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Two-way evolution of root form in the riverweed family Podostemaceae, with implications for phenotypic evolution

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Whether phenotypic evolution is always adaptive is a major issue in evolutionary biology. Based on the great morphological variation and slight ecological variation, Willis argued that Podostemaceae evolved in the absence of adaptation. Podostemaceae are ecological specialists with roots or holdfasts adhering to macrophyte-free (in Asia), soilless rock surfaces submerged in fast-flowing river currents. In Hydrobryum, the roots are foliose and rarely ribbon-like. The ribbon-like rooted species are almost always sympatric with foliose-rooted species. The matK phylogeny indicates that ribbon-like roots were derived recurrently from foliose roots. This late evolution is truly opposite to the early evolution, in which the foliose root of most Hydrobryum species was derived from the ribbon-like root of the common ancestor of the CladopusHydrobryum clade. The two-way evolution and subsequent sympatry suggests that root forms do not necessarily affect the microdistribution of the species.

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The illustrations are reproduced from Koi and Kato (2012)

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We thank T. Wongprasert, S. Suddee, L. Ampornpan, P. Werukamkul, N. Katayama and C. Tsutsumi for help in the field work and Y. Hirayama for technical assistance with the phylogenetic analysis. We also thank D. E. Boufford for the correction of the English of the manuscript. This study was supported by JSPS (KAKENHI Grant No. 26870502) and the Osaka City University Strategic Research Grant 2017 for young researchers to SK.

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SK and MK designed the study, conducted the research and wrote the paper.

Correspondence to Satoshi Koi.

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Online Resource 1. Materials used in molecular phylogenetic analysis. Voucher data include localities, voucher and DDBJ accession numbers; locality data are omitted for vouchers with registered sequence data.

Online Resource 2. Alignment of matK sequences used in phylogenetic analyses.

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Koi, S., Kato, M. Two-way evolution of root form in the riverweed family Podostemaceae, with implications for phenotypic evolution. Plant Syst Evol 306, 2 (2020). https://doi.org/10.1007/s00606-020-01635-1

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  • Adaptation
  • Epilithic root
  • Neutral evolution
  • Sympatry
  • Two-way evolution