Boechera, commonly called rockcress, is a genus of flowering plants in the mustard family (Brassicaceae), order Brassicales. It comprises roughly 100–142 accepted species, depending on the taxonomic authority, and is primarily a North American genus. The centre of diversity lies in the western United States, though the range extends northward into Greenland and eastward into the Russian Far East.
Members of Boechera were long classified under the closely related genus Arabis, but molecular and cytological studies revealed sufficient distinctions to justify separation. The most diagnostic cytological difference is the base chromosome number: Boechera consistently shows x=7, whereas Arabis carries x=8. Boechera is also closely allied to Arabidopsis, the genus that includes the model plant Arabidopsis thaliana, and the two share a number of genomic features that make Boechera a growing subject of evolutionary research.
Morphologically, most species are perennial herbs with leaves covered in stellate (star-shaped) trichomes, slender to narrow curved siliques (fruits), and small flowers ranging from white to purple, arranged in elongated racemes. Many taxa within the genus are triploid, and the genus is notable for widespread apomixis — asexual seed reproduction without fertilization. Microsatellite studies have shown that many of these apomictic lineages originated as hybrids between two or more sexual progenitor species, making Boechera an unusually rich system for studying the evolution of asexuality in plants.
Species delimitation within the genus is challenging: many species are morphologically cryptic and difficult to distinguish in the field, although some clearly distinct taxa are recognised. Relationships among eastern North American species, including Boechera laevigata, may represent a separate clade from the predominantly western core of the genus.
The genus was named in honour of the Danish botanist Tyge W. Böcher (1909–1983), a specialist in arctic and alpine plants.
Etymology
The genus name Boechera honours the Danish botanist Tyge W. Böcher (1909–1983), who was known for his research in alpine plants, including the mustard genera Draba and Boechera holboellii. The common name "rockcress" reflects the typical rocky, montane habitats many species favour.
Distribution
Boechera is primarily a North American genus, with the greatest species richness in the western United States. Its range extends northward to Greenland and westward to the Russian Far East. The genus is most commonly encountered in montane, subalpine, and arid rocky habitats across the American West.
Ecology
Several Boechera species are important host plants for the caterpillars of the butterfly Pieris oleracea (the mustard white), which rely on members of this genus as a primary food source. The genus is also ecologically notable for widespread apomixis, enabling many lineages to colonise and persist in habitats where sexual reproduction may be constrained.
Taxonomy Notes
Boechera was segregated from Arabis on the basis of molecular phylogenetic and cytological evidence. The key cytological marker is a base chromosome number of x=7 (versus x=8 in Arabis). The genus is closely related to Arabidopsis and sits within the tribe Boechereae of Brassicaceae. Species delimitation within Boechera is notoriously difficult due to morphological cryptism and the prevalence of apomictic hybrid lineages; the accepted species count varies among authorities (roughly 100–142). Some eastern North American species, including B. laevigata, may belong to a clade distinct from the predominantly western species.
