NOVON 9(3): 296-307. 1999.

Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 63166-0299, U.S.A.

Department of Biology, University of Northern Iowa, Cedar Falls, Iowa 50614-0421, U.S.A.

Department of Botany, University of Georgia, Athens, Georgia 30602, U.S.A.

Abstract. All 59 binomials previously assigned to Arabidopsis are critically evaluated and placed in 14 genera, of which Crucihimalya, Olimarabidopsis, and Pseudoarabidopsis are described as new. Nine new combinations in Crucihimalya, three in Olimarabidopsis, and one in Pseudoarabidopsis, as well as 12 new synonyms, are proposed. A key to all genera most commonly confused with Arabidopsis, including the three new proposed herein, is presented.

Generic delimitation in the Brassicaceae (Cruciferae) is one of the most difficult and often controversial aspects in the systematics of the family (Al-Shehbaz, 1973, 1984; Rollins, 1993; Schulz, 1936). Perhaps the two principal reasons for this are: (1) Convergence in basically every morphological character is so high that superficially very similar genera might well turn out to be remotely related or unrelated upon critical examination of so-called key generic characters and independent assessment of phylogenetic relationship using molecular comparisons (as in the genera herein segregated from Arabidopsis). (2) Although the family exhibits tremendous diversity in fruit morphology, other parts of the plant, especially the flowers, usually do not show much diversity and, therefore, there are few morphological characters that can be used to determine relationships. Characters of flowers and vegetative parts have often been ignored or overlooked. Because fruit morphology has traditionally been used in taxonomic treatments of the Brassicaceae, the problem becomes more acute among the numerous genera with relatively similar linear-shaped fruits. In cases like these, vegetative or floral characters could easily be of greater significance than fruit or seed characters in delimiting natural genera.

Given the great interest in Arabidopsis thaliana (L.) Heynhold as a model experimental organism, it is of particular value to define clear phylogenetic groupings among its related genera. The limits of Arabidopsis (DC.) Heynhold have been the subject of continuous controversy, and many authors (e.g., Al-Shehbaz, 1988; Hedge (in Hedge & Rechinger), 1968; Price et al., 1994) called for the need to establish well-defined boundaries between the genus and its relatives. Although O’Kane and Al-Shehbaz (1997) retained only nine species in Arabidopsis, the generic placement of 50 of the 59 binomials previously assigned to Arabidopsis remained to be established. The present paper addresses this problem, and keys for the determination of taxa most often confused with Arabidopsis are provided.

Molecular comparisons of both chloroplast DNA (Price et al., 1994, unpublished) and nuclear Internal Transcribed Spacer (ITS) regions (O’Kane et al., 1995, 1997, unpublished) have consistently supported dividing the core group of the broadly circumscribed Arabidopsis (e.g., table 1 of Price et al., 1994) into a small number of well-separated clades, most notably Arabidopsis sensu stricto (including Hylandra A. Löve and Cardaminopsis (C. A. Meyer) Hayek; see O’Kane & Al-Shehbaz, 1997; Mummenhoff & Hurka, 1995), A. pumila (Stephan) N. Busch and relatives (here newly described as Olimarabidopsis ), and A. himalaica (Edgeworth) O. E. Schulz and relatives (here newly described as Crucihimalya). All of these genera belong to a major terminal clade, including a number of other Eurasian and American genera such as Capsella Medikus, Neslia Desvaux, Erysimum L., Malcolmia R. Brown, and Halimolobos Tausch. Several other species sometimes placed in Arabidopsis have been found to belong to the distantly related genera Thellungiella O. E. Schulz (Al-Shehbaz & O’Kane, 1995; Galloway et al., 1998) and Neotorularia Hedge & J. Léonard (Al-Shehbaz & O’Kane, 1997), or in one case to the genus Erysimum (Al-Shehbaz, 1994), which is closely related to Olimarabidopsis. Although the taxonomy of Arabidopsis has now been worked out at the generic level (O’Kane & Al-Shehbaz, 1997), our research indicates that Arabis is polyphyletic and consists of at least three unrelated clades.

