Deciphering the sexual diploid members of the Boechera suffrutescens complex (Brassicaceae, Boechereae)

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.

A Hyb‐Seq phylogeny of Boechera and related genera using a combination of Angiosperms353 and Brassicaceae‐specific bait sets

“…As circumscribed by Alexander et al (2013), the suffrutescens group included three sexual diploid taxa ( B. constancei , B. rollei , and B. suffrutescens ), with B. constancei and B. suffrutescens supported as closest relatives. Subsequent analyses of microsatellite DNA data by Morin et al (2018) identified three additional sexual diploids, consisting of a genetically divergent population cluster within B. constancei (labeled constancei 2) and two largely allopatric segregates of B. suffrutescens ( B. botulifructa and B. duriuscula ). All six sexual diploids, now attributed to the B. suffrutescens clade, were included in our supermatrix analysis (Figure 3).…”

“…is monophyletic. In these closely related species, this may be the result of ongoing introgression in the central Sierra Nevada, potential evidence of which was noted in the microsatellite analyses of Morin et al (2018). See also Table 1.…”

The Boechera model system for evolutionary ecology

“…Phylogenetic analysis of 58 sexual species successfully identified subclades within Boechera , but relationships among these clades remain unresolved (Alexander et al, 2013). Recent studies have clarified relationships within several different clades, including the Boechera fendleri group (Alexander et al, 2015b), the Boechera suffrutescens group (Morin et al, 2018), and the Boechera puberula group (Schilling et al, 2018). Similar issues—backbone discordance and hybridization—have complicated phylogenetic resolution of other, similarly diverse ecological model systems such as Mimulus (Beardsley et al, 2004; Morales‐Briones et al, 2022) and Aquilegia (Fior et al, 2013) Two additional issues lend further complexity to phylogenetic resolution of Boechera : (1) the prevalence of polyploids (Schranz et al, 2005) and (2) the possibility of de novo evolution of sexual populations from rare recombination in facultatively asexual lineages (Carman et al, 2019), which would result in novel hybrid species like those found in other plant groups (e.g., Rieseberg et al, 2003).…”