution to Italian Botanist, researchers report the first chromosome counts for four subspecies of Centaurea aplolepa, a knapweed complex restricted to the Tyrrhenian portion of central-northern Italy, as well as the first count for an Italian population of Silene canescens, a catchfly of sandy southern European shores. The work, dated 24 June 2024, adds hard cytogenetic data to a flora whose taxonomy has long been debated on morphology alone.

Centaurea aplolepa is a species endemic to the Tyrrhenian side of central-northern Italy and, according to the taxonomic framework of Greuter from 2008, includes ten subspecies. The autonymic subspecies, Centaurea aplolepa subsp. aplolepa, is endemic to western Liguria, where it grows on garrigues and cliffs along the coastline. Seeds collected from plants on the rocky walls of Capo Noli in the province of Savona yielded a chromosome number of 2n = 18, reported here for the first time. The value accords with counts previously published for other subspecies of the complex by Viegi and colleagues in the 1970s, confirming that the westernmost member of the group shares the same karyological baseline as its relatives.

The second new count concerns Centaurea aplolepa subsp. bertolonii, endemic to the hills over and west of Genova. Its distribution range sits approximately between those of subsp. aplolepa to the west and subsp. levantina and subsp. lunensis to the east, a geographic pattern that has made it a key piece in reconstructing how the complex diversified along the Ligurian coast. Seeds from plants collected near Forte Sperone in Genova also gave 2n = 18, the first chromosome number ever reported for this subspecies. The consistency of the count across subspecies with distinct ecological preferences and disjunct ranges suggests that chromosome evolution has been conservative within the group, with differentiation proceeding at the level of ecology and geography rather than genome size.

A third subspecies, Centaurea aplolepa subsp. levantina, is described in Arrigoni’s 2003 taxonomic revision as a coastal ecotype endemic to eastern Liguria. Plants from small rocky outcrops near the pier at Sestri Levante in the province of Genova provided seeds that again revealed 2n = 18. The count, obtained from germinating seeds rather than from herbarium material, is reported for the first time and reinforces the emerging picture of a cytologically uniform species complex whose ten subspecies occupy a chain of specialized habitats along the Ligurian Tyrrhenian coast, from maritime cliffs to inland badlands.

The fourth subspecies, Centaurea aplolepa subsp. parvula, carries a particularly interesting biogeographic story. Although Arrigoni did not include it in his 2003 revision of the Centaurea paniculata group, recent Italian floristic checklists treat it as endemic to Liguria and Piemonte. In Liguria it is only reported for Rossiglione in the province of Genova, in a record dating back to Gola in 1912, while the only Piemontese record is the type locality at Acqui Terme in the province of Alessandria. During a field trip, the researchers found several populations growing on the badlands there, and seeds from these plants gave 2n = 18, the first count for the subspecies. The result extends the cytological uniformity of the complex beyond Liguria into Piemonte, supporting the view that subsp. parvula is a genuine member of the aplolepa group despite its earlier taxonomic neglect.

The fifth and final new count shifts from knapweeds to catchflies. Silene canescens is a psammophyte belonging to a species complex distributed throughout the sandy shores of southern Europe. Delineating its distribution range has long been difficult because of confusion with the closely related Silene colorata, and in Italy the species is considered taxonomically doubtful, although it is thought to occur along most of the Tyrrhenian and Adriatic coasts. Seeds collected at the retrodune of Marina di Torre del Lago in Viareggio, province of Lucca, and deposited at the germplasm bank of the Department of Biology of the University of Pisa, revealed a chromosome number of 2n = 2x = 24 for an Italian population, the first count of its kind for the country.

The Silene result is line with previous karyological data published by Blackburn and Morton in 1957, and it also matches counts available for the related Silene colorata compiled in later karyological databases. In practical terms, the diploid count of 24 for the Italian population helps anchor the cytological identity of a species whose Italian status has been uncertain, providing an independent line of evidence that can be compared with morphological and ecological data as botanists continue to disentangle the canescens-colorata species complex along the Italian coasts.

Technically, all five counts were obtained with a classical cytogenetic protocol applied to root tips from germinating seeds. Squash preparations were made on root tips pre-treated with 0.4% colchicine for three hours to arrest dividing cells at metaphase, then fixed in Carnoy solution for one hour. After hydrolysis in 1N hydrochloric acid at 60 degrees Celsius for seven and a half minutes for the Centaurea samples and eight minutes for the Silene sample, the tips were stained with leuco-basic fuchsine, a Schiff-based stain that colors chromosomes intensely while leaving the cytoplasm pale. The use of seeds rather than field-collected plants ensures that voucher specimens, deposited at the herbarium of the University of Pisa under barcodes including PI066169, PI066182, PI066170, PI066166 and PI063832, permanently document the source populations.

Why does a count of 2n = 18 or 2n = 24 matter beyond the herbarium? Chromosome numbers are among the most fundamental characters in plant systematics. They can reveal polyploidy, hybrid origins and reproductive isolation, and they often discriminate between morphologically similar species that occupy different habitats. For endemic complexes like Centaurea aplolepa, whose subspecies are confined to narrow ranges along the Ligurian coast, cytological data provide a baseline for conservation genetics and for understanding how local populations relate to one another. For taxonomically doubtful species like Silene canescens, a count from a documented Italian population can help settle whether the plants on Italian shores belong to the same biological species as those elsewhere in southern Europe.

The contribution is part of a serial effort to document chromosome numbers for the Italian flora, now at its fourteenth installment, and the authors invite further contributions: new chromosome data on Italian material, with indications on voucher specimens and methods used, can be submitted electronically to Antonio Giacò at the University of Pisa. As botanists continue to fill the cytogenetic map of Italy, each new count, however small it may seem, turns a morphological hypothesis about an endemic plant into a testable biological fact, one root tip at a time.

Subject of Research: Chromosome counts for Italian endemic Centaurea aplolepa subspecies and Silene canescens

Article Title: Chromosome numbers for the Italian flora: 14

Article References: Giacò, A., Carta, A., Franzoni, J., Mo, A., Pentassuglia, M., & Peruzzi, L. (2024). Chromosome numbers for the Italian flora: 14. Italian Botanist, 17, 69-75. https://doi.org/10.3897/italianbotanist.17.128997

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.17.128997

Keywords: chromosome counts, Centaurea aplolepa, Silene canescens, Italian flora, cytogenetics, endemic plants, karyology, Liguria, Asteraceae, Caryophyllaceae, taxonomy, biodiversity