Following up the suggestions of new phylogenetic groupings provided by molecular comparisons, we have thoroughly reexamined the morphology of the species previously placed in Arabidopsis in order to reassess morphological groupings of species and to try to find morphological characters distinguishing the groups indicated by molecular comparisons. Over the last seven years, we have critically examined more than 6000 specimens from numerous herbaria. We have found that differences in fruit morphology (terete vs. flattened) and seed morphology (incumbent vs. accumbent cotyledons and winged vs. unwinged seeds), which have been previously used (e.g., Busch, 1909; Ball, 1993; Jones, 1964; Mulligan, 1995; Rollins, 1993; Schulz, 1936) to separate the traditionally circumscribed genera Arabidopsis and Arabis, appear to be very unreliable in the delimitation of natural generic groups. Seven of the nine species of Arabidopsis sensu stricto have flattened fruits and accumbent cotyledons, while two have terete fruits and incumbent cotyledons (O’Kane & Al-Shehbaz, 1997). In contrast, differences in trichome branching, flower color, and nature of the cauline leaf base appear to be much more useful in defining natural generic groups among species previously placed in Arabidopsis sensu lato. Combinations of these characters, along with molecular phylogenetic data, support the retention of 9 species in Arabidopsis and the segregation of 13 species commonly placed in the genus into three new, well-defined genera herein proposed as Crucihimalya, Olimarabidopsis, and Pseudoarabidopsis. Based on chloroplast DNA sequencing (Price, unpublished), Olimarabidopsis is most closely related to Erysimum, and both are readily separated from Arabidopsis by having yellow or orange (rarely cream or purplish) flowers and malpighiaceous and/or sessile stellate trichomes (Erysimum) or submalpighiaceous and subsessile stellate trichomes (Olimarabidopsis).

Only 3 of the 14 genera to which the 59 Arabidopsis binomials belong are not included in the following key. These, Murbeckiella Rothmaler, Sisymbriopsis Botschantsev & Tzvelev, and Robeschia Hochstetter, are unrelated to Arabidopsis. Murbeckiella has auriculate cauline leaves, keeled valves, veined septa, and winged seeds, whereas Arabidopsis has petiolate cauline leaves, rounded or flat valves, veinless septa, and wingless seeds. Sisymbriopsis has pubescent, quadrangular fruits and prominently 3-veined valves, whereas Arabidopsis has glabrous, terete or flattened fruits and veinless or obscurely 1-veined valves. Finally, Robeschia has dendritic trichomes, 2-pinnatisect or 2-pinnate leaves, much thickened fruiting pedicels as thick as the fruit, and an obsolete style, whereas Arabidopsis has simple and stalked forked trichomes, undivided to pinnatifid leaves, slender fruiting pedicels narrower than the fruit, and distinct styles. These three genera have not yet been subjected to molecular studies, but should be analyzed in the near future.

The circumscriptions of Arabis and Halimolobos in the following key follow that of Rollins (1993). We are, however, aware that these genera, as presently delimited, represent very heterogeneous assemblages of species groups that will have to be re-assigned to other genera, most of which have already been proposed. We are currently working on these groups.

Artificial Key to the Genera with Members Formerly Placed in Arabidopsis Sensu Lato

1a. Plants completely glabrous; leaves and stems glaucous; plants often restricted to strongly saline and/or calcareous soil…Thellungiella

1b. Plants sparsely to densely hairy; leaves and stems not glaucous; plants usually on other soil types.

2a. Trichomes sessile and completely appressed, malpighiaceous and/or stellate with unbranched rigid straight rays…Erysimum

2b. Trichomes short- or long-stalked, simple or branched, if stellate and sessile then rays slender and/or branched.

3a. Scapose annuals without cauline leaves; fruiting pedicel nearly as thick as fruit…Drabopsis

3b. Nonscapose annuals, biennials, or perennials with few to many cauline leaves; very rarely perennials without cauline leaves; fruiting pedicels much narrower than fruit (except some Neotorularia).

4a. Fruits compressed; cotyledons accumbent.

5a. Cauline leaves short petiolate, neither auriculate nor sagittate at base; trichomes simple and 2- or 3(or 4)-forked, never dendritic or stellate; fruit valves with a prominent midvein; seeds usually wingless…Arabidopsis

5b. Cauline leaves auriculate to sagittate at base, rarely petiolate; at least some of leaf trichomes dendritic or stellate; fruit valve without or with obscure midvein; seeds often winged…Arabis

4b. Fruits usually terete or 4-angled; cotyledons incumbent.

6a. Inflorescence axis flexuous; leaves divided into 3 or 5 filiform to narrowly linear segments…Ianhedgea

6b. Inflorescence rachis not flexuous; leaves various but never divided into filiform or narrowly linear segments.

