Deciphering complex reticulate evolution of Asian Buddleja (Scrophulariaceae): insights into the taxonomy and speciation of polyploid taxa in the Sino-Himalayan region

Phylogenomic analyses revealed widely occurring hybridization events across Elsholtzieae (Lamiaceae)

Obtaining a robust phylogeny proves challenging due to the intricate evolutionary history of species, where processes such as hybridization and incomplete lineage sorting can introduce conflicting signals, thereby complicating phylogenetic inference. In this study, we conducted comprehensive sampling of Elsholtzieae, with a particular focus on its largest genus, Elsholtzia. We utilized 503 nuclear loci and complete plastome sequences obtained from 99 whole-genome sequencing datasets to elucidate the interspecific relationships within the Elsholtzieae. Additionally, we explored various sources of conflicts between gene trees and species trees. Fully supported backbone phylogenies were recovered, and the monophyly of Elsholtzia and Keiskea was not supported. Significant gene tree heterogeneity was observed at numerous nodes, particularly regarding the placement of Vuhuangia and the E. densa clade. Further investigations into potential causes of this discordance revealed that incomplete lineage sorting (ILS), coupled with hybridization events, has given rise to substantial gene tree discordance. Several species, represented by multiple samples, exhibited a closer association with geographical distribution rather than following a strictly monophyletic pattern in plastid trees, suggesting chloroplast capture within Elsholtzieae and providing evidence of hybridization. In conclusion, this study provides phylogenomic insights to untangle taxonomic problems in the tribe Elsholtzieae, especially the genus Elsholtzia.

Deep genome skimming reveals the hybrid origin of Pseudosasa gracilis (Poaceae: Bambusoideae)

Pseudosasa gracilis (Poaceae: Bambusoideae) is a temperate woody bamboo species endemic to South-central China with a narrow distribution. Previous phylogenetic studies revealed an unexpected, isolated phylogenetic position of Ps. gracilis. Here we conducted phylogenomic analysis by sampling populations of Ps. gracilis and its sympatric species Ps. nanunica and Sinosasa polytricha reflecting different genomic signals, by deep genome skimming. Integrating molecular evidence from chloroplast genes and genome-wide SNPs, we deciphered the phylogenetic relationships of Ps. gracilis. Both plastid and nuclear data indicate that Ps. gracilis is more closely related to Sinosasa, which is discordant with the taxonomic treatment. To further explore this molecular-morphological conflict, we screened 411 "perfect-copy" syntenic genes to reconstruct phylogenies using both the concatenation and coalescent methods. We observed extensive discordance between gene trees and the putative species tree. A significant hybridization event was detected based on 411 genes from the D subgenome, showing Ps. gracilis was a hybrid descendant between Sinosasa longiligulata and Ps. nanunica, with 63.56% and 36.44% inheritance probabilities of each parent. Moreover, introgression events were detected in the C subgenome between Ps. gracilis and S. polytricha in the same distribution region. Our findings suggest that sympatric hybridization and introgression play a crucial role in the origin of Ps. gracilis. By providing an empirical example of bamboo of hybrid origin using comprehensive analyses based on genomic data from different inheritance systems and morphological characters, our study represents a step forward in understanding of reticulate evolution of bamboos.

Transcriptome data reveals the conservation genetics of Cypripedium forrestii, a plant species with extremely small populations endemic to Yunnan, China

The Cypripedium forrestii is an orchid species with extremely small populations (PSESP) in Yunnan, China. C. forrestii is range-restricted and less-studied than many orchid species, and it is exposed to various threats to its survival. We investigated its potential habitats and collected 52 samples from eight locations, as well as two outgroup species for reference. We developed genetic markers (SNPs) for C. forrestii based on transcriptome sequencing (RNA-seq) data, and analyzed the genetic diversity, population structure, gene flow and demographic history of C. forrestii in detail. C. forrestii is a taxonomically independent species to protect. We found that the genetic diversity of C. forrestii was very low (1.7e-4) compared with other endangered species. We identified three genetic clusters, and several populations with distinct genetic backgrounds. Most genetic diversity was found within sampling sites (87.87%) and genetic clusters (91.39%). Gene flow has been greatly limited over the most recent generations, probably due to geographical distance, historical climate change and habitat fragmentation. We also detected a severe bottleneck event brought about by the recent population constraints. These factors, together with its reproductive characteristics, contribute to the population fragmentation and low genetic diversity of C. forrestii. Based on our findings, we suggest an integrative conservation strategy to protect and recover the genetic diversity of C. forrestii and a further comprehensive study of its ecological traits in the future.

Hybridization and polyploidization generate evolutionary diversification of the genus Buddleja in the Sino-Himalayan region. A commentary on 'Deciphering complex reticulate evolution of Asian Buddleja (Scrophulariaceae): insights into the taxonomy and speciation of polyploid taxa in the Sino-Himalayan region'

This article comments on:

Fengmao Yang, Jia Ge, Yongjie Guo, Richard Olmstead and Weibang Sun. Deciphering complex reticulate evolution of Asian Buddleja (Scrophulariaceae): insights into the taxonomy and speciation of polyploid taxa in the Sino-Himalayan region, Annals of Botany, Volume 132, Issue 1, 01 July 2023, Pages 15–28, https://doi.org/10.1093/aob/mcad022