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FNA | Family List | FNA Vol. 7 | Brassicaceae

11. Boechera Á. Löve & D. Löve, Bot. Not. 128: 513. 1976.

[For Tyge Wittrock Böcher, 1909-1983, Danish cytogeneticist who worked on subarctic flowering plants] [For Tyge Wittrock Böcher, 1909-1983, Danish cytogeneticist who worked on subarctic flowering plants]

Ihsan A. Al-Shehbaz, Michael D. Windham

Perennials or, rarely, biennials; (sexual or apomictic, caudex usually present, rarely absent); not scapose; usually glabrous or pubescent, rarely hirsute or hispid, trichomes simple or branched, 2-14-rayed, sometimes dendritic, not stellate. Stems erect, ascending, or decumbent, unbranched or branched distally. Leaves basal and cauline; petiolate or sessile; basal rosulate or not, petiolate, blade margins usually entire or dentate, rarely lyrate-pinnatifid; cauline usually sessile, rarely shortly petiolate, blade (base sometimes auriculate or sagittate), margins entire or dentate. Racemes (sometimes paniculate), often elongated in fruit. Fruiting pedicels erect, ascending, divaricate, or reflexed (secund or not), slender. Flowers: sepals ovate or oblong, (lateral pair slightly saccate or not basally, margins membranous); petals usually white, pink, lavender, or purple, rarely yellowish, red, or magenta, spatulate or oblanceolate, (claw shorter than sepals or undifferentiated from blade, apex obtuse); stamens tetradynamous; filaments not dilated basally; anthers ovate or oblong, (apex obtuse), [pollen ellipsoid (sexual plants) or spheroid (apomictic)]; nectar glands confluent, subtending bases of stamens, lateral glands semi-annular or annular. Fruits usually sessile, rarely shortly stipitate, usually linear, rarely oblong or lanceolate, straight or falcate, smooth or torulose; valves (papery), each with obscure or prominent midvein, usually glabrous, rarely pubescent; replum (visible), rounded; septum complete, (membranous, veinless); ovules 8-250 per ovary; (style sometimes obsolete); stigma capitate. Seeds usually uniseriate or sub-biseriate, rarely biseriate, flattened, winged, not winged, or margined, oblong or orbicular; seed coat (usually smooth or minutely reticulate, rarely papillate), not mucilaginous when wetted; cotyledons accumbent.

Species 111+ (109 in the flora): North America, n Mexico, e Asia (Russian Far East).

Boechera falcata (Turczaninow) Al-Shehbaz is known from eastern Asia (Russian Far East).

Boechera often is treated as a synonym of Arabis (e.g., R. C. Rollins 1993; S. L. Welsh et al. 2003) but it has become clear that morphological similarities between these groups are due to evolutionary convergence, not shared ancestry. Molecular analyses by M. Koch et al. (2001) and T. Mitchell-Olds et al. (2005) revealed that Arabis and Boechera belong to distantly related clades of Brassicaceae that diverged some 19-25 million years ago. A new tribal classification of the family (I. A. Al-Shehbaz et al. 2006) places them in different tribes (Arabideae and Boechereae, respectively), reflecting their substantial molecular divergence.

The taxonomic complexity of Arabis, in the broad sense, is legendary (R. C. Rollins 1941, 1993; G. A. Mulligan 1996). When the genus is split, most of the problematic taxa come to reside in Boechera. A rare confluence of hybridization, apomixis, and polyploidy makes this one of the most difficult genera in the North American flora. The sexual diploid species are relatively distinct from one another, but they hybridize wherever they come into contact. Through apomixis and polyploidy, the hybrids become stable, self-propagating lineages. Most of the hybrid derivatives in Boechera are triploids, but apomictic diploids are known as well. Thus, for any pair of sexual diploid species (e.g., AA and BB), this process can yield different intermediates, including AB apomicts and both possible apomictic triploids (AAB and ABB). The situation becomes even more challenging when a third sexual diploid enters the picture. To date, we have identified three taxa (B. divaricarpa, B. pinetorum, B. tularensis) that appear to be trigenomic triploids. Under these circumstances, even the most distinctive sexual diploid progenitors can become lost in a seemingly continuous range of morphological variability.

