A Synopsis of Arabidopsis (Brassicaceae) Steve L. O'Kane, Jr.Department of Biology, University of Northern Iowa, Cedar Falls, Iowa 50614-0421, U.S.A. Ihsan A. Al-Shehbaz Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 63166-0299, U.S.A.ABSTRACT. New combinations in Arabidopsis areproposed. Species previously placed in Cardami-nopsis are here transferred to Arabidopsis and taxapreviously recognized in Arabidopsis, other than A.thaliana and A. suecica, are excluded from the ge-nus. Distributions and a key to the nine speciesand five subspecies are presented. Based on analyses of rDNA sequences, the genusArabidopsis as understood prior to this study is notonly highly paraphyletic but also includes taxa thatare distant in the Brassicaceae (unpublished re-sults). This situation is particularly alarming giventhe central place that A. thaliana (L.) Heynholdplays in a myriad of current studies of genome evo-lution, developmental genetics, morphological evo-lution and development, etc. (Meyerowitz & Pruitt,1985; Endress, 1992; Maluszynska & Heslop-Har-rison, 1993; Crone & Lord, 1994; Larkin et al.,1994; Price et al., 1994; Teutonico & Osborn,1994; Zhang & Lechowica, 1994; Tsukaya, 1995).Without a well-documented phylogenetic recon-struction of the genus and a congruent taxonomy,studies that make assumptions about its relationsare likely to be inconclusive and to arrive at irrel-evant conclusions. A number of recent studies (e.g.,Maluszynska & Heslop-Harrison, 1993; Kamm etal., 1995; Tsukaya et al., 1997) have assumed closerelationships between A. thaliana and species cur-rently included in the genus that molecular data donot support (Price et al., 1994; O'Kane et al.,1995). The genus has been variously placed closeto several other genera (Arabis, Braya, Cardami-nopsis, Cymatocarpus, Drabopsis, Halimolobos, Hy-landra, Microsisymbrium, Nasturtiopsis, and Neo-torularia) based on morphological similarity(Hedge, 1968; Jafri, 1973; Al-Shehbaz, 1988; Ball,1993). Rather than showing a close relationshipamong these genera, our work indicates that thecircumscription of some of these genera and Ara-bidopsis is needed to better represent phylogeneticrelationships. Recently, for instance, A. erysimoideshas been moved to Erysimum (Al-Shehbaz, 1994),A. parvula (Schrenk) 0. E. Schulz has been trans-ferred to Thellungiella (Al-Shehbaz & O'Kane,1995), and both A. gamosepala Hedge and A. tue-murnica Kuan & An have been placed in Neoto-rularia (Al-Shehbaz & O'Kane, 1997). The nomenclatural changes proposed here aimto align the taxonomy of Arabidopsis with the resultsof recent analyses of rDNA sequences (O'Kane etal., 1997) and on-going phylogenetic analyses. Thechanges given here were anticipated by both Hy-lander (1957) and Ball (1993). Hylander (1957:602-603) stated that should Cardaminopsis and Ar-abidopsis be combined, Cardaminopsis "must bedropped into the latter genus [Arabidopsis], the lim-its of which would thereby be considerably wid-ened-or, perhaps more correctly, drawn in quiteanother way than e.g. by Schulz." Ball (1993: 322)echoed this by stating, "It seems probable that Car-daminopsis should be combined with Arabidopsis,and some species of Arabidopsis may have to beremoved from the enlarged genus." Furthermore,Jones (1964) suggested that Arabis cebennensis andA. pedemontana might better be placed in the genusCardaminopsis (here Arabidopsis). These earliermorphological predictions are now strongly sup-ported by two independent molecular studies: Priceet al. (1994) using chloroplast DNA restriction sitevariation and rbcL gene sequences, and O'Kane etal. (1997 and unpublished) using nuclear rDNA se-quences. Essentially, the nomenclatural changesproposed herein deal with the transfer of speciesfrom Cardaminopsis to Arabidopsis. Except for thenine species and five subspecies treated in this pa-per, all of the remaining 49 binomials variously as-signed to Arabidopsis are excluded from the genus.Work is in progress to assign those to other genera.Arabidopsis (DC.) Heynhold, in Holl & Heynhold, Fl. Sachsen 1: 538. 1842; nom. cons. TYPE: Arabidopsis thaliana (L.) Heynhold.Cardaminopsis (C. A. Meyer) Hayek, Fl. Steiermark 1: 477. 1908. Syn. nov. Basionym: Arabis sect. Carda-NovoN 7: 323-327. 1997.