Abstract
Boechera is a model genus that is of particular interest for understanding apomixis due to the presence of numerous apomictic diploid lineages that are tightly correlated with hybridisation events. Boechera includes many narrowly distributed endemics and apomictic hybrid lineages that obscure morphological boundaries amongst taxa. In this study, we focus on the Boechera suffrutescens complex, a phylogenetically well-supported but taxonomically complex north-western United States clade whose diploid species currently include the widespread B. suffrutescens and two narrowly distributed serpentine endemics, B. constancei and B. rollei. Using a 15-locus microsatellite dataset, we infer ploidy and sexual vs. apomictic reproduction for all individuals and then assess species limits for all sexual diploid samples. Our results support the recognition of B. rollei and B. constancei as distinct species and reveal three divergent sexual diploid lineages within B. suffrutescens sensu lato. The latter three lineages exhibit geographic, genetic and morphological coherence and consequently warrant recognition at the species rank. These include Boechera suffrutescens s.s., which is restricted to Idaho and eastern Oregon, Boechera botulifructa, a newly described species distributed along the Cascade Mountain Province from Lassen County, California north to Deschutes County, Oregon and the heretofore dismissed species Boechera duriuscula (basionym ≡ Arabis duriuscula), which occurs along the Sierra Nevada Province from Plumas County southwards to Fresno County, California. Our data also reveal substructure in B. constancei that is likely attributable to the highly fragmented distribution of its serpentine habitat. This refined taxonomic framework for the B. suffrutescens complex enhances Boechera as a model system, adds to our knowledge of speciation in edaphically extreme environments and provides information on ongoing conservation efforts for these taxa.
Highlights
The genus Boechera Á.Löve & D.Löve was first recognised in 1976, but it was not widely accepted as distinct from Arabis L. prior to 2003 (Al-Shehbaz 2003). This largely North American lineage represents a complex assemblage of ±83 sexual diploid (S2X) taxa that have given rise to hundreds of apomictic hybrids, a situation that has confounded morphological classification since the first species were described in the 1820s (Li et al 2017)
The confluence of these characteristics has attracted considerable attention and Boechera has become a focal point for studies of biogeography, speciation, adaptation, apomixis and ecological genomics
The stepwise process employed to first parse diploids from polyploids and S2X from A2X, identified 238 S2X individuals out of a total sample of 498 B. suffrutescens complex samples
Summary
The genus Boechera Á.Löve & D.Löve was first recognised in 1976, but it was not widely accepted as distinct from Arabis L. prior to 2003 (Al-Shehbaz 2003) This largely North American lineage represents a complex assemblage of ±83 sexual diploid (S2X) taxa that have given rise to hundreds of apomictic hybrids, a situation that has confounded morphological classification since the first species were described in the 1820s (Li et al 2017). Through a combination of molecular phylogenetic and population genetic studies, has genuine progress been made towards a coherent Boechera classification This has involved a modified “diploids first” approach (Brown et al 2002), acknowledging that it is nearly impossible to identify and study apomictic hybrids without an in-depth understanding of the sexual diploid species that gave rise to them. We apply this method to another poorly known group, the B. suffrutescens complex
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