In a genus characterized by the presence of polyploids and apomicts, it is essential to know which taxa represent the products of primary, divergent evolution (i.e., sexual diploids) and which are the result of secondary, reticulate evolution. Fortunately, a strong correlation between pollen morphology and ploidy level/reproductive mode facilitates the separation of sexual diploids from polyploids and apomicts in Boechera. Because of differences in meiosis, sexual diploids produce small (13-16 µm diam.), ellipsoid pollen grains with symmetrical colpi. In apomictic individuals, the pollen grains are significantly larger (20-30 µm diam.), and spheroid with asymmetrical colpi. The differences in pollen size and shape are so pronounced that the ploidy level and reproductive mode of most plants with flowers can be determined using a medium power (40×) dissecting microscope (see Fig. 1 in M. D. Windham and I. A. Al-Shehbaz 2006).

To facilitate the study of ploidy level and reproductive mode in Boechera and to allow direct comparison of taxa named by previous authors, we assembled at the Missouri Botanical Garden the holotypes and isotypes of all taxa originally described in Arabis and currently placed in Boechera (over 160 published basionyms). In addition to the types, another 12,000 specimens were examined to document morphological variability and geographic distribution. During this process, we identified additional morphological features (e.g., trichome branching patterns, number of ovules per ovary, pollen and seed morphology) overlooked or underused by previous authors. The result is a substantially revised taxonomy for the genus, the nomenclatural foundation for which was established in a series of papers (M. D. Windham and I. A. Al-Shehbaz 2006, 2007, 2007b).

In many cases, the species circumscriptions adopted here deviate significantly from those of previous authors. Where R. C. Rollins (1993) accepted 63 species with varieties, we recognize 109 species and two with two subspecies. Our treatment includes a total of 71 sexual species. These represent the morphological extremes of the complex and are often easily distinguished (when separated from the apomicts using pollen characters). Although it is likely that some diploid species remain to be discovered, we feel that this portion of the treatment is relatively complete.

Our coverage of the apomictic hybrids is much less comprehensive. There are literally hundreds of hybrids in Boechera with unique genomic combinations, each of which could be recognized at species level. Our treatment includes just 38 apomictic species, primarily taxa recognized at some level by other authors. Because some hybrid combinations are not formally recognized, it is inevitable that some names will be misapplied to superficially similar hybrids of different parentage. For example, plants of B. goodrichii (= B. retrofracta × B. gracilipes), B. consanguinea (= B. retrofracta × B. fendleri), and B. pinetorum (= B. retrofracta × B. rectissima × B. sparsiflora) are sufficiently similar that they might be considered a single taxon if their respective parentages and disjunct geographic ranges were not taken into account. Indeed, all three have been called Arabis holboellii var. pinetorum (e.g., R. C. Rollins 1993; S. L. Welsh et al. 2003), although the epithet consanguinea has priority.

The best way to avoid such misidentification is to pay close attention to the geographic distribution of apomictic taxa and their sexual progenitors. Apomictic hybrids in Boechera appear to be of relatively recent origin and generally have not migrated beyond regions where their parents are sympatric. Thus, users of this treatment should be wary of major range extensions for apomictic taxa; in most cases, these will turn out to be unique hybrid combinations not represented in the keys or descriptions. Because the use of hybrid binomials is potentially misleading, the best approach to identifying a hybrid is to provide a formula name based on the hypothesized parentage (e.g., B. fendleri × B. stricta or B. fendleri hybrid). This requires an accurate understanding of the sexual diploids occurring in the region of interest, which we hope the following keys and descriptions will provide.

Given the inherent taxonomic complexity of Boechera, it has been necessary to incorporate micromorphological characters such as pollen morphology and trichome branching patterns in the identification keys. Whenever possible, we have restricted such characters to later couplets, but microscopic observations are required to distinguish some species. Effective use of the keys also depends on having complete specimens bearing both flowers and fruits. In all cases, measurements of stem length are taken from fruiting plants, those of basal leaves from the largest in the basal rosette, for the fruiting pedicels from the longest in the infructescence, and for the stem trichomes from the largest near the base. Descriptions of the pedicels, flowers, and fruits are taken from the main inflorescence rather its lateral branches, the number of seed rows per locule is determined near the middle of the fruit, and the number of ovules is observed in mature fruits by counting the number of seeds plus the abortive ovules.