7a. Flowers yellow.

8a. Cauline leaves petiolate; fruit apex strongly recurved or contorted; fruiting pedicels stout, nearly as thick as fruit…Neotorularia

8b. Cauline leaves auriculate, rarely sessile; fruit apex neither recurved nor contorted; fruiting pedicels slender, narrower than fruit…Olimarabidopsis

7b. Flowers white, lavender, or purple, very rarely creamy white.

9a. Fruits glabrous.

10a. Cauline leaves petiolate; branched trichomes forked, rays always simple.

11a. Fruiting pedicels slender, narrower than fruit; seeds mucilaginous when wetted; fruits straight…Arabidopsis

11b. Fruiting pedicels stout, nearly as thick as fruit; seeds not mucilaginous when wetted; fruits often twisted…Neotorularia

10b. Cauline leaves sessile, often auriculate, sagittate, or amplexicaul, if short petiolate then plants canescent; at least some of the trichomes dendritic or stellate with some branched rays.

12a. All branched trichomes sessile; petals (6--)6.5--8(--9) mm; fruit short stipitate…Pseudoarabidopsis

12b. At least some of the branched trichomes distinctly stalked; petals 1.5--4(--5) mm; fruit sessile.

13a. Seeds biseriate; North America…Halimolobos

13b. Seeds uniseriate; Asia…Crucihimalya

9b. Fruit pubescent.

14a. Inflorescence bracteate at least on lower half…Crucihimalya

14b. Inflorescence ebracteate.

15a. Fruit with submalpighiaceous or short-stalked to subsessile stellate trichomes; septum lacking or perforated…Olimarabidopsis

15b. Fruit with other trichome types; septum complete.

16a. Seeds not mucilaginous when wetted, uniseriate; fruit often strongly torulose and/or twisted; stems often decumbent; predominantly Old World…Neotorularia

16b. Seeds mucilaginous when wetted, mostly biseriate; fruit often smooth; stems not decumbent; exclusively New World…Halimolobos

Folia caulina sessilia vel subsessilia, auriculata vel raro nonauriculata; pili ramosi stipitati stellati saepe pilis simplicibus vel furcatis praesentibus; racemi bracteati vel ebracteati, valde elongati; sepala oblonga, nonsaccata; petala alba vel purpurea; fructus lineares, teretes, saepe glabri; stipitum nullum; septum completum; semina uniseriata vel raro biseriata, (15--)20--60(--75) per locula, oblonga, mucilaginosa; cotyledones incumbentes.

Herbs annual or biennial, rarely perennial with a caudex. Trichomes stalked, 1- or 2-forked, often mixed with simple and/or stellate ones, never dendritic. Stems erect to ascending, simple or branched basally and/or apically. Basal leaves rosulate or not, simple, entire or dentate, rarely lyrate or pinnately lobed. Cauline leaves sessile or subsessile, auriculate, sagittate, or rarely cuneate into a petiole-like base, entire, dentate, or rarely pinnately lobed, rarely absent and plants scapose. Inflorescences several- to many-flowered, corymbose racemes, elongated considerably in fruit; rachis straight; bracts present along entire inflorescence, or restricted to lowermost flowers, or absent. Sepals oblong, deciduous, erect, pubescent, base of inner pair not saccate. Petals white, lavender, or purple, spatulate. Stamens 6, slightly tetradynamous; anthers ovate or oblong, sagittate at base, obtuse at apex. Nectar glands confluent and subtending bases of all stamens. Ovules (30--)40--120(--150) per ovary. Fruit dehiscent, linear, terete or somewhat 4-angled to rarely compressed parallel to septum; valves with a distinct midvein, glabrous or rarely stellate hairy or puberulent, smooth or torulose; gynophore absent; septum complete; style to 1 mm long; stigma capitate, entire. Seeds uniseriate or rarely biseriate, (15--)20--60(--75) per locule, wingless, oblong, plump; seed coat minutely reticulate, mucilaginous when wetted; cotyledons incumbent.

