Caladium is a member of the family Araceae, belonging to the tribe Caladieae in the subfamily Aroideae (Mayo et al., 1997). The genus is indigenous to the New World tropics, including Brazil, Colombia, Ecuador, Peru, and the West Indies. Like other members of the aroid family, caladiums develop an inflorescence composed of a spadix (an internal spike) and a spathe (an external modified leaf). The spathe is constricted centrally and divided into a lower tube and an upper limb, which unfurls at anthesis, allowing pollinators access to the spadix (Madison, 1981). The spadix consists of fertile male flowers at the top, sterile male flowers (the synandrode) in the middle, and female flowers at the base (Madison, 1981). The fertile male flowers pack tightly on the spadix and the stamens unite into a synandrium; the female flowers consist of only an ovary. The morphologies of inflorescences and flowers are commonly used in defining caladium species. Classification of caladium species and relationships among them have been a subject of debate for decades. The debate mainly concerns the classification of four species, C. bicolor, C. marmoratum, and C. picturatum, and C. steudneriifolium. Madison (1981) merged C. marmoratum, C. picturatum, and C. steudneriifolium into C. bicolor and reduced the number of species in the genus to seven. However, Croat (1994) kept the species status of each of the three species and included 17 species in the genus, and Mayo et al. (1997) reclassified the genus into 12 species (The Plant List, 2013). Subsequently two new species, C. clavatum and C. praetermissum, have been added to the genus.
Caladiums were introduced to Europe in the mid-18th century and became a popular hothouse plant there in the 19th century (Hayward, 1950). Caladium breeding was initiated in France in the early 1860s and advanced in Brazil and England in the 1880s (Hayward, 1950; Wilfret, 1993). Since the beginning of the 20th century, caladium breeding has been conducted mainly in Florida, resulting in many new cultivars (Hayward, 1950; Wilfret, 1993). For the last several decades, Florida has become the leading producer of caladium tubers, supplying as much as 95% of tubers used in pot plant production and landscapes worldwide. In recent years, caladium breeding is active in several other countries, especially in Thailand. Considering the possible hybrid origin of many caladium cultivars, Birdsey (1951) proposed the species name Caladium ×hortulanum for fancy-leaved caladium. Little information is available in the literature about the parentage of fancy-leaved caladium cultivars that were produced in other countries or those produced in Florida before 1976. The species that contributed to the development of caladium cultivars have been largely a matter of speculation. Caladium bicolor and C. marmoratum were thought to be the chief parent of fancy-leaved cultivars (Hayward, 1950; Wilfret, 1993), and C. picturatum or C. schomburgkii were suspected to provide the lanceolate leaf trait in some caladium cultivars (Graf, 1976; Hayward, 1950; Huxley et al., 1992; Wilfret, 1993).
In caladium taxonomy, morphological and anatomical differences are the primary basis for species classification. The characteristics commonly examined in caladium include floral and inflorescence morphologies, leaf shape and coloration pattern, and tuber morphology. Cytogenetic information such as chromosome number has not been used in the classification of caladium species, presumably as a result of the lack of such information. Although early chromosome counts exist in caladium (Kurakubo, 1940), chromosome numbers are not available for most caladium species according to the Index to Plant Chromosome Numbers [IPCN (Goldblatt and Johnson, 1979)]. In other plants, chromosome numbers have been used in cytotaxonomy (Guerra, 2008) and they have provided key evidence for redefining species boundaries or detecting misclassified species in some aroids (Petersen, 1989). Needless to say, information about chromosome number is useful for plant breeders to devise breeding strategies for introgressing traits through interspecific hybridization.
In recent years, there has been a rapid increase in the number of genome size estimates for many plant species. Such information is scarce for aroids, especially for caladiums (Bennett and Leitch, 2012). Knowledge of genome size is considered critical for many fields of research, including taxonomy and evolutionary studies (Doležel et al., 2007). For example, inter- and intraspecific genome size variation can complement chromosome counts and reveal taxonomic heterogeneity and incipient speciation (Loureiro et al., 2010). Genome size comparisons, based on nuclear DNA content and increasingly based on DNA sequencing data, have provided evidence for ancient genome-wide duplications in a diverse array of plants (Cui et al., 2006).
The objectives of this study were to document the chromosome numbers and genome sizes of caladium accessions representing 10 species and to pursue a better understanding of the variation in chromosome number and genome size in relation to caladium taxonomy and evolution.
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