The Phylogeny And Reticular Evolution Of The Genus Arrowba (Poaceae: Subfamily Bamboo) And Its Related Genera

Reticulate evolution involving hybridization and introgression has been recognized as a creative source of species and diversification in bamboos.Species of Fargesia Franch.emend.Yi nested within the Phyllostachys clade of tribe Arundinarieae,which are ecologically and economically important in mountainous forests.Previous phylogenetic studies revealed that Fargesia established mainly based on vegetative characteristics is not a monophyletic group.Most Fargesia species and Yushania should be merged into Fargesia(s.1.).However,the phylogenetic relationship between these two genera is still controversial depending on varying sampled taxa and molecular markers.Furthermore,the putative hybrid patterns of the major clades within Fargesia are also paid much attention due to incongruence between plastid and nuclear trees.Here,we used complete chloroplast genomes and multiple nuclear markers to reconstruct the phylogenetic relationship among Fargesia and its closely allies based on extensive sampling,to determine the monophyly of Fargesia,to detect the possible intergeneric or interspecific hybridization among Fargesia and its closely allies along with morphological evolution,and to examine the extent to which reticulate evolution has occurred at the interspecific level in bamboos.We reconstructed the phylogeny of Fargesia and its allies from a plastome sequence matrix of 46 species that contained 21 Fargesia and six Yushania species as ingroups,1 7 species from 10 other bamboo species plus Oryza sativa and Zea mays as outgroups.Fargesia and its allies were divided into eight clades.Fargesia damuniu and Thamnocalamus spathiflorus clustered in the Thamnocalamus clade,and Fargesia semiorbiculata,Fargesia collaris and Fargesia gyirongensis clustered in the Drepanostachyum+Himalayacalamus clade that rendered the polyphyly of Fargesia.The remaining six clades,including the Fargesia spathe clade,the Phyllostachys clade,the Ampelocalamus clade,Bashaniafargesii,the Yushania niitakayamensis clade,and the Fargesia macclureana clade,were identified.Bashania fargesii,the Phyllostachys clade and the Ampelocalamus clade are successive sisters to the Fargesia spathe clade.Meanwhile,Yushania species nested with Fargesia and grouped in the Yushania niitakayamensis clade,which is the paraphyletic group to the Fargesia macclureana clade.Combined with known inflorescences,Yushania might be included in Fargesia(s.l.),which share terminal inflorescences,racemose to paniculate,a series of leaf sheaths at the basis of inflorescences regardless of size.A total of 130 bamboo species were chosen as Fargesia and its allies,including 71 Fargesia species(72 individuals),39 Yushania species,18 species from other genera,and Dendrocalamus latiflorus belonging to Bambusatae.Three nuclear genes(gpa1?pabp1 and rpb2)and nrITS sequences were used to reconstruct the evolutionary history among Fargesia(s.l.)and its allies to discuss the causes of the incongruence between the plastid and nuclear trees of the major clades within Fargesia.Phylogenetic trees based on nrITS sequences revealed seven major clades despite some unclear lineage affinities.Temperate woody bamboos(tribe Arundinarieae)are allotetraploid with subgenomes designated A and B of three nuclear genes,respectively.In the phylogenetic trees based on combined homeologs A sequences of nuclear genes,Bashania species nested within Fargesia(s.l.),which is further divided into two groups.The Fargesia spathe clade,the Alpine Bashania clade,and Fargesiayajiangensis comprise a Group A-I,while the Bashania fargesii clade and the remaining Fargesia(s.l.)species form the Group A-II,which was divided into the Yushania niitakayamensis clade,the Yushania brevipaniculata clade,the Yushania qiaojiaensis f.nuda clade,and the Fargesia macclureana clade.Clades revealed in combined homeologs A phylogeny were not well resolved in combined homeologs B phylogeny as they formed a polytomy.Incomplete lineage sorting,hybridization/introgression,plastid capture,and recent radiation among different clades within Fargesia(s.l.)confound the nuclear and chloroplast phylogenies.The Fargesia spathe clade may have a hybrid origin.Results of PhyloNetwork revealed multiple introgression within Fargesia(s.l.).We also detected several putative hybrid species in Fargesia(s.l.)based on alleles of homeologs A or B of nuclear genes.Overall,evidence was found of different degrees of hybridization,ranging from interspecific gene flow to hybridization or introgression at the clade level.Rapid radiation and reticulate evolution have played a prominent role in Fargesia(s.l.)evolution,which could,in part,account for the taxonomic difficulty associated with Fargesia(s.l.)and other alpine bamboos.The study also underscores the importance of hybridization in the evolution of bamboos,at both intergeneric and intrageneric levels.