The sister genera Berberis L. and Mahonia Nutt. represent the two largest groups within the family Berberidaceae, consisting of ≈400 and 100 species, respectively (Ahrendt, 1961; Kim et al., 2004). This highly ornamental group of shrubs and small trees is valued for their evergreen or multicolored leaves, brilliant flowers, and often showy fruit. The two genera have also been recognized for their pharmaceutical and medicinal properties (Alvarez et al., 2009) as well as their use in the printing and dyeing industry (Yan-Jun et al., 2006). Distribution of the two genera is nearly worldwide with centers of diversity in southern Asia as well as Central and South America and with minor representation in North America, Europe, Africa, and the Pacific Islands (Ahrendt, 1961).
Taxonomic standing of Mahonia and Berberis as distinct genera has been the subject of much debate among botanists and horticulturists. Before the development of DNA-based phylogenetics, morphological characters such as leaf and stem complexity, inflorescence structure, and floral anatomy had served to distinctly separate Mahonia from Berberis. However, Mahonia section Horridae Fedde (approximately nine species), which includes M. freemontii (Torr.) Fedde, M. haematocarpa (Wooton) Fedde, M. nevinii (A. Gray) Fedde, and M. trifoliolata (Moric.) Fedde, exhibits a blend of taxonomic features intermediate between Mahonia and Berberis (Ahrendt, 1961; Whittemore, 1997). These morphological inconsistencies have led some (Laferriere, 1997; Marroquin, 1993; Whittemore, 1997) to adopt a unified treatment of all Berberis and Mahonia species within Berberis. Nevertheless, the obvious difference in physical appearance between the two genera, with compound leaves within Mahonia versus simple leaves within Berberis, makes a unified circumscription hard to reconcile, and consequently, the horticultural field generally maintains the two groups as separate genera (Ahrendt, 1961; Dirr, 2009; Hinkley, 2009; Huxley et al., 1992; Yan-Jun et al., 2006).
When the taxonomy is viewed sensu Ahrendt, Berberis and Mahonia are each broken down into two subgenera set forth by Ahrendt (1961) and Schneider (1905). Within Berberis, the Australes C.K. Schneid. includes all the species from Central and South America; the remaining species are placed in the Septentrionales C.K. Schneid. and occur entirely in the northern hemisphere save for two in East Africa and one in Java and Sumatra (Ahrendt, 1961). Conversely, Mahonia are grouped longitudinally with those of the Eastern hemisphere in subgenus Orientales Ahrendt and all those of the Western hemisphere [with the notable exception of M. nervosa (Pursh) Nutt.] in subgenus Occidentales Ahrendt.
Recent phylogenetic analysis based on internal transcribed spacer (ITS) sequencing (Kim et al., 2004) has yielded further insight into the taxonomic relationships and evolutionary history of Mahonia and Berberis. For example, the postulation of Ahrendt (1961) of Mahonia as the progenitor of Berberis was supported. Examining the contemporary dispersal of the two genera from South America northward reveals that although the distinctive compound-leaved Mahonia is first encountered in Central America, a number of Berberis characters persist within Mahonia much further north. These transitional species, representing the aforementioned Mahonia section Horridae, showed a closer relationship with Berberis and thus a paraphyletic subgenus Occidentales (Kim et al., 2004). Although ITS phylogeny supported the subgenera proposed by Ahrendt (1961) and Schneider (1905), groupings below the subgeneric levels were not supported (Kim et al., 2004). Furthermore, M. nervosa was retained within the Orientales, albeit with weak support.
Along with the monotypic herb Ranzania japonica T. Ito, Berberis and Mahonia form a monophyletic clade within Berberidaceae in which base chromosome number is x = 7 (Kim and Jansen, 1998). As Dermen (1931) noted in his cytological studies, chromosomes among widespread species of both genera are of similar size. Furthermore, artificial intergeneric hybridization events between Mahonia and Berberis originated in Europe as early as 1854 (Dirr, 2009). Despite a number of successful intergeneric hybrids (×Mahoberberis C.K. Schneid.), the resulting progeny have been horticultural curiosities at best, typically regarded as inferior to both parent taxa (Phillips and Barber, 1981). ×Mahoberberis tend to exhibit numerous leaf-morphs among single plants, and in general flowering and fruiting of the hybrids is known to be rare or nonexistent (Dirr, 2009; Wyman, 1958). In addition, all ×Mahoberberis hybrids have been comprised of only one species of Mahonia [M. aquifolium (Pursh) Nutt.], and the cross appears uni-directional with Mahonia only functioning as the maternal parent (Dirr, 2009; personal observation), further suggesting that the two genera are largely incompatible. Conversely, hybrids among species of Berberis and among species of Mahonia are commonplace (Huxley et al., 1992; personal observation).
