title: Past hybridization between two East Asian long-tailed tits (Aegithalos bonvaloti and A. fuliginosus).
abstract: BACKGROUND
  Incomplete lineage sorting and hybridization are two major nonexclusive causes of haplotype sharing between species. Distinguishing between these two processes is notoriously difficult as they can generate similar genetic signatures. Previous studies revealed that the mitochondrial DNA (mtDNA) differentiation between two East Asian long-tailed tits (Aegithalos bonvaloti and A. fuliginosus) was extremely low, even lower than intraspecific differentiation in some other long-tailed tits. Using a combination of multilocus and coalescent analyses, we explored the causes of the anomalous lack of mtDNA differentiation between the two species.
  
  
  RESULTS
  The mtDNA divergence between the two species was shallow, while the nuclear DNA (nuDNA) divergence was considerably deeper. The IMa analyses based on the mtDNA dataset suggested relatively high gene flow from A. fuliginosus to A. bonvaloti, while negligible gene flow in the opposite direction. In contrast to mtDNA, the migration rates at autosomal and Z-linked nuDNA loci were negligible or much lower. The NEWHYBRIDS analysis assigned all individuals except one to pure parental species with high posterior probability. The Bayesian skyline plot showed that both species underwent population expansions during the Last Glacial Maximum (LGM), and the ecological niche modelling suggested that their ranges overlapped more during the LGM than at present.
  
  
  CONCLUSIONS
  We suggest that historical hybridization, in combination with selective sweep and/or genetic drift might be the main causes of the extremely low mtDNA differentiation between the two species. The hybridization probably occurred mainly between A. fuliginosus females and A. bonvaloti males. The LGM distribution expansion might have facilitated hybridization, while the post-LGM distribution contraction could have facilitated some mtDNA sorting. Ongoing hybridization between the two species might be very limited, but further studies with more samples from the contact zone are needed to test this conclusion.
authors.affiliation: Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, No.1 Beichen West Road, Chaoyang District, 100101 Beijing, China ; Center for Watershed Ecology, Institute of Life Science, Nanchang University, 330031 Nanchang, China.
authors.forename: Wenjuan
authors.initials: W
authors.lastname: Wang
authors.affiliation: School of Chemistry and Life Sciences, Guizhou Normal College, 550001 Guiyang, China.
authors.forename: Chuanyin
authors.initials: C
authors.lastname: Dai
authors.affiliation: Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, No.1 Beichen West Road, Chaoyang District, 100101 Beijing, China ; Swedish Species Information Centre, Swedish University of Agricultural Sciences, SE-75007 Uppsala, Sweden.
authors.forename: Per
authors.initials: P
authors.lastname: Alström
authors.affiliation: Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, No.1 Beichen West Road, Chaoyang District, 100101 Beijing, China.
authors.forename: Chunlan
authors.initials: C
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, No.1 Beichen West Road, Chaoyang District, 100101 Beijing, China.
authors.forename: Yanhua
authors.initials: Y
authors.lastname: Qu
authors.affiliation: Department of Life Science, National Taiwan Normal University, Taipei, Taiwan.
authors.forename: Shou-Hsien
authors.initials: SH
authors.lastname: Li
authors.affiliation: State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, 650223 Kunming, China.
authors.forename: Xiaojun
authors.initials: X
authors.lastname: Yang
authors.affiliation: Key Laboratory of Ecological Impacts of Hydraulic-Projects and Restoration of Aquatic Ecosystem of Ministry of Water Resources, Institute of Hydroecology, Ministry of Water Resources and Chinese Academy of Sciences, 430079 Wuhan, China.
authors.forename: Na
authors.initials: N
authors.lastname: Zhao
authors.affiliation: Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, No.1 Beichen West Road, Chaoyang District, 100101 Beijing, China.
authors.forename: Gang
authors.initials: G
authors.lastname: Song
authors.affiliation: Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, No.1 Beichen West Road, Chaoyang District, 100101 Beijing, China.
authors.forename: Fumin
authors.initials: F
authors.lastname: Lei
keywords: Ecological niche modelling; Incomplete lineage sorting; Introgression; Isolation-with-migration; Multilocus analyses