SELECTED REFERENCES Al-Shehbaz, I. A. 2003b. Transfer of most North American species of Arabis to Boechera (Brassicaceae). Novon 13: 381-391. Windham, M. D. and I. A. Al-Shehbaz. 2006. New and noteworthy species of Boechera (Brassicaceae) I: Sexual diploids. Harvard Pap. Bot. 11: 61-88. Windham, M. D. and I. A. Al-Shehbaz. 2007. New and noteworthy species of Boechera (Brassicaceae) II: Apomictic hybrids. Harvard Pap. Bot. 11: 257-274. Windham, M. D. and I. A. Al-Shehbaz. 2007b. New and noteworthy species of Boechera (Brassicaceae) III: Additional sexual diploids and apomictic hybrids. Harvard Pap. Bot. 12: 235-257.


Group 4

1 Fruiting pedicels usually abruptly recurved at bases   (2)
+ Fruiting pedicels usually not abruptly recurved at bases (except B. subpinnatifida)   (7)
       
2 (1) Fruits 3-6 mm wide; ovules 20-30 per ovary; racemes 6-12-flowered.   101 Boechera suffrutescens (in part)
+ Fruits 0.9-2.5 mm wide; ovules 46-126 per ovary; racemes 10-140-flowered   (3)
       
3 (2) Stems proximally with branched trichomes only   (4)
+ Stems proximally with simple and branched trichomes, or simple trichomes only   (5)
       
4 (3) Fruits (2-)2.2-2.5 mm wide, descending, not appressed to rachises, strongly secund; Greenland.   44 Boechera holboellii
+ Fruits 0.9-1.5 mm wide, reflexed, usually appressed to rachises, usually not secund, rarely so; not Greenland.   86 Boechera retrofracta (in part)
       
5 (3) Stems proximally with simple trichomes only; petals 3-4 mm; ovules 46-80 per ovary.   84 Boechera rectissima (in part)
+ Stems proximally with simple and branched trichomes; petals 4-6 mm; ovules 70-126 per ovary   (6)
       
6 (5) Basal leaf blade surfaces with 5-8-rayed trichomes; fruits 0.9-1.5 mm wide; seeds 1-1.4 × 0.8-1 mm; plants sexual, pollen ellipsoid.   12 Boechera collinsii
+ Basal leaf blade surfaces with 2-4 (or 5)-rayed trichomes; fruits 1.5-2 mm wide; seeds 1.5-1.8 × 1.2-1.5 mm; plants apomictic, pollen spheroid.   73 Boechera pinetorum (in part)
       
7 (1) Fruiting pedicels sparsely to densely pubescent   (8)
+ Fruiting pedicels usually glabrous, rarely with few isolated trichomes present   (23)
       
8 (7) Stems proximally with simple and branched trichomes, or simple trichomes only   (9)
+ Stems proximally with branched trichomes only   (14)
       
9 (8) Stems proximally with simple trichomes only; fruits 2.5-4 mm wide; seeds with wing 0.4-1 mm wide.   9 Boechera canadensis (in part)
+ Stems proximally with simple and branched trichomes; fruits 0.8-2.5 mm wide; seeds with wing 0.05-0.3 mm wide   (10)
       
10 (9) Petals purple to magenta, 8-12 × 2.5-4 mm; stems elevated above ground surface on woody base; caudices with persistent (peg-like) leaf bases.   49 Boechera koehleri (in part)
+ Petals lavender to white, 4-8 × 0.5-2 mm; stems from near ground surface, without woody base; caudices without persistent leaf bases   (11)
       
11 (10) Fruits 2.5-4 cm; ovules 36-54 per ovary; stems usually 2-6 per caudex branch, arising from margins of rosettes; racemes 7-15-flowered; plants sexual, pollen ellipsoid.   40 Boechera gunnisoniana (in part)
+ Fruits 4-10.5 cm; ovules 56-162 per ovary; stems usually 1 per caudex branch, arising from centers of rosettes; racemes 10-63-flowered; plants apomictic, pollen spheroid   (12)
       