Crucihimalya, which means "cross Himalaya," is readily distinguished from Arabidopsis by having at least some stalked stellate trichomes (these sometimes mixed with simple or forked ones), sessile and auriculate to sagittate cauline leaves rarely subsessile into a petiole-like base, and bracteate or ebracteate inflorescences. Arabidopsis always has stalked forked trichomes, petiolate cauline leaves, and ebracteate inflorescences. Four species of Crucihimalya (C. axillaris, C. himalaica, C. lasiocarpa, and C. stricta) have inflorescences bracteate at least on the lower half, and most of the remaining species often have the lowermost flower bracteate. However, this character can be very variable, and the presence of stellate trichomes and/or auriculate to sagittate cauline leaves should help in the separation of such species from Arabidopsis.

Key to the Species of Crucihimalya

1a. Fruit valves densely and coarsely stellate; pedicels pubescent all around; fruits often subappressed to rachis…4. C. lasiocarpa

1b. Fruit valves glabrous or very rarely puberulent; pedicels glabrous adaxially, rarely (C. wallichii) pubescent all around; fruits not appressed to rachis.

2a. Cauline leaves distinctly auriculate or amplexicaul at base.

3a. Inflorescences bracteate at least along the lower portion; annuals or biennials…6. C. himalaica

3b. Inflorescences ebracteate; perennials.

4a. Fruits divaricate; plants biennial; adaxial surface of lower cauline leaves predominantly with forked trichomes…2. C. ovczinnikovii

4b. Fruits erect; plants perennial; adaxial surface of lower cauline leaves predominantly with stalked stellate trichomes…1. C. mollissima

2b. Cauline leaves sessile or subsessile into a petiole-like base, neither auriculate nor amplexicaul, sometimes absent.

5a. Plants scapose; cauline leaves absent or only 1; Mongolia…3. C. mongolica

5b. Plants with well-developed stems and several cauline leaves; Himalaya and southwest Asia.

6a. Lowermost flowers of main inflorescence bracteate; basal leaves entire to dentate, not canescent, withering by flowering or fruiting.

7a. Stem leaves adaxially with simple and forked trichomes, ovate to elliptic or oblong, rarely oblanceolate; plants 4--15(--20) cm tall; main inflorescence bracteate nearly throughout…7. C. axillaris

7b. Stem leaves adaxially with stellate stalked trichomes, linear-lanceolate; plants (18--)30--85(--120) cm tall; only lowermost flowers of main inflorescence bracteate…5. C. stricta

6b. Lowermost flowers of main inflorescence ebracteate; basal leaves lyrate to pinnatifid, often canescent, persistent in flower and fruit.

8a. Petals (2--)2.5--3.5(--4.5) mm long; stem base and petioles of basal leaves often with simple trichomes (0.5--)0.8--1.5(--2) mm long; base of lower cauline leaves usually minutely auricled; plants from Nepal and Tibet east into Iran …8. C. wallichii

8b. Petals 4.5--5.5 mm long; stem base and petioles of basal leaves without simple trichomes; base of lower cauline leaves not auricled; plants of Sinai and western Saudi Arabia…9. C. kneuckeri

Distribution. Afghanistan, China, India, Kashmir, Kazakstan, Mongolia, Pakistan, Russia (Siberia), Tajikistan.

Reports of Crucihimalya mollissima (as Arabidopsis) from Bhutan by Grierson (1984) and from the Chinese provinces Sichuan and Yunnan by An (1987) were based on misidentified plants of C. himalaica. An also reported the species from Gansu and Shaanxi, but we were unable to verify those records.

Schulz (1924) divided the species into eight varieties, of which var. yunnanensis O. E. Schulz is based on an immature plant of a species of Arabis, var. afghana O. E. Schulz is Crucihimalya wallichii, and var. griffithiana (Boissier) O. E. Schulz is Arabis bijuga Watt. The remaining varieties were based on minor continuous differences in the indumentum and leaf margin and, therefore, do not merit recognition. None of Schulz’s eight varieties were recognized in any subsequent taxonomic treatment.

Distribution. Endemic to Tajikistan.

The species is most closely related to and sometimes difficult to separate from Crucihimalya mollissima. The most reliable characters that separate the two species are listed in the key above.

Distribution. Endemic to Mongolia.

Crucihimalya mongolica is most closely related to C. mollissima; it is readily distinguished from this and the remaining species of the genus by the scapose inflorescences and the lack of or presence of a single non-auriculate cauline leaf. The species has incumbent cotyledons, and Botschantsev’s (1975) original placement of the species in Arabis is erroneous because this genus always has accumbent cotyledons. M� s¥ … ek and Soj< k (1995) were correct in associating the species with C. mollissima.

