Comparing with allopolyploid speciation, there are fewer cases of homoploid hybrid speciation. Although transient homoploid hybridization events were revealed in many plant genera, solid evidences from genomic data are scare.

Reticulate evolution caused by interspecific hybridization has greatly contributed to speciation of Actinidia. Actinidia zhejiangensis, a species published in 1982, distributes in Jiangxi, Zhejiang and Fujian province in China with a small population size. Previous studies speculated that A. zhejiangensis originated from hybridization between Actinidia eriantha and Actinidia hemsleyana or Actinidia rufa based on limited molecular markers and scarce number of sampled taxa. The hybridization origin of A. zhejiangensis, and phylogenetic relationship between it and other Actinidia species, still cannot be determined with morphological characteristics and the reported molecular evidence.

Researchers from Wuhan Botanical Garden, together with the collaborators from Lushan Botanical Garden and Forest Resources Monitoring Center of Zhejiang province, generated chromosome-scale reference genome assemblies of A. zhejiangensis and A. hemsleyana to reveal the transient homoploid hybridization of A. zhejiangensis.

The chromosomes of A. zhejiangensis were confidently assigned to two sets of haplomes. Combined with a published A. eriantha genome, the study found these two haplomes originated from A. eriantha and A. hemsleyana, respectively. Based on resequencing data from A. zhejiangensis, A. eriantha and A. hemsleyana individuals, they discovered that A. zhejiangensis were mainly F1 hybrids of A. hemsleyana and A. eriantha, and A. hemsleyana and A. eriantha were the constant paternal and maternal parents, respectively. Thus, A. zhejiangensis was not a stabilized independent hybrid species, even though gene flow initiated about 0.98 million years ago, implying strong reproductive barriers between A. hemsleyana and A. eriantha.

Five inversions containing genes involved in pollen germination and pollen tube growth might affect the fertility of hybrids between A. hemsleyana and A. eriantha. Despite its distinct morphological traits and long recurrent hybrid origination, A. zhejiangensis did not initiate speciation.

In summary, this study, for the first time, demonstrates dead-end hybridization between two deeply divergent kiwifruit species rather than homoploid hybrid speciation, englightening Acitinida speciation and generating valuable genomic resources for future evolutionary and functional genomics studies of kiwifruit.

It was supported by grants from the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Foundation of Hubei Hongshan Laboratory.

Results have been published on The Plant Journal entitled “Genomic analyses reveal dead-end hybridization between two deeply divergent kiwifruit species rather than homoploid hybrid speciation”. (First published: 31 May 2023)

 Morphological characteristics, genome features and geographic distribution of A. zhejiangensis, A. hemsleyana and A. eriantha. (a) Morphological characteristics for fruits and female flowers of A. zhejiangensis, A. hemsleyana and A. eriantha. (b) Circos plot of the A. zhejiangensis, A. hemsleyana and A. eriantha genomes (Image by YU Xiaofen)