12 (11) Fruits reflexed; fruiting pedicels with few simple and 2-rayed trichomes.   73 Boechera pinetorum (in part)
+ Fruits pendent, horizontal, divaricate, ascending, or descending; fruiting pedicels with many 2- or 3-rayed trichomes   (13)
       
13 (12) Petals 4-5 mm; fruits 0.8-1.2 mm wide; ovules 56-78 per ovary; cauline leaf blades with auricles 0.5-1.5 mm; basal leaf blade surfaces with 3-6-rayed trichomes 0.07-0.3 mm.   57 Boechera macounii (in part)
+ Petals 5-8 mm; fruits 1.5-2.2 mm wide; ovules 80-162 per ovary; cauline leaf blades with auricles (1-)3-10 mm; basal leaf blade surfaces with 2-5-rayed trichomes 0.3-0.6 mm.   66 Boechera pauciflora (in part)
       
14 (8) Stems arising from margins of rosettes, elevated above ground surface on woody bases.   89 Boechera rollinsiorum
+ Stems arising from centers of rosettes, produced near ground surface, usually without woody bases (except B. californica)   (15)
       
15 (14) Ovules 14-44 per ovary (-64 in B. cobrensis); plants sexual, pollen ellipsoid   (16)
+ Ovules (56-)68-180 per ovary; plants apomictic, pollen spheroid   (19)
       
16 (15) Fruits 0.7-0.8 mm wide; seeds 0.7-1.1 × 0.5-0.6 mm, not winged; basal leaf blade surfaces with simple trichomes adaxially.   19 Boechera dentata (in part)
+ Fruits 1.6-3 mm wide; seeds 1.3-3.5 × 1-2.2 mm, winged; basal leaf blade surfaces with (2-)4-10-rayed trichomes adaxially   (17)
       
17 (16) Petals 9-14 mm; basal leaf blade margins (at least some) prominently dentate or subpinnatifid; cauline leaves (10-)20-60.   100 Boechera subpinnatifida (in part)
+ Petals 3.5-8 mm; basal leaf blade margins entire; cauline leaves 2-12(-14)   (18)
       
18 (17) Petals lavender to white, 3.5-6 mm; styles to 0.2 mm; ovules 34-64 per ovary; sandy habitats in California, Idaho, Nevada, Oregon, Wyoming.   11 Boechera cobrensis (in part)
+ Petals purple, 6-8 mm; styles 1.5-2 mm; ovules 20-24 per ovary; serpentine outcrops in nw California.   94 Boechera serpenticola (in part)
       
19 (15) Petals 9-14 mm; fruits (6-)8-12 cm; ovules 140-180 per ovary; s California.   8 Boechera californica (in part)
+ Petals 5-8.5 mm; fruits 3-6.5 cm; ovules 56-134 per ovary; California, Colorado, Nevada, New Mexico, Utah   (20)
       
20 (19) Cauline leaves 5-9, not concealing stem proximally; styles 0.5-1 mm; ovules 62-80 per ovary; seeds uniseriate.   28 Boechera falcifructa
+ Cauline leaves (7-)12-72, often concealing stem proximally; styles 0.05-0.5 mm; ovules 74-136 per ovary; seeds usually biseriate or sub-biseriate (rarely uniseriate in B. inyoensis)   (21)
       
21 (20) Stems proximally with 7-12-rayed trichomes.   47 Boechera inyoensis (in part)
+ Stems proximally with 2-8-rayed trichomes   (22)
       
22 (21) Fruiting pedicels 8-14 mm; stems proximally with 2-6-rayed trichomes; fruits 1-2 mm wide; Four Corners region.   13 Boechera consanguinea (in part)
+ Fruiting pedicels (10-)15-28 mm; stems proximally with 4-8-rayed trichomes; fruits 1.5-2.9 mm wide; e Nevada, w Utah.   36 Boechera goodrichii (in part)
       
23 (7) Seeds biseriate or sub-biseriate   (24)
+ Seeds usually uniseriate   (33)
       
