Goldenrod herbariomics: Hybrid-sequence capture reveals the phylogeny of Solidago.

Premise: The phylogenetic relationships among the ca. 138 species of goldenrods (Solidago; Asteraceae) have been difficult to infer due to species richness, and shallow interspecific genetic divergences. This study aims to overcome these obstacles by combining extensive sampling of goldenrod herbarium specimens with the use of a custom Solidago hybrid-sequence capture probe set.

Methods: A set of tissues from herbarium samples comprising ca. 90% of Solidago species was assembled and DNA was extracted. A custom hybrid-sequence capture probe set was designed, and data from 854 nuclear regions were obtained and analyzed from 209 specimens. Maximum likelihood and coalescent approaches were used to estimate the genus phylogeny for 157 diploid samples.

Results: Although DNAs from older specimens were both more fragmented and produced fewer sequencing reads, there was no relationship between specimen age and our ability to obtain sufficient data at the target loci. The Solidago phylogeny was generally well-supported, with 88/155 (57%) nodes receiving ≥95% bootstrap support. Solidago was supported as monophyletic, with Chrysoma pauciflosculosa identified as sister. A clade comprising Solidago ericameriodes, Solidago odora, and Solidago chapmanii was identified as the earliest diverging Solidago lineage. The previously segregated genera Brintonia and Oligoneuron were identified as placed well within Solidago. These and other phylogenetic results were used to establish four subgenera and fifteen sections within the genus.

Conclusions: The combination of expansive herbarium sampling and hybrid-sequence capture data allowed us to quickly and rigorously establish the evolutionary relationships within this difficult, species-rich group.