Caladiums are ornamental aroids widely grown as pot plants or used in landscapes as accent or border plants. Commercial pot caladium plants are produced by forcing tubers, while dry tubers are available for garden or landscape planting (Evans et al., 1992). More than 95% of the caladium tubers used in the world for container forcing and dry sales are produced in Florida (Deng et al., 2005b). Pioneering breeding of caladium occurred in Europe in the 1860s, but since the beginning of the 20th century, breeding of this crop has been conducted primarily in Florida (Wilfret, 1993). It is generally believed that cultivated caladiums (C. ×hortulanum) resulted from intraspecific or interspecific hybridizations among several Caladium Vent. species, including Caladium bicolor (Aiton) Vent., Caladium marmoratum Mathieu, Caladium picturatum C. Koch, and Caladium schomburgkii Schott, which are native to the tropical regions of South America and Central America (Birdsey, 1951; Hayward, 1950; Wilfret, 1993). Caladiums are diploids with 2n = 2x = 30 chromosomes (in Darlington and Wylie, 1955). They seem to be quite heterozygous genetically (Z. Deng, personal observation), which is expected as all commercial cultivars are asexually propagated through tuber division (Wilfret, 1993).
The ornamental value of caladium in the container or in the landscape depends, to a large extent, on its leaf characteristics, including leaf shape, color, and color pattern. Improving these characteristics or creating novel combinations of these characteristics has been one of the most important objectives in caladium breeding (Deng and Harbaugh, 2006; Wilfret, 1993). Often, more than 10 years are needed to develop a new caladium cultivar that is acceptable to both field growers engaged in large-scale commercial production of caladium tubers and to greenhouse growers producing pot caladium plants. To improve breeding efficiency and shorten this long breeding process, it has become increasingly important to have a good understanding of the mode of inheritance of major morphological and physiological traits that determine the ornamental and production values of caladium. Information of this kind can assist breeders in choosing appropriate parents and parent combinations, determining population sizes, and developing applicable screening or selection schemes.
In caladium, leaf shape has been shown to be controlled by one locus with two codominant alleles (Deng and Harbaugh, 2006; Wilfret, 1983, 1986) and main vein color by an independent locus with three alleles (Deng and Harbaugh, 2006; Wilfret, 1986). Leaf spots are another important trait involved in defining leaf color and coloration pattern. A number of major cultivars express leaf spots, resulting in intriguing coloration patterns on leaves with increase in ornamental and economic values. Tremendous efforts (at least in an aroid) have been made to understand the genetic control of leaf spots and their genetic relationship with other foliar traits. Based on the segregation of leaf spots and colors at the central leaf area in the progeny of a cross between two cultivars, Zettler and Abo El-Nil (1979) proposed that one locus controlled the vein color, vein pattern, and spotting in caladium and that ‘Painter's Palette’, a plant collected by W. Zettler from a homesite near Campville, FL (Gager, 1991), carried alleles R and W for its red and white spots, while ‘Poecile Anglais’, a commercial cultivar, had the allele R′ in a homozygous state for its red center on the leaf. Subsequently, Wilfret (1983, 1986) showed that caladium leaf vein pattern and color were controlled by separate genetic systems. To resolve this discrepancy, Gager (1991) performed crosses between ‘Painter's Palette’ and ‘Aaron’ or ‘Florida Cardinal’, sib-mated their progeny, and analyzed the segregation of leaf spots, spot color, and vein pattern. The conclusion was that a single locus controlled the expression of spotting, with two codominant alleles, Sr for red spots and Sw for white spots, and a recessive allele s for no spots. It was believed that both progeny with pink spots and progeny with red and white spots had the same genotype (Sr/Sw), but the former resulted from the overlapping of red spots and white spots at the same locations on the leaf, while the latter was due to the expression of red and white spots at different locations on the leaf (Gager, 1991). Gager speculated that there is a separate gene that controls the location of spots (isolated, border touching, or overlapping). However, the observed segregation deviated from the expected ratios in 21 out of 45 crosses. This was thought to be caused by the presence of a factor with lethal effects on genotype Sr/s, especially on genotypes Sr/Sr, Sw/Sw, and Sr/Sw. To understand the genetic relationship between veins and spots, the joint segregation of these two traits were analyzed (Gager, 1991). The observed ratios did not fit to the expected 9:3:3:1 because one of the four phenotype classes (green-veined, no spots) was always missing in the progeny. Again, it was suspected that some form of lethality was involved and caused the deviation.
The objectives of this study were to 1) understand the inheritance of leaf spots, 2) determine the genetic relationship of spots with leaf shape and vein color, and 3) determine the leaf spot, shape, and vein color genotype of several important caladium cultivars.
Color spots are expressed also on the leaves of several other major ornamental aroids, such as aglaonema (Aglaonema costatum N.E. Br.) and calla lily (Zantedeschia Spreng.), and on the flowers of important cut flowers, flowering pot plants, or bedding plants, such as alstroemeria (Alstroemeria L.), lily (Lilium L.), orchid (Orchidaceae), and pansy (Viola ×wittrockiana Gams.)/viola (Viola cornuta L.). Information on the inheritance of the spots in these ornamental crops is not available or very limited as well.
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