Distribution. Bhutan, China, India, Nepal.

Balakrishnan (1970) proposed the name Sisymbrium bhutanicum to replace S. lasiocarpum J. D. Hooker & Thomson because he erroneously believed that S. lasiocarpum F. Mueller was the earlier homonym.

Schulz’s (1927) original description of Microsisymbrium duthiei matches that of C. lasiocarpa in every morphological detail, and it appears that Schulz (1924) never examined any material of C. lasiocarpa, a species he placed in Arabidopsis. Schulz (1924, 1936) depended solely on the presence vs. absence of seed mucilage to separate Microsisymbrium from Arabidopsis, and it is likely that he did not observe seed mucilage in the material he described as M. duthiei.

Distribution. China, India, Kashmir, Nepal, Pakistan.

An’s (1987) report of Crucihimalya stricta (as Arabidopsis stricta) from Sichuan is based on misidentification of plants of C. himalaica. The reports in Schulz (1924), Jafri (1973), and Hajra et al. (1993) of the species from Afghanistan were based on collections (Aitchinson 210 and 251, BM and K) from Pakistan. A duplicate specimen at K of the Jacquemont collection from India (no locality was given, but it bears the numbers 2188 and "(786)") is definitely C. himalaica because it has large-auricled leaves and bracts. It is not part of the type collection of C. stricta, a species that never has auriculate leaves or bracts.

Crucihimalya stricta was not reported from Nepal (Hara, 1979), though Schulz (1927) cited one collection (Duthie 5352) from Budhi village in western Nepal. We have examined another collection, Stainton, Sykes & Williams 3365 (BM, E, G), that was collected from Jagat and misidentified as Arabidopsis mollissima.

Distribution. Afghanistan, Bhutan, China, India, Kashmir, Nepal, Pakistan, Sikkim.

Arabis brevicaulis was transferred by Jafri (1973) to Arabidopsis. He admitted that the species could conceivably be placed in A. mollissima. The specimens cited by Jafri under A. mollissima clearly represent a mixture of that species and A. himalaica. One of the specimens, Bowes Lyon 849 (BM), was annotated by Jafri as the holotype of Arabidopsis chitralica Jafri, a name that was never published. The specimen was cited by Jafri (1973) as A. mollissima, and it is most likely the same as the type of Arabis brevicaulis.

On the basis of cited specimens, both Arabidopsis himalaica and A. mollissima were confused by Schulz (1924), Hedge (in Hedge & Rechinger, 1968), and Jafri (1973). The principal character used by these authors to separate these two species is the presence in A. himalaica of bracts along the entire length of the inflorescences and the lack in A. mollissima of bracts or their restriction to the lowermost few flowers of the inflorescence. However, this character shows tremendous variability in the same population or even on the same plant. In most plants of A. himalaica the uppermost portion of the inflorescence is ebracteate. For example, in Lowndes 1166 (BM) one plant has the lowermost 24 flowers bracteate and another has only the lowermost 4 bracteate. In Stewart 17986 (US) one plant has almost ebracteate inflorescences and another has fully bracteate inflorescences. Plants of Crucihimalya himalaica are annual or biennial with coarse stellate and forked trichomes, at least basally bracteate inflorescences, slender fruits (0.4--)0.5--0.8(--1) mm wide, and seeds 0.5--0.8 mm long. By contrast, plants of C. mollissima are perennial with soft stellate trichomes, ebracteate inflorescences, fruits 1--1.5 mm wide, and seeds 0.8--1.1 mm long.

Plants from Chitral (Pakistan) treated by Jafri (1956, 1973) as Arabis brevicaulis and Arabidopsis brevicaulis, respectively, differ from typical C. himalaica in being perennials with only the basal one or two flowers bracteate, but they are perfectly at home in the species in trichome type, fruit width, and seed length. They might be recognized at the subspecific rank, but more material is needed for a sound conclusion to be reached.

Distribution. Bhutan, China, India, Kashmir, Nepal, Sikkim.

Schulz (1924) recognized plants with puberulent fruits as variety dasycarpum, but both glabrous and puberulent fruits can be found in the same population. Robust plants of this species were described by Jafri (1956) as Microsisymbrium bracteosum, while those with short pedicels and ebracteate uppermost flowers were described as M. axillare var. brevipedicellatum. However, these characters show continuous variation and, therefore, these two taxa do not merit recognition.