Polyploidization is a significant phenomenon in the plant kingdom that can play a role in rapid genomic rearrangement, development of novel traits and adaptations, reproductive isolation, and can ultimately lead to speciation (Adams and Wendel, 2005; Soltis and Burleigh, 2009). Furthermore, polyploidy is an important consideration in plant breeding because it can influence crossability, morphology, fertility, and gene expression (Chen and Ni, 2006; Soltis et al., 2004). Sampling of ploidy levels has been very limited for Mahonia taxa. Mahonia aquifolium, M. napaulensis DC., M. repens (Lindl.) G. Don., and M. japonica (Thunb.) DC. have been reported to be diploid with 2n = 2x = 28 (Dermen, 1931; Xu et al., 1992). In other cases, M. aquifolium and M. nervosa were reported to be tetraploid with 2n = 4x = 56 (Taylor and Taylor, 1997). Reports on 45 Berberis species found diploids, 2n = 2x = 28, including B. koreana Palib., B. seiboldii Miq., B. thunbergii DC., B. vulgaris L., and B. yunnanensis Franch. as well as tetraploids, 2n = 4x = 56, including B. buxifolia Lam., B. heterophylla Juss. ex Poir., and B. turcomanica Kar. (Bottini et al., 2000; Dermen, 1931).
Independent of variations in ploidy level, information on base genome size (base DNA content) can be used as an indicator of genome evolution and taxonomic relationships (Greilhuber, 1998; Vinogradov, 1994; Zonneveld and Duncan, 2010; Zonneveld and Van Iren, 2001), lending insight into species evolution and potential breeding applications. As it relates to breeding, disparities in genome sizes can reflect differences in chromosome sizes and arrangement that may influence crossability and fertility of hybrid progeny (Zonneveld, 2009). There are no published reports of Mahonia genome size, and those of Berberis are extremely limited in both number of taxa and species diversity. Previous reports of genome size among Berberis were determined using Feulgen microspectrophotometry with diploid species constituting a range of 1.5 pg for B. bidentata Lechler to 3.6 pg for B. empetrifolia Lam. (Bottini et al., 2000). A desirable alternative to microspectrophotometry is flow cytometry, which allows for much greater ease in sample preparation, rapid determination of genome size, and can be accurately performed using a variety of plant tissues (Doležel and Bartos, 2005; Doležel et al., 1998). For closely related taxa, in which genome sizes are relatively conserved, flow cytometry can also be used for determination of ploidy level. Although a number of different fluorochromes may be used to stain DNA, many including 4′,6-diamidino-2-phenylindole, Hoechst 33258, and olivomycin are exclusive to either AT or CG bps, whereas propidium iodide (PI) is known to be largely non-specific with only a slight preference toward CG (Doležel et al., 1998; Vinogradov, 1994).
Considering the tremendous diversity and crossability found in Berberis and Mahonia, the potential for breeding improved hybrids is considerable. However, a greater understanding of genome sizes and ploidy levels within these genera would greatly enhance future breeding efforts. Although basic information on chromosome numbers, genome sizes, and ploidy levels have been reported for some Berberis and Mahonia, sampling has been limited and little is known about ploidy levels of specific clones or cultivars. The objectives of this research were to conduct an extensive survey of genome sizes and ploidy levels of species, hybrids, and cultivars of Berberis and Mahonia using a combination of flow cytometry and traditional cytology. Taxa included for this survey exhibit attributes of value for the ornamental plant breeder and are representative of each major phylogenetic clade. As a result of the unresolved nature of the generic classification, and for purposes of comparison, we accept the treatment of Ahrendt (1961), whom conducted the last thorough review of the genera.
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