24 (23) Stems proximally with subsessile, submalpighiaceous trichomes; seeds 2-2.5 × 1.1-1.5 mm; (s Sierra Nevada).   104 Boechera tularensis
+ Stems proximally with simple and/or stalked, branched trichomes; seeds 0.7-1.8 × 0.5-1.2 mm   (25)
       
25 (24) Stems proximally with 2-8-rayed trichomes; plants apomictic, pollen spheroid   (26)
+ Stems proximally with simple, spurred, or 2- or 3-rayed trichomes; plants sexual, pollen ellipsoid (except B. languida)   (29)
       
26 (25) Fruiting pedicels divaricate-ascending to horizontal; stems proximally with simple and 2-4-rayed trichomes   (27)
+ Fruiting pedicels reflexed or divaricate-descending; stems proximally with 2-8-rayed trichomes   (28)
       
27 (26) Sepals pubescent; fruiting pedicels divaricate-ascending; fruits 1.7-2 mm wide; ovules 60-96 per ovary; Colorado Plateau and vicinity.   37 Boechera gracilenta (in part)
+ Sepals glabrous; fruiting pedicels horizontal; fruits 1.8-2.5 mm wide; ovules 100-160 per ovary; s New Mexico, w Texas.   77 Boechera porphyrea (in part)
       
28 (26) Fruiting pedicels 8-14 mm; stems proximally with 2-6-rayed trichomes; fruits1-2 mm wide; Four Corners region.   13 Boechera consanguinea (in part)
+ Fruiting pedicels (10-)15-28 mm; stems proximally with 4-8-rayed trichomes; fruits (1.5-)2-3 mm wide; e Nevada, w Utah.   36 Boechera goodrichii (in part)
       
29 (25) Cauline leaves 30-65; fruiting pedicels (15-)20-47 mm; ovules 130-210 per ovary.   38 Boechera gracilipes
+ Cauline leaves 3-25; fruiting pedicels 3-15(-23) mm; ovules 40-128 per ovary   (30)
       
30 (29) Cauline leaves usually concealing stem proximally; ovules 90-128 per ovary   (31)
+ Cauline leaves not concealing stem proximally; ovules 40-70(-90) per ovary   (32)
       
31 (30) Petals 5-9 × 1-2 mm; fruiting pedicels 9-18(-23) mm; Colorado Plateau, s Nevada.   30 Boechera fendleri
+ Petals 3-3.7(-4) × 0.5-0.8 mm; fruiting pedicels 6-10(-15) mm; mountains of c Colorado, nc North Mexico.   98 Boechera spatifolia
       
32 (30) Basal leaf blade surfaces densely pubescent, trichomes simple and 2-4-rayed; seeds sub-biseriate; cauline leaf blades with auricles 0.5-1.5 mm; racemes 10-20-flowered; plants apomictic, pollen spheroid; s Wyoming.   51 Boechera languida
+ Basal leaf blade surfaces sparsely pubescent, trichomes simple and 2-rayed; seeds biseriate; cauline leaf blades with auricles to 0.7 mm; racemes 4-14-flowered; plants sexual, pollen ellipsoid; Arizona, California, Colorado, Nevada, Utah, Wyoming.   69 Boechera pendulina (in part)
       
33 (23) Stems proximally with simple trichomes only.   9 Boechera canadensis (in part)
+ Stems proximally usually with simple and/or branched trichomes (rarely all simple in B. pusilla)   (34)
       
34 (33) Fruits 3-5(-6) mm wide; seeds winged, wings 0.8-1.5 mm wide; stems elevated above ground surface on woody bases.   101 Boechera suffrutescens (in part)
+ Fruits 0.7-3.5 mm wide; seeds not winged, or wings 0.05-1 mm wide; stems usually not from above ground surface on woody bases (except B. perennans and B. lignifera)   (35)
       
35 (34) Fruits reflexed to closely pendent.   73 Boechera pinetorum (in part)
+ Fruits descending to ascending or widely pendent   (36)
       
36 (35) Fruits 0.7-0.8 mm wide; petals 2-3.5 mm; basal leaf blade surfaces with simple trichomes adaxially.   19 Boechera dentata (in part)
+ Fruits 1-3.5 mm wide; petals 3.5-8(-9) mm; basal leaf blade surfaces with branched trichomes adaxially   (37)
       