Distribution. Afghanistan, Bhutan, China, India, Iran, Kashmir, Kazakstan, Kyrgyzstan, Nepal, Pakistan, Tajikistan, Turkmenistan, Uzbekistan.

Crucihimalya wallichii is the most variable species in the genus, especially in the density of indumentum, division of basal and cauline leaves, length of fruiting pedicels, and length and degree of compression of fruit. The continuous variation in these characters has been adequately described by Hedge (in Hedge & Rechinger, 1968), and it is evident that no infraspecific taxa can be recognized. However, Jafri (1973) segregated two additional species, as Arabidopsis taraxacifolia (T. Anderson) Jafri and A. russelliana Jafri, based primarily on differences in the degrees of stem branching, style length, and petal size. In our opinion, these highly variable characters are unreliable and, therefore, these segregates do not merit recognition at any rank.

Crucihimalya wallichii is closely related to C. kneuckeri, and the two species can be separated only by the characters in the key above.

Schulz (1927), in his original description of Microsisymbrium flaccidum O. E. Schulz, cited two syntypes, Duthie11055 and Inayat 19172, both of which are deposited at DD, with photos and fragments at B. The former collection was designated by Jafri (1973) as the lectotype, and there is an isolectotype at K. This is definitely an immature plant of a species of Arabis. The second collection, Inayat 19172, is a glabrescent form of Crucihimalya wallichii with slightly lyrate leaves and incumbent cotyledons.

Distribution. Saudi Arabia, Egypt (Sinai).

Schulz (1924) mixed the flower size of Crucihimalya kneuckeri with that of C. wallichii. The latter has flowers as small as 2 mm long. More material of C. kneuckeri is needed to fully assess its overall variation.

Folia caulina sessilia, minute vel valde auriculata; pili ramosi sessiles vel minute stipitati malpighiacei vel stellati, ramis simplicibus; racemi ebracteati, valde elongati; sepala oblonga, nonsaccata; petala flava; fructus lineares, teretes, saepe stellati vel malpighiacei; stipitum nullum; septum nullum vel perforatum vel completum; semina uniseriata, 9--30 per locula, oblonga, mucilaginosa; cotyledones incumbentes.

Herbs annual. Trichomes short-stalked or sessile, malpighiaceous and 3- or 4-rayed stellate. Stems erect to ascending, sometimes decumbent, simple or branched basally and/or apically. Basal leaves not rosulate, simple, entire or very rarely pinnately dissected. Cauline leaves sessile, minutely to conspicuously auriculate, entire to dentate. Inflorescences few- to several-flowered, ebracteate, corymbose racemes, elongated considerably or rarely slightly elongated in fruit; rachis straight. Sepals oblong, deciduous, erect, glabrous or pubescent, base of inner pair not saccate. Petals yellow or yellowish white, oblanceolate. Stamens 4 or 6 and only slightly tetradynamous; anthers oblong, rounded at base, obtuse at apex. Nectar glands confluent and subtending bases of all stamens. Ovules 18--60 per ovary. Fruit dehiscent, linear, terete; valves with a distinct midvein, pubescent with exclusively malpighiaceous and/or short-stalked stellate trichomes; gynophore absent; septum complete, perforated, or reduced to a rim; style obsolete or distinct and to 1 mm long; stigma capitate, entire. Seeds uniseriate, wingless, oblong, plump; seed coat minutely reticulate, slightly mucilaginous or not mucilaginous when wetted; cotyledons incumbent.

As delimited here, Olimarabidopsis, which means "formerly Arabidopsis," consists of three species all of which were previously placed in Arabidopsis. One of those species was described twice under two genera (as Trichochiton umbrosum Botschantsev & Vvedensky and 20 years later (see below) as Arabidopsis eseptata Hedge). The two questions most relevant to Olimarabidopsis are: First, is Trichochiton distinct from the closely related and earlier published Cryptospora Karelin & Kirilow? Second, is T. umbrosum correctly assigned to this genus? All species of Cryptospora have 2-lobed stigmas, large seeds 2.5--4.5 mm long, indehiscent fruits that break up at maturity into 1-seeded segments, some simple trichomes on the leaves and/or stems, white to lavender flowers, and non-auriculate cauline leaves (Botschantsev, 1963). The type species of Trichochiton, T. inconspicuum Komarov, has all of these characters. Therefore, we fully agree with Schulz (1936) and Rechinger (in Hedge & Rechinger, 1968) in reducing Trichochiton to synonymy of Cryptospora and in recognizing its type as C. inconspicua (Komarov) O. E. Schulz.

