The genus Codiaeum A. Juss. belongs to the family Euphorbiaceae and encompasses 17 species native to tropical forests from Indonesia and the Philippines to New Guinea and Australia (Govaerts et al., 2000). Codiaeum variegatum (L.) Blume, commonly referred to as crotons, are small evergreen trees and shrubs with varied leaf shapes and vivid foliage colors displayed as shades, blends, combinations, or solid patches of red, pink, orange, yellow, lavender, black, and green. Leaf sizes range from small (6 cm long × 1 cm wide) to large (35 cm long × 13 cm wide), and margins can be entire or trifurcate. Leaf shapes vary from simple ovate to linear; some are slightly or deeply cut, and others are connected with the blade only by the midrib. Thus, crotons are valued mainly for their attractive foliage. Currently, more than 300 cultivars are reported in the world, and they are cultivated as either landscape plants or containerized foliage plants (Chen et al., 2005).
Native to the Mollucan Islands of Indonesia (Brown, 1995), crotons were first introduced to England in 1804, but hybrids were developed mainly in Belgium and France in the 1800s. Many of the European hybrids were introduced into the United States in 1871. At least 70 hybrids, commonly known as Florida hybrids, were developed in south Florida during the 1920s and 1930s. Crotons are monoecious with poor seed set, and seed propagation results in plants with different phenotypes (Sharma and Bal, 1958). In commercial production, once a hybrid selection is made, it is propagated asexually through stem cuttings (Chen and Stamps, 2006).
The Croton Society (2010) categorizes croton cultivars into nine types based on their leaf morphology: 1) broad leaf; 2) oak leaf; 3) semioak leaf; 4) spiral leaf; 5) narrow leaf; 6) very narrow leaf; 7) small leaf; 8) interrupted leaf; and 9) recurved leaf. However, information regarding the genetic relationships of cultivated crotons is incomplete. Cytological studies of cultivars produced in India (Sharma and Bal, 1958), the Philippines (Pancho and Hilario, 1963), Nigeria (Ogunwenmo et al., 2007), and Florida (Deng et al., 2010) showed that crotons are predominantly polyploid with 2n = 60 to 120. Endoreduplication, irregular chromosome segregation, spindle multipolarity, and unusual nucleus shape were observed in crotons and proposed to be the mechanisms underlying the wide range of chromosome variation (Chennaveeraiah and Wagley, 1985). As a result, genetic relationships among cultivars have not been established (Deng et al., 2010; Sharma and Bal., 1958), which has hampered our efforts on croton germplasm conservation and new cultivar development.
Amplified fragment length polymorphism (AFLP), a polymerase chain reaction-based technique for DNA fingerprinting (Vos et al., 1995), has been widely used for studying genetic relationships among cultivars of different species or genera. The advantages of this technique include reproducibility, high levels of polymorphism detection, genomewide distribution of markers, and no requirement of prior knowledge of the genome being studied (Mueller and Wolfenbarger, 1999; Prabhu and Gresshof, 1994). The AFLP technique is particularly valuable for studying intraspecific variation and also plants that are polyploid (Meudt and Clarke, 2007). This technique has been successfully used for studying cultivar differences of Euphorbia pulcherrima (Parks and Moyer, 2004) and detecting sports and somaclonal variants of Syngonium podophyllum (Chen et al., 2006). AFLP has also been used successfully for studying allopolyploidy among cultivars or collections of Cucumis ×hytivus (Chen et al., 2007), Gossypium spp. (Liu et al., 2001), and Triticum spp. (Ozkan et al., 2002) and autopolyploidy among Dactylorhiza spp. (Hedren et al., 2001), accessions of Solanum tuberosum (Milbourne et al., 1997; Spooner et al., 2005), and collections of Saxifraga cernua, including aneuploid lines (Kjolner et al., 2004).
This study was undertaken to analyze the genetic relationships of popular croton cultivars using the AFLP technique and determine possible factors contributing to the wide morphological variation of the cultivars.
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