| Encompassing ca.450 species from alpine and arctic areas in the Northern Hemisphere,Saxifraga is the most species-rich and taxonomically complex genus in Saxifragaceae.The genus is an ideally suited study system to alpine radiations and levels of diversity.Sect.Irregulares is a special group in the genus characterized by zygomorphic flowers.This section includes only 15-20 species,all of which are perennial herbs and flowering stems leafless or small bracts.Members of sect.Irregulares are widely distributed in eastern Asian,and mainly in China and Japan.Therefore,it is important to perform phylogenetic and biogeographic analyses of this section for a better understading of the whole genus.The main goal of this study is to reconstruct the phylogenetic relationships of sect.Irregulares and infer its biogeographic history,with highlights for their implications in understading the evolutionary history of the whole genus and even the family.1.Determination of the status of a new speciesSaxifraga kegangii D.G.Zhang,Y.Meng & M.H.Zhang was found from the Lüdong Mountain in Baojing county of Hunan province.The new species is closely related to S.epiphylla Gornall & H.Ohba and S.daqiaoensis F.G.Wang & F.W.Xing.These three species share such features(which distinguish them from the other sect.Irregulares species)as leafs abaxially with spotted,smaller petals with yellow spotted.S.daqiaoensis is characterized by peltate leaves,S.epiphylla is characterized by leaf blade with foliar embryos,however,the new species leaf is fan-shaped to orbicular and abaxially with yellow-green spotted,margin entire or 8–10-crenate and without foliar embryos,this combination of features differs from other species.The phylogenetic relationships tree based on nuclear gene(ITS)and chloroplast DNA regions(psbA-trnH and trnL-F)showed that S.daqiaoensis,S.epiphylla and the new species formed a clade(PP=1.00,BP=86,LP=93),and two accessions of new species were grouped together and sister to S.daqiaoensis with strong supports(PP=1.00,BP=100,LP=100).2.Molecular phylogenetic relationshipsBased on phylogenetic analyses of ITS and three chloroplast markers(trnL-F,psbA-trnH and matK),our phylogenetic results indicate that Sect.Irregulares is well supported as monophyletic and composed of two main lineages corresponding to their morphological features and biogeographic distributions.Clade A comprises only S.stolonifera Curtis and S.nipponica Makino,and Clade B contains the remaining taxa further grouped into two subclades.One consists of about 4 species(S.imparillis I.B.Balfour,S.fortune J.D.Hooker,S.rufescens I.B.Balfour,S.serotine Sipliv.),and the other containing 7 species(S.daqaoensis,S.kegangii,S.epiphylla,S.mengtzeana,S.kwangsiensis,S.sp1,S.sp2),all of which are endemic to China.(3)Biogeographic origin and evolutionary historyDivergence times estimation of Sect.Irregulares were carried out using the concatenated dataset(psbA-trnH,trnL-F,matK and ITS)based on three fossil calibration points.Biogeographic analyses indicated that the origin of Saxifraga sect.Irregulares was dated to 27.07 million years ago(Ma)with a split with sect.Heterisia in the late Oligocene with subsequent diversification in eastern Asia into two extant lineages(Clade A and Clade B)at 16.85-20.07 Ma.We hypothesized that sect.Irregulares originated in North America along with sect.Heterisia before diversifying in north-eastern Asia(probably in Japan),and then migrating into south part of eastern Asia with major diversifications in SW China,with recent migrations from south to north in eastern Asia.Compared with previous studies,our results with expanded samples well resolved the relationships within sect.Irregulares,and inferred the possible history of biogeographical evolution pattern.Though Sect.Irregulares is a group in the Saxifraga with fewer members,but it is a relatively old lineage with unique morphological characteristics and geographical distribution,which has important insights into our understanging for Saxifrage and other taxa in the family. |
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