The answer to the second question is no. Trichochiton umbrosum has entire stigmas, small seeds to 1.3 mm long, readily dehiscent fruits, exclusively stellate and malpighiaceous (never simple) trichomes, pale yellow flowers, and minutely auriculate cauline leaves. In our opinion, these differences are substantial, and T. umbrosum (= Arabidopsis eseptata) is herein assigned to a new genus, Olimarabidopsis, of three very closely related species.

Olimarabidopsis is readily distinguished from Arabidopsis by having yellow flowers, auriculate cauline leaves, and pubescent fruits. In contrast, Arabidopsis has white to lavender flowers, short petiolate stem leaves, and glabrous fruits. Olimarabidopsis is distinguished from small-flowered species of the closely related genus Erysimum by its auriculate cauline leaves and short-stalked branched trichomes. All species of Erysimum have petiolate cauline leaves and malpighiaceous and/or sessile 3--5-rayed stellate trichomes.

Key to the Species of Olimarabidopsis

1a. Trichomes on fruit valve exclusively submalpighiaceous; seeds 1--1.3 mm long; fruit attenuate to apex; style obsolete; stamens 4 or rarely 6…3. O. umbrosa

1b. At least some of the fruit trichomes 3- or 4-rayed; seeds less than 1 mm long; fruit cuneate to apex; style distinct; stamens 6.

2a. Septum perforate; fruit 0.4--1(--1.5) cm long, 5--18(--20)-seeded…2. O. cabulica

2b. Septum complete; fruit 1.5--3.2(--4) cm, (15--)22--40(--60)-seeded…1. O. pumila

Distribution. Afghanistan, Armenia, Azerbaijan, China, Georgia, India, Iran, Iraq, Israel, Jordan, Kazakstan, Kyrgyzstan, Lebanon, Oman, Pakistan, Russia, Syria, Tajikistan, Turkmenistan, Turkey, Uzbekistan.

Sisymbrium griffithianum Boissier was transferred to Microsisymbrium by Schulz (1924). An examination of the type and numerous other collections clearly shows that the species is a minor variant of Olimarabidopsis pumila with strongly reflexed instead of divaricate to ascending fruits. In fact, plants with reflexed and divaricate to ascending fruits are commonly found in the same population, as evidenced by Botschantsev 111 (LE) from Uzbekistan, Hedge & Wendelbo 2901 (E) from Afghanistan, and Lammond 1026 (E, LE) from Pakistan. It is surprising, therefore, to have the same species cited by Schulz (1924) under two different genera, as M. griffithianum and Arabidopsis pumila. We agree with Hedge (in Hedge & Rechinger, 1968) in reducing S. griffithianum to synonymy of Olimarabidopsis pumila (as A. pumila).

Distribution. Afghanistan, W China, Kyrgyzstan, Tajikistan.

The type and other collections annotated by Botschantsev as Arabidopsis korshinskyi are indistinguishable from the type of Sisymbrium cabulicum. The latter was reduced by Jafri (1973) to synonymy of Arabidopsis pumila. However, the perforate septum, shorter fruits, and fewer seeds per locule (see the key above) readily distinguish plants of the two species herein placed in Olimarabidopsis. Furthermore, O. cabulica (reported as A. korshinskyi) is a hexaploid (2n = 48), whereas O. pumila is tetraploid (2n = 32) (Aryavand, 1983; Ginter & Ivanov, 1968; Polatschek, 1971).

An (1987) reported the species (as Arabidopsis pumila (Stephan) N. Busch var. alpina (Korshinsky) O. E. Schulz) from Xinjiang, but we have not seen any material from China.

Distribution. Afghanistan, Tajikistan, Uzbekistan.

Arabidopsis eseptata Hedge was correctly reduced to synonymy of Trichochiton umbrosum by Pachomova (1974) and Junussov (1978), but Pachomova erroneously cited the authorship of the species by considering it as a new combination based on Komarov’s (1896) T. inconspicuum var. umbrosum Komarov. Although they selected the same epithet for the species, Botschantsev and Vvedensky (1948) treated T. umbrosum as a new species and cited a holotype collected 22 years after Komarov’s publication.

