Flow cytometric and karyological analyses of Calendula species from Iberian Peninsula

Calendula L. (Asteraceae) is a taxonomically and cytologically complex genus due to its high morphological and karyological variation. To gather consistent cytological information aiming to consolidate the existing knowledge, sustain the taxonomic revision of the genus and explore the evolutionary relationships among species, the genome size and chromosome number of the Iberian Peninsula representatives of this genus were assessed. The study included 11 taxa that occur in the Iberian Peninsula, one in Madeira and two from Morocco. Chromosome counts were made using the squash technique in root tips and flower buds, while nuclear DNA contents were assessed using propidium iodide flow cytometry. The following chromosome numbers are reported: 2n = 44 for C. arvensis, 2n = 30 for C. tripterocarpa, and 2n = 32 for the remaining Iberian taxa. The genome size of Calendula species was assessed for the first time and ranged from 1.75 pg/2C in C. maroccana to 5.41 pg/2C in C. arvensis. Within the complex formed by C. incana and C. suffruticosa, a gradient of genome size values was obtained. Intraspecific variation in genome size was detected in some taxa. The obtained genome size values and their variation are discussed in the light of the theories proposed for the speciation of the genus, with events of hybridization, genome duplication and dysploidy being hypothesized to play a major role in the evolution of this genus.