Analysis of the chloroplast genomes of four Pinus species in Northeast China:Insights into hybrid speciation and identification of DNA molecular markers

Species of the Pinus genus provide a classical model for studying hybrid speciation.Although studies on two narrowly distributed species(P inus funebris and P.takahasii)concluded that they originated from two widespread species(P.sylvestris and P.densiflora)via hybrid speciation,the conclusion was based on a low number of informative restriction sites.In this study,we analyzed the sequences of four Pinus chloroplast(cp)genomes(P.sylvestris,P.densiflora,P.funebris and P.takahasii)to clarify whether hybrid speciation was involved.The complete cp-genomes of Pinus species ranged in size from 119,865 to 119,890 bp,similar to other Pinus species.Phylogenetic results based on the whole cp-genomes showed P.sylvestris clustered with P.funebris and P.takahasii,which suggested that P.sylvestris was the paternal parent in hybridization events.In an analysis of simple sequence repeats(SSRs),we detected a total of 69 SSRs repeats among the four Pinus cp-genomes;most were A or T bases.In addition,we identified divergent hotspot regions among the four Pinus cp-genomes(trnE-clpP,cemA-ycf4,petD-rpoA,psbD-trnT,and trnN-chlL),in P.sylvestris(psbD-trnT,trnN-chlL,psbB and rps8)and in P.densiflora(trnE-clpP,petD-rpoA,ycf3 intron,psbD-trnT,and trnN-chlL).The genome information found in this study provides new insights into hybrid speciation in P inus and contributes to a better understanding of the phylogenetic relationships within the Pinus genus.

Hybrid speciation via inheritance of alternate alleles of parental isolating genes

It is increasingly realized that homoploid hybrid speciation(HHS),which involves no change in chromosome number,is an important mechanism of speciation.HHS will likely increase in frequency as ecological and geographical barriers between species are continuing to be disrupted by human activities.HHS requires the establishment of reproductive isolation between a hybrid and its parents,but the underlying genes and genetic mechanisms remain largely unknown.In this study,we reveal by integrated approaches that reproductive isolation originates in one homoploid hybrid plant species through the inheritance of altemate alleles at genes that determine parental premating isolation.The parent species of this hybrid species are reproductively isolated by differences in flowering time and survivorship on soils containing high concentrations of iron.We found that the hybrid species inherits alleles of parental isolating major genes related to flowering time from one parent and alleles of major genes related to iron tolerance from the other parent.In this way,it became reproductively isolated from one parent by the difference in flowering time and from the other by habitat adaptation(iron tolerance).These findings and further modeling results suggest that HHS may occur relatively easily via the inheritance of alternate parental premating isolating genes and barriers.

Genomics of plant speciation

Studies of plants have been instrumental for revealing how new species originate.For several decades,botanical research has complemented and,in some cases,challenged concepts on speciation developed via the study of other organisms while also revealing additional ways in which species can form.Now,the ability to sequence genomes at an unprecedented pace and scale has allowed biologists to settle decades-long debates and tackle other emerging challenges in speciation research.Here,we review these recent genome-enabled developments in plant speciation.We discuss complications related to identification of reproductive isolation(RI)loci using analyses of the landscape of genomic divergence and highlight the important role that structural variants have in speciation,as increasingly revealed by new sequencing technologies.Further,we review how genomics has advanced what we know of some routes to new species formation,like hybridization or whole-genome duplication,while casting doubt on others,like population bottlenecks and genetic drift.While genomics can fast-track identification of genes and mutations that confer RI,we emphasize that follow-up molecular and field experiments remain critical.Nonetheless,genomics has clarified the outsized role of ancient variants rather than new mutations,particularly early during speciation.We conclude by highlighting promising avenues of future study.These include expanding what we know so far about the role of epigenetic and structural changes during speciation,broadening the scope and taxonomic breadth of plant speciation genomics studies,and synthesizing information from extensive genomic data that have already been generated by the plant speciation community.