The genus Cercis L. (Fabaceae: Caesalpinoideae: Cercideae), also known as redbud, is a valuable commodity in the North American landscape industry and can be found growing in temperate environments across the globe. Cercis consists of ≈10 species (Davis et al., 2002; Fritsch et al., 2009), which can be found in North America (C. canadensis L., Cercis occidentalis Torr. ex A. Gray), Asia (C. chinensis Bunge, Cercis chingii Chun, Cercis chuniana P.F. Metcalf, Cercis gigantea W.C. Cheng & Keng f., Cercis glabra Pamp, Cercis racemosa Oliv., and Cercis siliquastrum L.), and the Middle East (Cercis griffithii Boiss.). Redbud is recognized for a variety of interesting morphological characteristics, many of which make them ideal ornamental specimens. Valuable insight into angiosperm evolution can be obtained through genetic surveys of this valuable landscape commodity.
Fabaceae, one of the most successful lineages of flowering plants (Legume Phylogeny Working Group et al., 2013) has long been the subject of genomics and genetic research. In particular, species like Lotus japonicus (Regel) K. Larsen and Medicago truncatula Gaertner have been adopted internationally as genetic models for legume-based research thanks to their model characteristics (Udvardi et al., 2005). Caesalpinoideae, a subfamily in which Cercideae resides, contains much of the evolutionary and genetic diversity found in all of Fabaceae. However, recent studies have focused on cultivated legume crops, all of which have diverged relatively recently (Young et al., 2003). This information covers only a fraction of the great diversity that can be found within Fabaceae (Doyle and Luckow, 2003) and could be further supplemented by studying a basal, non-nitrogen fixing member of Fabaceae such as Cercis.
Cercis, an ancient member of Caesalpinoideae, has fossil records that date back to the Eocene era (Jia and Manchester, 2014), and is therefore a prime candidate for a comprehensive study of genome size as it relates to legume systematics and taxonomy. Most aspects of legume biology, from ploidy number to floral diversity, can be further examined through the evolutionary relationships that exist among leguminous taxa (Young et al., 2003). This information could grant valuable insight into species evolution and provide potential breeding applications (Rounsaville and Ranney, 2010) in future Cercis hybridization projects.
Information derived from a genome survey of Cercis will be useful as it relates to a better understanding of the evolution of genome size and ploidy distribution within the legumes. Three cultivars of Cercis warrant particular interest in regard to ploidy variation. Traveller is a unique cultivar of C. canadensis var. texensis possessing both male and female sterility. The basis of this sterility is unknown, but potentially could be based on triploidy. Triploid plants often have reduced fertility or sterility. Likewise, ‘Don Egolf’ is a female sterile form of C. chinensis and will be investigated to determine if its sterility is due to triploidy. Finally, ‘Tom Thumb’ a diminutive sterile form of C. canadensis with extremely small leaves and flowers will be investigated to determine if its unique characters are potentially due to haploidy. Haploid plants have been shown to exhibit dwarfism in other woody species (Yahata et al., 2005). This survey will also contribute to the knowledge of the taxonomic relationship of Bauhinia to redbud. Cercis has been documented as having seven chromosome pairs with 2n = 2x = 14 (Curtis, 1976; Goldblatt, 1981). Bauhinia is thought to be a tetraploid (2n = 4x = 28) relative of Cercis (Doyle and Luckow, 2003) with 14 chromosome pairs (Turner, 1956). As the closest living relative of redbud (Coskun and Parks, 2009), Bauhinia will serve as an interesting comparison with the relative DNA estimations of Cercis in this study.
Cercis chinensis possesses a relatively small genome size of 350 million bps (Mb) (De Mita et al., 2014), which corresponds with the phylogenetic position of significant antiquity (Zou et al., 2008) that Cercis occupies within Fabaceae. Except for C. chinensis, there are currently no other reports of genome size of Cercis. The objectives of this study were to examine the genome size of a comprehensive collection of species, botanical varieties, and cultivars of Cercis. Additionally, the genome size of ‘Tom Thumb’, demonstrating morphological characters suggesting haploidy, and of ‘Traveller’ and ‘Don Egolf’, sterile cultivars suggesting triploidy, will be examined.
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