In the conventional view,species are separate gene pools delineated by reproductive isolation(Rl).In an alternative view,species may also be delineated by a small set of'speciation genes'without full RI,a view that has gained broad acceptance.A recent survey,however,suggested that the extensive literature on'speciation with gene flow'is mostly(if not all)about exchanges in the early stages of speciation.There is no definitive evidence that the observed gene flow actually happened after speciation is completed.Here,we wish to know whether'good species'(defined by the'secondary sympatry'test)do continue to exchange genes and,importantly,under what conditions such exchanges can be observed.De novo whole-genome assembly and re-sequencing of individuals across the range of two closely related mangrove species(Rhizophora mucronata and R.stylosa)reveal the genomes to be well delineated in allopatry.They became sympatric in northeastern Australia but remain distinct species.Nevertheless,their genomes harbor～4000-10 000 introgression blocks averaging only about 3-4 Kb.These fine-grained introgressions indicate continual gene flow long after speciation as non-introgressable'genomic islets,'～1.4 Kb in size,often harbor diverging genes of flower or gamete development.The fine-grained introgression in secondary sympatry may help settle the debate about sympatric vs.micro-allopatric speciation.In conclusion,true'good species'may often continue to exchange genes but the opportunity for detection is highly constrained.

Xinfeng Wang;Ziwen He;Zixiao Guo;Ming Yang;Shaohua Xu;Qipian Chen;Shao Shao;Sen Li;Cairong Zhong;Norman C.Duke;Suhua Shi

State Key Laboratory of Biocontrol,Guangdong Key Laboratory of Plant Resources,School of Life Sciences,Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai),Sun Yat-sen University,Guangzhou 510275,China;Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering,School of Life Sciences,Fudan University,Shanghai 200438,China;Department of Laboratory Medicine&Pathology,University of Washington,Seattle,WA 98195,USA;Hainan Academy of Forestry(Hainan Academy of Mangrove),Haikou 571100,China;Centre for Tropical Water and Aquatic Ecosystem Research,James Cook University,Townsville,QLD 4811,Australia

国家科学评论（英文版）

[1]Xinfeng Wang;Ziwen He;Zixiao Guo;Ming Yang;Shaohua Xu;Qipian Chen;Shao Shao;Sen Li;Cairong Zhong;Norman C.Duke;Suhua Shi-.Extensive gene flow in secondary sympatry after allopatric speciation)[J].国家科学评论（英文版）,2022(12):18-30

sympatry,Rl,mucronata,allopatry,introgressions,introgressable

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