Folia caulina sessilia, sagittato-amplexicaulia vel raro auriculata; pili ramosi sessiles stellati, ramis ramosis; racemi ebracteati, valde elongati; sepala oblonga, subsaccata; petala alba vel lavandula; fructus lineares, teretes, glabri; stipitum distinctum; septum completum; semina biseriata, 30--50 per locula, oblonga, mucilaginosa; cotyledones incumbentes.

Herbs biennial or perennial. Trichomes sessile, stellate, 4- or 5-rayed, with at least some rays branched. Stems erect to ascending, simple or branched basally. Basal leaves rosulate, often withered by fruiting time, simple, entire or dentate. Cauline leaves sessile, deeply sagittate-amplexicaul or rarely auriculate, entire to dentate. Inflorescences several-flowered, ebracteate, corymbose racemes, elongated considerably in fruit; rachis straight. Sepals oblong, deciduous, erect, glabrous or pubescent, base of inner slightly saccate. Petals white to lavender, spatulate. Stamens 6, prominently tetradynamous; anthers oblong, sagittate at base, obtuse at apex. Nectar glands confluent, subtending bases of all stamens, surrounding those of lateral ones. Ovules 60--100 per ovary. Fruit dehiscent, linear, terete; valves with obscure midvein, glabrous; gynophore distinct; septum complete; style distinct and to 1 mm long; stigma capitate, entire or slightly 2-lobed. Seeds biseriate, 30--50 per locule, wingless, oblong to ovoid, plump; seed coat minutely reticulate, slightly mucilaginous when wetted; cotyledons incumbent.

Pseudoarabidopsis is readily distinguished from Arabidopsis and Erysimum by having sessile, sagittate-amplexicaul cauline leaves, exclusively sessile, stellate trichomes with branched rays, and a distinct gynophore. In Arabidopsis the leaves are petiolate and never amplexicaul or sagittate, the trichomes are simple mixed with stalked forked ones, and the fruits are sessile or subsessile. In its stellate trichomes and sagittate-amplexicaul cauline leaves, P. toxophylla resembles Capsella bursa-pastoris (L.) Medikus. Although drastically different in fruit morphology, these two species show remarkable affinities on the basis of ITS results.

Distribution. Afghanistan, western China, Kazakstan, Russia, Tajikistan.

Generic Placement of Species Previously Included in Arabidopsis

The following 59 binomials previously assigned to Arabidopsis are assigned to 14 genera, including Arabidopsis. Accepted taxa are in boldface, and synonyms, excluded, or doubtful taxa are in italics. Binomials marked with (*) are proposed in this paper.

Acknowledgments. Research and fieldwork were supported by the National Science Foundation (grant DEB-9208433) and the National Geographic Society (grant 5068-93). The following people were gracious hosts or invaluable field companions: Abdulla Abbas, An Zheng-xi, Katsutoshi Arai, Nogman Aralbaev, Isa O. Baitulin, Alexandra Berkutenko, Deng Yanbin, Fu Dezhi, Gheorghe Dihoru, Vladimir Dorofeev, Yang Guang, Qin Haining, Sun Hang, Josef Holub, Peng Hua, Dilxat Kasm, H. Kato, the late Sigizmund Kharkevich, Franta Krahulec, Zbigniew Mirek, Hanna Kuciel, Lou Lianli, Karol Marhold, Klaus Mummenhoff, Xia Nianhe, Noriaki Murakami, Zvetlana Nesterova, Hideaki Ohba, Nonna Pavlova, Ching-I Peng, Nina Probatova, Boris Syomkin, Hirokazu Tsukaya, and Guilin Zhu. Barbara Schaal graciously made her laboratory available for many of the molecular studies leading to this paper. We are grateful to Henk van der Werff for correcting the Latin, Zhu Guanghua for help with Chinese literature, Tatiana Shulkina for translating some Russian texts, and Michael G. Gilbert and Anthony R. Brach for providing much-needed literature. We thank the directors and curators of the following herbaria (abbreviations follow Holmgren et al. (1990) and Fu et al. (1993)) for the loans and/or for allowing the study of their holdings: A, AA, B, BH, BKL, BM, CAS, CDBI, COLO, E, F, G, GH, HAST, HNWP, IBSC, K, KATH, KUN, LE, M, MHA, MO, MW, NAS, NY, NYS, P, PE, S, SHI, SZ, TAI, TI, TNS, UC, US, W, WU, WUK, XJA, XJBI, XJU, and Z.

Literature Cited