Boechera sierraensis: a newly discovered apomictic species from the Sierra Nevada with exceptional stress tolerance

Abstract This study introduces a new species, Boechera sierraensis , recently discovered in the Sierra Nevada of California. Previous research has shown that this taxon possesses exceptional tolerance to environmental stressors, including heat, drought, and high light intensity, which make it a valuable research subject in the face of accelerating climate change. However, species delimitation within the genus Boechera is notoriously difficult due to extensive morphological convergence and complex evolutionary dynamics. While polyploidy has often been cited as a major driver of speciation in the genus, repeated interspecific hybridization coupled with rampant apomictic reproduction also plays major roles in the process. These factors contribute to blurred species boundaries and underscore the need for careful taxonomic evaluation. We conducted cytogenetic, pollen, microsatellite DNA, and morphological analyses to investigate the ploidy, genetic makeup, and evolutionary origin of this Sierran endemic originally identified as Arabis lemmonii var. depauperata . Specimens were compared to 5014 samples from the Boechera Microsatellite Website. Chromosome spreads were prepared and 15 microsatellite loci studied. Morphological and geographical data were analyzed in light of the microsatellite results. Our analyses revealed these plants to be apomictic triploids containing three divergent subgenomes derived from B. lemmonii , B. paupercula , and B. retrofracta . The presence of this trihybrid species has since been confirmed at multiple locations in the Sierra Nevada, and morphological analyses allow separation from closely related taxa. The identification key and description provided here will hopefully facilitate further research into the broader ecological and evolutionary significance of Boechera sierraensis and related species.