Delimitation of genera is one of the most commonly faced problems in the systematics of the Brassicaceae. The vast majority of genera are established primarily on a few differences in fruit characters. Of the 338 genera in the family, about 225 (ca. 67%) contain no more than four species (Al-Shehbaz et al., 2006). As shown in the molecular studies briefly discussed below, most of the smaller genera are nested within and should be united with the larger ones. Therefore, it is expected that the total number of genera in the Brassicaceae will likely be reduced substantially.
A classic example of generic reduction is the South African Heliophila Linnaeus, a genus of about 80 species in which nested are the monospecific Brachycarpaea de Candolle, Schlechteria Bolus, and Silicularia Compton and the dispecific Cycloptychis E. Meyer ex Sonder and Thalspeocarpa A. C. Smith (Mummenhoff et al., 2005). Based on this molecular study and a closer examination of morphology, these five genera were united with Heliophila by Al-Shehbaz and Mummenhoff (2005). The five segregates differ from Heliophila primarily by the reduction in seed number and the development of indehiscent vs. dehiscent fruits—features that have evolved independently many times in the family (Al-Shehbaz et al., 2006).
In their molecular study of Smelowskia C. A. Meyer, Warwick et al. (2004a) found that seven smaller genera are nested within, and after a critical evaluation of the differences separating them, the eight genera were united into one (Al-Shehbaz and Warwick, 2006). Another important study involves the union of Cardaria Desvaux (three spp.), Coronopus Zinn (ten spp.), and Stroganowia Karlin & Kirilow (21 spp.) with Lepidium Linnaeus, in which they were nested, to form a monophyletic genus of about 223 spp. (Mummenhoff et al., 2001; Al-Shehbaz et al., 2002). Also, the monospecific Californian endemic Twisselmannia Al-Shehbaz and the presumably extinct Chilean endemic Agallis Philippi were nested within and differed in their ITS and ndhF by a single base pair substitution from the dispecific Tropidocarpum W. J. Hooker (Baja California and SW California), a genus with which they were united (Al-Shehbaz and Price, 2001; Al-Shehbaz, 2003b). Finally, the phylogenetic study of Eutrema R. Brown and five smaller genera nested within (Warwick et al., 2006a) prompted Al-Shehbaz and Warwick (2005) to treat the complex as one genus. Warwick et al. (2006a) added a new native North American Arabidopsis (de Candolle) Heynhold (A. arenicola (Richardson) Al-Shehbaz et al.), a species recognized for well over a century in Arabis Linnaeus. In all of the above cases, the smaller genera were united with the larger ones only after critical re-evaluations of morphology determined the insignificance of the differences on which those segregates were based.
Different conclusions were reached in the phylogenetic studies of Arabis and Arabidopsis, although these cases are less common compared to the examples listed above. Arabidopsis was believed to include more than 50 species centered in the Himalayas, but NSF-supported molecular studies (O’Kane et al., 1997; O’Kane and Al-Shehbaz, 2003) amply demonstrated that the genus includes nine species primarily centered in Europe (O’Kane and Al-Shehbaz, 1997), and the remaining species were excluded to several genera, including three new (Al-Shehbaz et al., 1999). In Arabis, as many as 180 species were recently recognized (Appel and Al-Shehbaz, 2003), but molecular studies (Koch et al., 1999a, 2000) supported the transfer of many species to about 20 genera of various tribes (Al-Shehbaz, 2003a, 2005; German and Al-Shehbaz, 2008b; Windham and Al-Shehbaz, 2006, 2007a, 2007b). For example, the North American Arabis was treated by Rollins (1993) to include 80 species and 64 varieties with base chromosome numbers of x = 6, 7, and 8. Treatment of this complex in the forthcoming Flora of North America (vol. 7) includes 131 species in six genera of five tribes. Of these, 109 species belong to Boechera A. Löve & D. Löve (Boechereae), 16 to Arabis (Arabideae), two to Pennellia Nieuwland (Halimolobeae), two to Arabidopsis and one to Turritis Linnaeus (formerly Camelineae but needs a new tribal placement), and one to Streptanthus Nuttall (Thelypodieae). Thus, the two common features used by Rollins (1993) to delimit Arabis, the latiseptate fruits (flattened parallel to the septum) and accumbent cotyledons, evolved independently in at least 54 genera of the Brassicaceae (Al-Shehbaz, unpublished).
