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Many recent studies have shown that most morphological structures in the Brassicaceae show considerable convergence, making it rather difficult to construct phylogenies based solely on morphology (Al-Shehbaz, 1984; Price et al., 1994; Appel & Al-Shehbaz, 2003; Koch et al., 2003a; Mitchell-Olds et al., 2005; Al-Shehbaz et al., 2006; Koch & Al-Shehbaz, 2009). Prior to the first utilization of molecular data in phylogenetic classifications by Al-Shehbaz et al. (2006), almost all works followed Schulz’s (1936) highly artificial classification, and some (e.g., Rollins 1993; Appel & Al-Shehbaz, 2003) resorted to enumerate taxa alphabetically.
The type of embryo curvature and relative position of the radicle to the cotyledons, first utilized by de Candolle (1921a), have significantly influenced both generic and tribal delimitations to the present. By contrast, trichomes types and floral and vegetative morphology were largely neglected. Certainly, embryo type often shows considerable convergence, and many genera (e.g., Schizopetalon Sims, Lepidium Linnaeus) may have more than one type. For example, spirally coiled embryos, which are rather rare in the Brassicaceae, evolved independently in Bunias Linnaeus (Buniadeae), Erucaria Gaertner (Brassiceae), Heliophila (Heliophileae), and Schizopetalon (Schizopetaleae). Perhaps the fruits show far more convergence than any other structure in the family, especially in the type of flattening, dehiscence vs. indehiscence, development of the fruit wing, reduction is seed number, perforation or absence of septum, and development of appendages.
Floral architecture of Brassicaceae is quite conservative, though one finds substantial variations within and among genera that can be of considerable taxonomic and phylogenetic value. Several genera (e.g., Iberis Linnaeus, Ornithocarpa Rose, Schizopetalon, Stanleya Nuttall, Warea Nuttall, Stenopetalum R. Brown) are easily determined based on floral morphology alone, and others (e.g., Alyssum Linnaeus, Heliophila, Lepidium, and Streptanthus Nuttall) show a tremendous floral diversity that can be used in the delimitation of lineages within. However, as in other morphological aspects of the family, floral features also show convergence such as zygomorphy between Iberis and Streptanthus or within the latter genus.
Although, the importance of trichome morphology in the classification of the Brassicaceae was first utilized some 118 years ago (Prantl, 1891), it was largely neglected by subsequent authors until Rollins and Banerjee (1975, 1976) showed that it holds substantial value in taxonomic work. Trichome development in Arabidopsis thaliana (e.g., Beilstein and Szymanski, 2004; Hülskamp, 2000; Schwab et al., 2000, Szymanski et al., 2000,) have determined a number of genes that control trichome development. It would be a major advancement to our knowledge of the family if such genes are sequenced across a large spectrum of the family to determine their utilities in phylogenetic studies. Here again, trichomes also exhibit homoplasy in the family, as evidenced by the presence of malpighiaceous trichomes in at least eight tribes, or the independent evolution of lepidote, webbed trichomes in the Alysseae and Physarieae.