A synopsis of Arabidopsis (Brassicaceae)

New combinations in Arabidopsis are proposed. Species previously placed in Cardaminopsis are here transferred to Arabidopsis and taxa previously recognized in Arabidopsis, other than A. thaliana and A. suecica, are excluded from the genus. Distributions and a key to the nine species and five subspecies are presented. Based on analyses of rDNA sequences, the genus Arabidopsis as understood prior to this study is not only highly paraphyletic but also includes taxa that are distant in the Brassicaceae (unpublished results). This situation is particularly alarming given the central place that A. thaliana (L.) Heynhold plays in a myriad of current studies of genome evolution, developmental genetics, morphological evolution and development, etc. (Meyerowitz & Pruitt, 1985; Endress, 1992; Maluszynska & Heslop-Harrison, 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 reconstruction of the genus and a congruent taxonomy, studies that make assumptions about its relations are likely to be inconclusive and to arrive at irrelevant conclusions. A number of recent studies (e.g., Maluszynska & Heslop-Harrison, 1993; Kamm et al., 1995; Tsukaya et al., 1997) have assumed close relationships between A. thaliana and species currently included in the genus that molecular data do not support (Price et al., 1994; O'Kane et al., 1995). The genus has been variously placed close to several other genera (Arabis, Braya, Cardaminopsis, Cymatocarpus, Drabopsis, Halimolobos, Hylandra, Microsisymbrium, Nasturtiopsis, and Neotorularia) based on morphological similarity (Hedge, 1968; Jafri, 1973; Al-Shehbaz, 1988; Ball, 1993). Rather than showing a close relationship among these genera, our work indicates that the circumscription of some of these genera and Arabidopsis is needed to better represent phylogenetic relationships. Recently, for instance, A. erysimoides has been moved to Erysimum (Al-Shehbaz, 1994), A. parvula (Schrenk) O. E. Schulz has been transferred to Thellungiella (Al-Shehbaz & O'Kane, 1995), and both A. gamosepala Hedge and A. tuemurnica Kuan & An have been placed in Neotorularia (Al-Shehbaz & O'Kane, 1997). The nomenclatural changes proposed here aim to align the taxonomy of Arabidopsis with the results of recent analyses of rDNA sequences (O'Kane et al., 1997) and on-going phylogenetic analyses. The changes given here were anticipated by both Hylander (1957) and Ball (1993). Hylander (1957: 602-603) stated that should Cardaminopsis and Arabidopsis be combined, Cardaminopsis "must be dropped into the latter genus [Arabidopsis], the limits of which would thereby be considerably widened-or, perhaps more correctly, drawn in quite another way than e.g. by Schulz. " Ball (1993: 322) echoed this by stating, "It seems probable that Cardaminopsis should be combined with Arabidopsis, and some species of Arabidopsis may have to be removed from the enlarged genus. " Furthermore, Jones (1964) suggested that Arabis cebennensis and A. pedemontana might better be placed in the genus Cardaminopsis (here Arabidopsis). These earlier morphological predictions are now strongly supported by two independent molecular studies: Price et al. (1994) using chloroplast DNA restriction site variation and rbcL gene sequences, and O'Kane et al. (1997 and unpublished) using nuclear rDNA sequences. Essentially, the nomenclatural changes proposed herein deal with the transfer of species from Cardaminopsis to Arabidopsis. Except for the nine species and five subspecies treated in this paper, all of the remaining 49 binomials variously assigned to Arabidopsis are excluded from the genus. Work is in progress to assign those to other genera. Arabidopsis (DC.) Heynhold, in Holl & Heynhold, F1. Sachsen 1: 538. 1842; nom. cons. TYPE: Arabidopsis thaliana (L.) Heynhold. Cardaminopsis (C. A. Meyer) Hayek, F1. Steiermark 1: 477. 1908. Syn. nov. Basionym: Arabis sect. CardaNovoN 7: 323-327. 1997. This content downloaded from 207.46. 13.128 on Tue, 06 Sep 2016 06:08:56 UTC All use subject to http://about.jstor. org/terms