Phylogenomic analysis of target enrichment and transcriptome data uncovers rapid radiation and extensive hybridization in the slipper orchid genus Cypripedium.

Background And Aims: Cypripedium is the most widespread and morphologically diverse genus of slipper orchids. Despite several published phylogenies, the topology and monophyly of its infrageneric taxa remained uncertain. Here, we aimed to reconstruct a robust section-level phylogeny of Cypripedium and explore its evolutionary history using target capture data for the first time.

Methods: We used the orchid-specific bait set Orchidaceae963 in combination with transcriptomic data to reconstruct the phylogeny of Cypripedium based on 913 nuclear loci, covering all 13 sections. Subsequently, we investigated discordance among nuclear and chloroplast trees, estimated divergence times and ancestral ranges, searched for anomaly zones, polytomies and diversification rate shifts, and identified potential gene (genome) duplication and hybridization events.

Key Results: All sections were recovered as monophyletic, contrary to the two subsections within sect. Cypripedium. The two subclades within this section did not correspond to its subsections but matched the geographical distribution of their species. Additionally, we discovered high levels of discordance in the short backbone branches of the genus and within sect. Cypripedium, which can be attributed to hybridization events detected based on phylogenetic network analyses, and incomplete lineage sorting caused by rapid radiation. Our biogeographical analysis suggested a Neotropical origin of the genus during the Oligocene (~30 Ma), with a lineage of potentially hybrid origin spreading to the Old World in the Early Miocene (~22 Ma). The rapid radiation at the backbone probably occurred in Southeast Asia around the Middle Miocene Climatic Transition (~15-13 Ma), followed by several independent dispersals back to the New World. Moreover, the glacial cycles of the Pliocene-Quaternary may have contributed to further speciation and reticulate evolution within Cypripedium.

Conclusions: Our study provides novel insights into the evolutionary history of Cypripedium based on high-throughput molecular data, shedding light on the dynamics of its distribution and diversity patterns from its origin to the present.