Phylogenetic Position And Conservation Genetics Of The Endangered Plant Iris Narcissiflora

Iris narcissiflora is endemic to China and narrowly distributed at high-altitude areas of the Hengduan Mountains in western Sichuan.Serious contraction of distribution range has been detected in this species due to habitat degradation and human disturbances,and its natural populations only exist in Daofu County and Kangding City,Sichuan Province.Like other alpine plants,I.narcissiflora is facing increasing threats from global climate changes.However,the lack of information about its systematics and genetic variations will challenge the design of a practical and effective conservation strategy.In this study,the phylogenetic relationships between I.narcissiflora and its relatives were reconstructed using both plastid genomes and nuclear ribosomal DNA sequences.Moreover,comparative plastid genomics and time-calibrated phylogeny analyses were performed using the dataset containing both our assembled and the published plastid genomes.Finally,the spatial genetic variations of I.narcissiflora populations were delineated based on self-isolated nuclear microsatellites.The aim of this study thus was to evaluate the endangerment status of this species,providing insights into conservation practices.The main findings are as follows:（1）The phylogenetic trees constructed by both plastid and nuclear DNA sequences supported that I.narcissiflora belongs to the subgenus Iris section Pseudoregelia and is closely related with I.goniocarpa,I.goniocarpa var.tenella,and I.cuniculiformis.Compared with other clades,the plastid genomes in this taxon showed highly conserved structures and sequences,further supporting its monophyletic origin.These phylogenetic results are also consistent with morphological characteristics and geographic distributions.（2）Nuclear-cytoplasmic conflicts were detected between I.goniocarpa var.tenella and I.cuniculiformis,challenging the previous species delimitation.This inconsistency may be the result of natural hybridization or interspecific gene flow among I.goniocarpa,I.goniocarpa var.tenella,and I.cuniculiformis due to their overlapped distributions.The phylogenetic analysis showed no evidence of hybridization events between I.narcissiflora and its relatives,possibly because of its extremely narrow distribution range and distinct habitat preference compared with its relatives.（3）I.narcissiflora and its relatives diverged in the Mid Pliocene（3.97 Ma,95%PHD:1.84-6.45）,likely attributing to the rapid evolutionary radiations caused by the intense tectonic activities of the Hengduan Mountains.（4）The species level genetic diversity of I.narcissiflora is relatively high（H_E=0.558）,with no significant signals of inbreeding(F_IS=0.026).This could be attributed to the gene flow between neighboring populations and life history traits such as long generation time and outcrossing.Besides,a significant bottleneck signature was detected in the YLX3 population,resulting in the loss of some alleles and heterozygosity.This implies that recent human disturbance might have led to a rapid reduction of its effective population size,and genetic diversity is likely to decline in the future.（5）The genetic differentiation among I.narcissiflora populations is relatively high(F_ST=0.172).Both STRUCTURE and principal component discriminant analysis divided them into two genetic clusters:the Daofu cluster and the Kangding cluster.These two genetic clusters diverged in the Late Pleistocene（0.61 Ma,95%HPD:0.10-1.30）,indicating that long-term geographic isolation led to genetic differentiation.Based results of genetic structure analysis（K=2 and K=3）,we further found the absence of recent gene flow between genetic clusters,illustrating that dispersal can barely overcome geographic distances and mountain ranges.In summary,phylogenetic trees based on both nuclear and plastid data simultaneously support the phylogenetic position of I.narcissiflora.I.narcissiflora diverged from its relatives recently and is a relatively young taxon.Although the genetic diversity of I.narcissiflora is relatively high,the detection of a recent population bottleneck highlights the urgency to restore its disturbed habitats to prevent further population decline.Moreover,the strong population genetic structure indicates limited large-scale gene flow.Therefore,human-assisted gene flow in some populations is required to maintain genetic diversity and increase fitness in the case of inbreeding depression.In addition,ex situ conservation is also necessary to preserve the valuable genetic resources.