Cercis canadensis, a leguminous tree, is native to North America and cultivated widely as an ornamental. Flowers emerge directly from the stem or trunk before the leaves early in the spring. Petal colors range from purple to pink to red or white. A double-flowered cultivar is available. Growth habits include weeping forms, dwarf types, and small- to medium-sized types. Leaves can be glossy to pubescent and leaf color from green to purple or variegated. There are more than three dozen C. canadensis cultivars commercially available in the United States that encompass the major phenotypic variants. Ornamental traits are usually simply inherited, and novel combinations of traits are expected from breeding and selection (Werner, 2006).
There are three recognized botanical varieties of C. canadensis, which account for the high degree of morphological variation in the cultivated forms (Isley, 1975). C. canadensis var. canadensis is found in the eastern United States and is noted for dull green leaves with acute apices. It has glabrous branchlets and leaves that can be glabrous or pubescent. C. canadensis var. texensis (S. Watson) M. Hopkins grows in Texas and Oklahoma and has thick, glossy leaves and branchlets that are also glabrous. C. canadensis var. mexicana (Rose) M. Hopkins is found in northern Mexico and southern Texas. Leaves are thick and shiny with rounded apices with branchlets that are pubescent. In an effort to resolve the phylogeny, several taxonomic studies were previously conducted with morphological characters and DNA sequences (Davis et al., 2002). Most of the morphological characters are continuous over the geographic ranges, and variation, particularly in leaf morphology, may be a response to different climatic regions (Davis et al., 2002). Characters such as pubescence are thought to have evolved along temperature and moisture clines (Fritsch et al., 2009). DNA studies support geographic distributions, but internal transcribed spacer (ITS) sequence data have low resolution within the populations sampled (Davis et al., 2002; Fritsch and Cruz, 2012).
In situations in which botanical varieties of C. canadensis are growing side by side in the wild, little introgression is recorded, possibly as a result of differences in timing of flowering. However, all three botanical varieties are sexually compatible as evidenced by complex controlled hybridizations in ornamental breeding programs (Werner, 2006). In cultivated forms, variation in leaf traits generally corresponds to genetic background; however, introgression of traits among these three botanical varieties is becoming more common. Complex pedigrees make taxonomic placement relatively ambiguous based on morphological characters alone, although true var. canadensis forms are typically determined by their dull, thin leaves. Although not widely grown as an ornamental tree, C. occidentalis Torr. Ex A. Gray also is native to the North America, including parts of California, Arizona, Utah, and Nevada (Isley, 1975).
Two other species of Cercis, both native to Asia, are cultivated as ornamental plants in the United States. C. chingii is found in southeastern China in mixed temperate or warm forests and C. chinensi is widely distributed in southern China (Li, 1944). Both species have desirable ornamental traits. C. chinensis, commonly known as chinese redbud, grows quickly and produces flowers at an earlier age than other species in production. It has prolific bud set and a growth habit of a shrub or compact small tree. C. chingii has large pink flowers that are the first to open of all of the redbuds (Dirr, 1998). There are five additional Cercis species found in Asia including C. griffithii Bioss, C. glabra Pamp., C. gigantea F.C. & Keng f., C. chuniana F.P. Metcalf, and C. racemosa Oliv. (Hopkins, 1942). C. siliquastrum L. is found in the Mediterranean region from France to Turkey (Hopkins, 1942; Isley, 1975). All species except C. chuniana are represented in the study presented here.
Genetic relatedness, particularly among botanical varieties and species, is an important consideration when estimating the use and practicality of wide hybridizations in ornamental plant breeding. Continuous, qualitative, and wide-ranging morphological traits are an important source of variation for plant breeding programs but are not the most useful characters for phylogenetic estimates. DNA sequences such as the nuclear ribosomal ITS region have proven useful in resolving phylogenetic questions about plant species (for review, see Baldwin et al., 1995). However, ITS sequences are less useful in plant breeding when confirming pedigrees, establishing markers for linkage to important traits, and genetic mapping. In this respect, molecular markers that are randomly dispersed throughout the nuclear genome have the required distribution and frequency to provide information at the population and single plant levels. Simple sequence repeat (SSR) markers are widely used because of their high repeatability between laboratories, codominant nature, and potential transferability across related species.
The objectives of this study were to use SSRs from C. canadensis to accomplish the following: 1) determine the cross-species transfer within Cercis; 2) independently reconstruct species relationships within Cercis using SSR data and ITS sequence data; and 3) address at what taxonomic level the SSR loci are useful for breeding and genetics within Cercis in the context of the results.
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