Molecular Systematics Of Ambrosia

Inferring phylogeny and speciation processes of closely related species is one of the most fundmantal problems in plant evolutionary biology,and understanding phylogenetic relationships and speciation patterns is a prerequisite to explain origin and variation of species diversity.It is generally difficult to explore accurate phylogeny and speciation of closely related species,such as Ambrosia L.,due to evolutionary forces at the molecular level,for instance introgressive hybridization, lineage sorting,herizotal gene transfer,hidden paralogy,as well as morphological similarity among them.Ambrosia L.is a medium-sized genus with approximately forty-two species found predominantly in the New World,and principally in North America.The plants are distributed mostly in the desert regions of the southwestern United States and adjacent Mexico.Primitive species are shrubs of the deserts and arid regions,while more specialized and derived species are herbs often found as weeds of cultivation or as strand species of the Pacific and Caribbean beaches.In this study,the phylogeny of ragweed genus(Ambrosia) were explored using sequences of four DNA fragments from chloroplast(psbM-petD,psb and trnL-trnF) and nuclear genome(g3pdh).On these basis,we estimated the value of gaps and discussed coalescent data used in phylogenetic analyses.1.Phylogenetic reconstruction of AmbrosiaPhylogenetic analyses indicate that:Ambrosia falls into two main clades.The first clade includes A.trifida,A.bidentata,A.pumila and A.chamissonis.The second clade includes A.psilostachya,A.conferlillora,A.artemisiifolia and A.tomentosa. A.artemisiifolia is closely related with A.psilostachya,aroused by introgressive hybridization.And A.trifida is closely related with A.bidentata.2.Systematic values of gapsAnalyses implemented by maximum parsimony methods based on nuclear sequence and all genes data show that total of trees decrease and bootstrap increase. These results indicate that the gaps,when treated appropriately,can provide additional information in phylogenetic study.3.Coalescent data used in phylogenetic analyses.Coalescent data using three chloroplast fragments reveal that bootstrap of each clade markedly increase.By analyzing of the conflict between chloroplast gene tree and nuclear gene tree,introgression between A.artemisiifolia and A.psilostachya which cause the conflict were discovered.Coalescent of all the data could cover the noise in matrix.But the different evolutional history between chloroplast genome and nuclear genome would depress the bootstrap rate.When considering the discrepancy from different genome,coalescent data could reflect systematic relationship better.