Interspecific genome size and chromosome number variation shed new light on species classification and evolution in caladium

Abstract

The genus Caladium Vent. is a member of the family Araceae; some of its species are cultivated as ornamentals. The present study was conducted to determine the genome size, somatic chromosome number, and their variation within 63 accessions representing 10 species of Caladium. Caladium genome sizes estimated using propidium iodide staining and flow cytometry ranged from 2.98 pg/2C in Caladium lindenii Engl. to 9.89 pg/2C in Caladium ×hortulanum Birdsey 'Chang Suek'. Two genome size groups (large and small) were evident among the 63 caladium accessions. The average genome size of 36 caladium accessions in the large genome size group was 9.29 pg/2C, roughly twice that of the 27 accessions in the small genome size group (4.50 pg/2C). Microscopic examination of squashed root tip cells revealed seven somatic chromosome numbers among 39 caladium accessions, including 2n = 18, 20, 24, 26, 30, 34, and 38, and provided the first chromosome counts for four caladium species new to Caladium. The results support the species status of C. marmoratum Mathieu ex K. Koch, C. picturatum K. Koch&C.D. Bouché, and C. steudneriifolium Engl. that were merged into C. bicolor (Aiton) Vent. previously and also support the species status of C. clavatum Hett., Bogner&J. Boos, and C. praetermissum Bogner & Hett., two species recently established in or transferred to Caladium. The results suggest that C. bicolor and C. schomburgkii Schott, not C. picturatum or C. marmoratum, are the chief parents of the fancy-leaved caladium (C. ×hortulanum). Four caladium cytotype groups (CCG-1 to -4) were identified in scatterplot of chromosome number vs. genome size. The genome size of C. bicolor, C. schomburgkii, and C. ×hortulanum in the CCG-4 is approximately twice that of C. humboldtii (Raf.) Schott and C. picturatum in the CCG-2, and the chromosome number of C. clavatum and C. marmoratum in the CCG-3 is close to twice that of C. humboldtii and C. picturatum in the CCG-2, both suggesting possible genome duplication or tetraploidization events in Caladium. However, the chromosome number of the CCG-4 species does not correspond to an expected 2n = 36 or 40, and the genome size of the CCG-3 species does not correspond to an expected 8.98 pg/2C. Conflicts between genome size and chromosome number indicate that genome duplication events were likely followed by chromosome fusions/losses in the formation of CCG-4 species and DNA losses likely followed tetraploidization in the formation of the CCG-3 species. The high level of cytological diversity found within Caladium affects germplasm collection and preservation efforts as well as breeding programs in the genus.