Microsatellite, Hyb-Seq, and morphological studies confirm the enigmatic mustard Boechera tiehmii as an intergeneric...

The mustard genus Boechera (Brassicaceae) is notorious for rampant intra- and interclade hybridization, which has produced a complex reticulate network of apomictic diploid, triploid, and tetraploid hybrids. A recent hypothesis suggested the possibility of intergeneric hybridization between Boechera and other closely related genera. Here we explored the origin of Boechera tiehmii, a putative hybrid between Boechera and Nevada.

To test the hybrid hypothesis, we leveraged a unique methodological approach that integrates results from 15 microsatellite loci from the Boechera Microsatellite Website (BMW), 1114 phased Hyb-Seq loci from a recently published phylogeny of tribe Boechereae, and a detailed investigation of morphological variation.

Our study supported the hypothesis that B. tiemhii is an intergeneric hybrid between Boechera lemmonii and Nevada holmgrenii. The placement of phased hybrid tips within the sexual diploid phylogeny was unequivocal, the microsatellite loci exhibited near perfect additivity, and B. tiehmii was shown to be morphologically intermediate between the proposed parents.

By affirming the hybrid nature of B. tiehmii, this work contributes to a deeper understanding of the evolutionary processes that shape biodiversity within the tribe Boechereae. It also highlights the importance of applying a broad range of analytical tools when addressing taxonomic ambiguities in groups where hybridization is rampant.