Temperature is not responsible for left-right reversal in pelagic unicellular zooplanktons

Many holoplanktons disperse passively without active habitat choice. Their morphology may vary over wide distribution ranges by phenotypic plasticity or allelic variation. Planktic foraminifera, which are unicellular holoplanktons and occur in every ocean, could be an excellent system to study diversity and evolution in cellular responses to the environment. They uniquely exhibit single-cell asymmetry in the coiled shell. Their handedness has long been said to change phenotypically by habitat temperature without statistical evidence. We tested temperature dependence of coil-morph frequency within species of pelagic foraminifera in global scale for the first time. Our analyses of molecular phylogeny indicate that five monophyletic clades in Globorotalia truncatulinoides represent genetically isolated species from one another. Morph frequency varies across wide ranges of water temperature within three of the five species but shows no dependence on temperature. Contrarily, morph frequency exhibited apparent dependence when we pooled all specimens of the five species. This suggests that the correlations with temperature have classically been observed because of taxonomical confusions and interspecific differences in distribution. The present results against the classical hypothesis by thorough examinations rather argue for a possible presence of genetic basis for coiling direction in foraminifera. Our results provide a base to explore the evolution of left-right asymmetry in unicellular eukaryotes.