37 (36) Stems proximally with simple and branched trichomes   (38)
+ Stems proximally with branched trichomes only   (41)
       
38 (37) Stems proximally with sessile or subsessile, 2- or 3(-6)-rayed trichomes.   39 Boechera grahamii (in part)
+ Stems proximally with stalked, 2-5-rayed trichomes   (39)
       
39 (38) Basal leaf blade margins dentate, 3-15(-20) mm wide; fruiting pedicels (6-)10-25 mm; ovules 60-96 per ovary; stems (1.5-)2-7 dm.   71 Boechera perennans (in part)
+ Basal leaf blade margins entire, 1-3(-4) mm wide; fruiting pedicels 2-7 mm; ovules 20-54 per ovary; stems 0.5-2(-2.5) dm   (40)
       
40 (39) Basal leaf blade surfaces densely pubescent, trichomes 3-6-rayed; fruits not secund, 1-1.5 mm wide; ovules 36-54 per ovary; seeds with continuous wing 0.1-0.15 mm wide; plants sexual, pollen ellipsoid; c Colorado.   40 Boechera gunnisoniana (in part)
+ Basal leaf blade surfaces sparsely pubescent, trichomes 2- or 3-rayed; fruits secund, 1.5-2 mm wide; ovules 20-32 per ovary; seeds not winged or with distal wing 0.05-0.1 mm wide; plants apomictic, pollen spheroid; c Wyoming.   81 Boechera pusilla (in part)
       
41 (37) Fruits secund   (42)
+ Fruits not secund   (45)
       
42 (41) Petals white; Quebec.   83 Boechera quebecensis (in part)
+ Petals purple to lavender; not Quebec   (43)
       
43 (42) Stems proximally with sessile and subsessile, 2- or 3-rayed trichomes; petals 6-8 mm; plants usually short-lived perennials, without woody caudices.   23 Boechera drepanoloba (in part)
+ Stems proximally with stalked, 2-6-rayed trichomes; petals 3.5-6 mm; plants long-lived perennials, with woody caudices   (44)
       
44 (43) Cauline leaf blades with auricles 0.5-1.5 mm; petals 1.5-2 mm wide; fruiting pedicels 4-11 mm; fruits 2-3 mm wide; ovules 40-54 per ovary; seeds 2-2.5 × 1.7-2 mm; sc Oregon.   45 Boechera horizontalis (in part)
+ Cauline leaf blades with auricles to 0.5 mm; petals 1-1.5 mm wide; fruiting pedicels 2-6 mm; fruits 1.6-2.3 mm wide; ovules 28-44 per ovary; seeds 1.3-2 × 1-1.5 mm; Alaska, Alberta, British Columbia, California, Colorado, Idaho, Montana, Nevada, Oregon, Utah, Washington, Wyoming, Yukon.   53 Boechera lemmonii (in part)
       
45 (41) Basal leaf blade margins dentate; stems proximally with 2-5-rayed trichomes   (46)
+ Basal leaf blade margins entire; stems proximally with (3 or) 4-10-rayed trichomes   (47)
       
46 (45) Basal leaf blade surfaces with trichomes 0.05-0.2 mm; fruiting pedicels 5-10 mm; fruits 1.5-1.8 mm wide; seeds with distal wing; plants apomictic, pollen spheroid; nw Wyoming.   33 Boechera fructicosa
+ Basal leaf blade surfaces with trichomes 0.2-0.4 mm; fruiting pedicels (6-)10-25 mm; fruits 1.7-2.1 mm wide; seeds with continuous wing; plants sexual, pollen ellipsoid; Arizona, California, Nevada, New Mexico, Texas, Utah.   71 Boechera perennans (in part)
       
47 (45) Stems usually 1 per caudex branch, arising from centers of rosettes; petals 0.7-1 mm wide; seeds with wings 0.25-0.5 mm wide; fruits 1.7-2.5 mm wide.   11 Boechera cobrensis (in part)
+ Stems usually 2-5 per caudex branch, arising laterally proximal to sterile shoots; petals 1-1.5 mm wide; seeds with wings 0.1-0.15 mm wide; fruits 1.2-2 mm wide.   54 Boechera lignifera

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