A chronotype effect: circadian-period variation underlies the local adaptation of photoperiodism in the short-day plant Lemna aequinoctialis

Phenotypic variation is the basis for trait adaptation via evolutionary selection.[1][1], [2][2], [3][3] However, the driving forces behind the quantitative trait variations in natural settings remain unclear owing to their complexity at the molecular level.[4][4], [5][5], [6][6] This study focused on the natural variation of the free-running period (FRP) of the circadian clock as FRP is a determining factor of the internal clock phase (chronotype), which is responsible for physiological timing during a day.[7][7] Although natural variations in FRP have been widely reported,[8][8], [9][9], [10][10], [11][11] few studies have shown the association between FRP and adaptive temporal traits. As a clock-dependent physiological process, photoperiodism is a typical target of local adaptation.[12][12], [13][13] Lemna aequinoctialis in Japan is a paddy-field duckweed exhibiting a latitudinal cline of critical day-lengths (CDLs) for short-day flowering.[14][14] To investigate the relationship between FRP and CDL, we collected 72 strains of L. aequinoctialis within a latitudinal range between 31.5°N to 43.8°N. We found a significant correlation ( P = 7.5E-8) between FRPs and locally adaptive CDLs, confirming that the variation in FRP-dependent chronotypes underlies geographically differentiated photoperiodism. Diel transcriptome analysis revealed that the induction timing of a florigen gene is key for connecting chronotypes to photoperiodism at the molecular level. Based on these results, we propose a fundamental rule concerning the “chronotype effect” in evolution: the variation of circadian period functions as a resource for the variation of temporal traits. This study highlights the adaptive significance of circadian-period variation and provides a reason for the maintenance of period variation in natural populations. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-4 [5]: #ref-5 [6]: #ref-6 [7]: #ref-7 [8]: #ref-8 [9]: #ref-9 [10]: #ref-10 [11]: #ref-11 [12]: #ref-12 [13]: #ref-13 [14]: #ref-14