Another classic example is Sisymbrium L., a genus previously believed to include 90 species, the majority of them in South America (Appel and Al-Shehbaz, 2003). However, detailed molecular and morphological studies (Warwick et al., 2002, 2006b) have demonstrated that the genus is almost exclusively Eurasian, and all except one of the New World species belong to other genera (Warwick and Al-Shehbaz, 2003; Al-Shehbaz, 2006).
The examples discussed above demonstrate the valuable contribution of molecular phylogeny in the delimitation of monophyletic genera and how the combination of phylogeny and traditional botany yields a clearer, more concise understanding of the taxonomy and morphology of the family. In particular, results to date indicate that convergence is widespread in almost every conceivable morphological character in the family. Thus, efforts to establish genera without a combination of molecular studies and critical evaluation of morphology will likely lead to unwarranted, non-monophyletic genera and tribes, hindering the considerable progress already achieved.
Comparative sequence data of rapidly evolving markers (e.g., the chloroplast ndhF gene and the nuclear ITS region) show that many related species have remarkable sequence similarities but considerably different embryo types or fruit morphologies (Warwick and Black 1994; Crespo et al. 2000; Mummenhoff et al. 2001a, 2005 ; Beilstein et al. 2006). These findings suggest that major differences in fruit morphology can take place rather rapidly and independent of molecular or other morphological features. Developmental genetics in Arabidopsis thaliana (see below) demonstrates that few gene mutations cause significant alterations in fruit morphology, and the same may well apply elsewhere in the family. It is safe to conclude, therefore, drastic bursts of fruit evolution may occur rather rapidly and independent other morphological characters or the rapidly evolving parts of the genome. Such drastic fruit differences can easily lead to erroneous generic delimitations.
Several botanists (e.g., Ferrandiz et al., 1999, 2000; Ferrandiz, 2002; Dinneny &Yanofsky, 2004; Liljegren et al., 2000, 2004; Polster, 2005) focused on fruit development in Arabidopsis thaliana and demonstrated that a few genes (e.g., MADS-box, FRUITFUL, SHATTERPROOF) are responsible for altering some aspects of fruit morphology such as silique vs. silicle, length/width ratio, or dehiscence vs. indehiscence. They identified at least six genes that control fruit dehiscence in A. thaliana and showed that one or double mutant genes can change the fruits from dehiscent to indehiscent. These outstanding discoveries should alarm us from the use of fruit dehiscence vs. indehiscence as the main feature for defining genera in the family. Several examples can be discussed, but a couple stands as classic. Cardaria Desvaux (3 spp.) differs from the much larger Lepidium Linnaeus (223 spp.), in which it is nested (Mummenhoff, 1995; Mummenhoff et al., 2001a), by the indehiscent vs. dehiscent fruits. Similarly, the monospecific Boleum Desvaux (indehiscent fruits) is nested within Vella Linnaeus (Warwick & Black, 1994, Crespo et al., 2000) that has dehiscent fruits. Therefore, the union of Cardaria with Lepidium (Al-Shehbaz et al., 2002) and Boleum with Vella (Warwick and Al-Shehbaz, 1998) are fully supported.
Finally, Thlaspi Linnaeus was previously recognized as a genus of nearly 100 species delimited primarily on angustiseptate fruits (flattened at a right angle to the septum), two to may seeds per locule, and simple or no trichomes. Meyer (1973, 1979) divided Thlaspi into 12 segregates based mainly on differences in on seed-coat anatomy, and these smaller genera were not accepted in latter systematic studies (Hedge, 1976; Al-Shehbaz, 1986; Appel & Al-Shehbaz, 2003). However, molecular data (Mummenhoff & Koch, 1994; Zunk et al., 1996; Mummenhoff et al., 1997a, b; Koch & Mummenhoff, 2001) strongly support the
recognition of some of Meyer’s segregates. As a result, Al-Shehbaz et al. (2006) assigned Thlaspi (now only six species) to the tribe Thlaspideae and Noccaea Moench (82 spp.) and a few smaller segregates are placed in the tribe Noccaeeae (see these tribes on this web site).
Three basic conclusions were forwarded by Al-Shehbaz et al. (2006: 93) regarding the delimitation of genera: “First, monotypic or oligotypic genera should not be established without prior molecular studies and critical evaluation of morphology. Second, because of widespread convergence in most morphological characters, especially fruit types and embryo position, these characters should be used with extreme care in establishing generic boundaries. Finally, major differences in fruit morphology can be misleading, and the examples of Heliophila, Tropidocarpum, and Vella should be a constant reminder about the dangers of making erroneous taxonomic conclusions by overemphasizing fruit morphology at the expense of other, potentially very useful vegetative and floral characters.”